NIWA Client report · 2020-06-05 · three times the median flow at the Kauaeranga River site. 16 Figure 3-8: Ohinemuri River site. 17 Figure 3-9: Ohinemuri River flow from 1 January

Waikato Regional Council Technical Report 2017/28

Periphyton and macrophytes in seven Hauraki-Coromandel rivers www.waikatoregion.govt.nz ISSN 2230-4355 (Print) ISSN 2230-4363 (Online)

Prepared by: Fleur Matheson and Rohan Wells (NIWA) For: Waikato Regional Council Private Bag 3038 Waikato Mail Centre HAMILTON 3240 July 2017 Document #:10818626

Doc # 10818626

Peer reviewed by: Bill Vant

Date August 2017

Approved for release by: Date December 2018 Tracey May

Disclaimer This technical report has been prepared for the use of Waikato Regional Council as a reference document and as such does not constitute Council’s policy. Council requests that if excerpts or inferences are drawn from this document for further use by individuals or organisations, due care should be taken to ensure that the appropriate context has been preserved, and is accurately reflected and referenced in any subsequent spoken or written communication. While Waikato Regional Council has exercised all reasonable skill and care in controlling the contents of this report, Council accepts no liability in contract, tort or otherwise, for any loss, damage, injury or expense (whether direct, indirect or consequential) arising out of the provision of this information or its use by you or any other party.

Periphyton and macrophytes in seven Hauraki-Coromandel rivers Initial assessment of seasonal abundance, provisional

attribute state and management options

Prepared for Waikato Regional Council

July 2017

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Prepared by: Fleur Matheson Rohan Wells

For any information regarding this report please contact: Dr Fleur Matheson Biogeochemist Aquatic Plants +64-7-856 1776 [email protected] National Institute of Water & Atmospheric Research Ltd PO Box 11115 Hamilton 3251 Phone +64 7 856 7026

NIWA CLIENT REPORT No: 2017243HN Report date: July 2017 NIWA Project: EVW16216

Quality Assurance Statement

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Formatting checked by: Alison Bartley

Approved for release by: David Roper

Periphyton and macrophytes in seven Hauraki-Coromandel rivers

Contents

Executive summary ............................................................................................................... 7

1 Introduction ................................................................................................................ 8

2 Methods ...................................................................................................................... 9

3 Results ....................................................................................................................... 11

3.1 Kauaeranga River .................................................................................................... 11

3.2 Ohinemuri River ...................................................................................................... 17

3.3 Waiwawa River ....................................................................................................... 21

3.4 Piako River – Kiwitahi .............................................................................................. 26

3.5 Piako River – Paeroa-Tahuna Rd ............................................................................. 32

3.6 Waihou River .......................................................................................................... 38

3.7 Waitoa River ........................................................................................................... 43

4 Discussion ................................................................................................................. 49

4.1 Provisional NOF periphyton attribute states .......................................................... 49

4.2 Substituting cover for biomass ............................................................................... 51

4.3 Options to manage D band sites ............................................................................. 53

5 Conclusions ............................................................................................................... 55

6 Acknowledgements ................................................................................................... 56

7 Glossary of abbreviations and terms .......................................................................... 57

8 References................................................................................................................. 58

Appendix A Periphyton and macrophyte summary data ....................................... 60

Tables Table 2-1: Periphyton cover categories used in the monitoring protocol. 9 Table 2-2: Substrate size categories used in the monitoring protocol. 10 Table 3-1: Kauaeranga River monthly summary data for shade, water depth,

substrate, chlorophyll a and the number of days since an event greater than or equal to three times the median flow. 14

Table 3-2: Ohinemuri River monthly summary data for shade, water depth, substrate, chlorophyll a and number of days since an event greater than or equal to three times the median flow. 19


Table 3-3: Waiwawa River monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times the median flow. 23

Table 3-4: Piako River - Kiwitahi monthly summary data for shade, water depth, substrate, chlorophyll a and number of days since an event greater than or equal to three times median flow. 28

Table 3-5: Piako River - Paeroa-Tahuna Rd monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times the median flow. 34

Table 3-6: Waihou River monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times the median flow. 40

Table 3-7: Waitoa River monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times median flow. 45

Table 4-1: Periphyton biomass data for Hauraki-Coromandel rivers showing interpolated missing values. 49

Table 4-2: Provisional periphyton attribute states assigned to Hauraki-Coromandel rivers. 50

Figures Figure 3-1: Kauaeranga River site. 11 Figure 3-2: Kauaeranga River flow from 1 January 2016 to 23 March 2017. 11 Figure 3-3: Kauaeranga River periphyton cover on monthly sampling dates from February

2016 to March 2017. 12 Figure 3-4: Derived versus measured derived chlorophyll a values for Kauaeranga River

site. 13 Figure 3-5: Periphyton total cover versus measured periphyton biomass as chlorophyll a at

Kauaeranga River site. 13 Figure 3-6: Periphyton cover versus shade at the Kauaeranga River site. 15 Figure 3-7: Periphyton total cover versus days since an event greater than or equal to

three times the median flow at the Kauaeranga River site. 16 Figure 3-8: Ohinemuri River site. 17 Figure 3-9: Ohinemuri River flow from 1 January 2016 to 23 March 2017. 17 Figure 3-10: Ohinemuri River periphyton cover on monthly sampling dates from February

2016 to January 2017. 18 Figure 3-11: Periphyton cover versus shade at the Ohinemuri River site. 20 Figure 3-12: Waiwawa River site. 21 Figure 3-13: Waiwawa River flow from 1 April 2016 to 23 March 2017. 21 Figure 3-14: Waiwawa River periphyton cover on monthly sampling dates from April 2016

to March 2017. 22 Figure 3-15: Periphyton cover versus shade at the Waiwawa River site. 24 Figure 3-16: Periphyton biomass as chlorophyll a versus number of days since an event

greater than or equal to three times the median flow at the Waiwawa River site. 25

Figure 3-17: Piako River - Kiwitahi site. 26


Figure 3-18: Piako River - Kiwitahi flow from 1 February 2016 to 23 March 2017. 26 Figure 3-19: Piako River - Kiwitahi periphyton cover on monthly sampling dates from

February 2016 to March 2017. 27 Figure 3-20: Piako River - Kiwitahi percentage of channel cross-sectional area or volume

(CAV) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. 29

Figure 3-21: Piako River - Kiwitahi macrophyte percentage of channel water surface area (WSA) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. 30

Figure 3-22: Percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes versus days since an event greater than or equal to three times the median flow at the Piako River - Kiwitahi site. 31

Figure 3-23: Measured periphyton biomass as chlorophyll a versus the percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes. 31

Figure 3-24: Piako River - Paeroa-Tahuna Rd site. 32 Figure 3-25: Piako River - Paeroa-Tahuna Rd flow from 1 February 2016 to 23 March

2017. 32 Figure 3-26: Piako River - Paeroa-Tahuna Rd periphyton cover on monthly sampling dates

from February 2016 to March 2017. 33 Figure 3-27: Derived versus measured periphyton biomass as chlorophyll a at the Piako

River - Paeroa-Tahuna Rd site. 35 Figure 3-28: Cover of long filaments vs measured periphyton biomass as chlorophyll a at

the Piako River - Paeroa-Tahuna Rd site. 35 Figure 3-29: Piako River - Paeroa-Tahuna Rd percentage of channel cross-sectional area or

volume (CAV) occupied by macrophytes on monthly sampling dates from Feb 2016 to Jan 2017. 36

Figure 3-30: Piako River - Paeroa-Tahuna Rd percentage of channel water surface area (WSA) occupied by macrophytes on monthly sampling dates from Feb 2016 to Jan 2017. 36

Figure 3-31: Periphyton and macrophyte metrics versus days since an event greater than or equal to three times the median flow at the Piako River - Paeroa-Tahuna Rd site. 37

Figure 3-32: Waihou River site. 38 Figure 3-33: Waihou River flow from 1 February 2016 to 23 March 2017. 38 Figure 3-34: Waihou River percentage of channel cross-sectional area or volume (CAV)

occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. 41

Figure 3-35: Waihou River percentage of channel water surface area (WSA) by macrophytes on monthly sampling dates from February 2016 to March 2017. 41

Figure 3-36: Percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes versus days since an event greater than or equal to three times the median flow at the Waihou River site. 42

Figure 3-37: Waitoa River site. 43 Figure 3-38: Waitoa River flow from 1 February 2016 to 23 March 2017. 43 Figure 3-39: Waitoa River periphyton cover on monthly sampling dates from February 2016

to March 2017. 44


Figure 3-40: Derived versus measured periphyton biomass as chlorophyll a at the Waitoa River site. 46

Figure 3-41: Cover of long filaments versus measured periphyton biomass as chlorophyll a at the Waitoa River site. 46

Figure 3-42: Waitoa River percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. 47

Figure 3-43: Waitoa River percentage of channel water surface area (WSA) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. 47

Figure 3-44: Percentage of channel cross-sectional area or volume (CAV) or water surface area (WSA) occupied by macrophytes versus days since an event greater than or equal to three times the median flow at the Waitoa River site. 48

Figure 4-1: Relationships between measured periphyton biomass and cover, and measured and derived periphyton biomass, in Hauraki-Coromandel rivers. 52

Periphyton and macrophytes in seven Hauraki-Coromandel rivers 7

Executive summary The National Policy Statement for Freshwater Management (NPS-FM) identifies periphyton as the key attribute for assessing ecosystem health and trophic state in rivers. Waikato Regional Council (WRC) sought to gain a better understanding of periphyton and macrophyte abundance and seasonal dynamics in a selection of representative Hauraki-Coromandel rivers to inform an upcoming Plan Change (“Healthy Rivers #2”). WRC also wished to examine the applicability of periphyton cover to biomass conversion factors established for Canterbury rivers to the Hauraki-Coromandel system to ensure that current visual observation procedures would comply with NPS-FM requirements. There was also interest in the abundance of macrophytes, which are the primary attachment surface for periphyton in soft-bottom rivers which are common in the Hauraki-Coromandel system.

NIWA were engaged by WRC to undertake monthly monitoring of periphyton and macrophyte abundance for one year at the following river sites: Kauaeranga River at Smiths Cableway, Waiwawa at Rangihau Ford, Ohinemuri River at Karangahake, Waihou River at Te Aroha, Waitoa River at Mellon Road, Piako River at Kiwitahi and Piako River at Paeroa-Tahuna Road. Three of the sites were hard-bottom and four sites were soft-bottom. For periphyton, cover classes were assessed using the WRC protocol for wadeable streams and samples for biomass were collected and analysed in line with national protocols. For macrophytes the percentages of channel cross-sectional area or volume (CAV) and water surface area (WSA) occupied by individual species, and in total, were assessed. Monitoring at these sites also included assessments of channel width, water depth, dominant substrate type and shading. Flow records for the sites were also obtained and examined.

Our monitoring data indicate that two of the four soft-bottom river sites monitored (Piako – Kiwitahi and Waitoa) have a provisional NOF periphyton attribute state below the national bottom line. These two sites, and Piako River at Paeroa-Tahuna Road, also have nuisance macrophytes occupying more than 50% of the channel cross-sectional area or volume (and up to 90%) during the summer-autumn period which exceeds provisional guidelines to protect aesthetic, angling, ecological and flow conveyance values. The three hard-bottom river sites monitored (Kauaeranga, Ohinemuri and Waiwawa) and the fourth soft-bottom river site (Waihou) have provisional NOF periphyton attribute states in the A and B bands, indicating excellent and good condition, respectively.

The main drivers of periphyton and macrophyte abundance in rivers are flow, light, nutrients, grazing and temperature. Manipulation of channel lighting and temperature via riparian planting (to produce >60% shading across a large proportion of the river channel where it is < c. 14 m) is the most promising approach for reducing nuisance periphyton and macrophyte abundance in Hauraki-Coromandel soft-bottom rivers to comply with NOF requirements to implement remedial management actions at locations that fall below national bottom lines. Riparian planting also has a number of significant co-benefits. It is unlikely that the other main drivers of abundance (i.e., flow, nutrients and grazing) could be manipulated sufficiently to exert an influence in these rivers.

Waikato Regional Council’s State of Environment monitoring programme (for wadeable streams) currently assesses periphyton abundance as cover rather than biomass. The results of this investigation suggest that using conversion factors derived from Canterbury streams, or the overall cover metric to biomass regression equations derived using this Hauraki-Coromandel river data set, would likely provide only crude approximations of biomass at quite a coarse scale.

8 Periphyton and macrophytes in seven Hauraki-Coromandel rivers

1 Introduction The National Policy Statement for Freshwater Management (NPS-FM) identifies periphyton as the key attribute for assessing ecosystem health and trophic state in rivers. For example, regular and/or extended-duration nuisance blooms of periphyton (as chlorophyll a >200 mg/m2) are regarded as reflecting high nutrient enrichment and/or significant alteration of the natural flow regime or habitat. In soft-bottom rivers, macrophytes are a primary attachment surface for periphyton and usually contribute substantially to primary production. High levels of primary production (periphyton and/or macrophytes) can exert a strong influence on river dissolved oxygen levels and pH. Large diel fluctuations in these parameters are often associated with excessive primary production generating supersaturated oxygen and high pH levels during the day and very low levels of dissolved oxygen at night. Depleted dissolved oxygen levels are particularly harmful to fish and stream invertebrates.

Waikato Regional Council (WRC) sought to gain a better understanding of periphyton and macrophyte abundance and temporal dynamics in a selection of representative streams and rivers to inform the Hauraki-Coromandel Plan Change (“Healthy Rivers #2”). In addition, WRC wished to examine the applicability of the periphyton cover to biomass conversion factors established in Canterbury rivers (Kilroy et al. 2013) to the Hauraki-Coromandel system to ensure that procedures used from making visual observations will meet National Policy Statement requirements (as per Ministry for the Environment’s recent draft guidance for attributes, MfE 2015). NIWA were thus engaged by WRC to undertake monthly monitoring of periphyton and macrophyte abundance at the following river sites: Kauaeranga River at Smiths Cableway, Waiwawa at Rangihau Ford, Ohinemuri River at Karangahake, Waihou River at Te Aroha, Waitoa River at Mellon Road, Piako River at Kiwitahi and Piako River at Paeroa-Tahuna Road.

All sites selected for this study have matching flow and water quality data enabling examination of relationships between periphyton, flow and nutrients. Light availability is also a key driver of periphyton abundance, so measurements of shading were included in the monthly monitoring protocol. For future management of rivers based on the periphyton attribute, options to regulate biomass include: riparian planting and shading to limit light availability and reduce stream temperatures (most feasible for narrower channels), controls on point and diffuse source nutrient inputs and limits on water abstraction. Better understanding the relationships between periphyton and these drivers (particularly at regional and local scales) is critical for informing these management actions.


2 Methods Monitoring was carried out monthly for 14 months as river flow conditions allowed. Monitoring of all sites except Waiwawa commenced in February 2016. Monitoring at Waiwawa began in April 2016.

To assess periphyton cover we used the NIWA RAM-2 protocol (Biggs and Kilroy 2000), as modified by Waikato Regional Council (Collier et al. 2014), which visually assesses cover of different periphyton categories (Table 2-1). We made assessments using an underwater viewer (or snorkelling mask in deeper water) at five equidistant points in each of five transects within a 100 m reach located in ‘run’ habitat. In total, 25 observations of periphyton cover are made at each site on each sampling date. Nuisance abundance is defined as >30% long filaments (LF) or >60% thick mats (TM) (MfE 2000), or a weighted composite cover >30% (i.e., PeriWCC = LF + (TM/2) (Matheson et al. 2012a). We also calculated periphyton total cover as the sum of short and long filaments plus medium and thick mats.

Table 2-1: Periphyton cover categories used in the monitoring protocol.

Periphyton cover categories

Mat Filament

Thin film/mat

Medium mat – green Short filaments

Medium mat – light brown Long filaments - green

Medium mat – black or dark brown Long filaments – brown or red

Thick mat – green or light brown

Thick mat – black or dark brown

Thin film/mats are <0.5mm thick, medium mats are 0.5 to 3mm thick, thick mats are >3mm thick and long filaments are >2cm long.

At sites where macrophytes were present we assessed macrophyte abundance across the same transects as periphyton using a modified form of the Collier et al. (2014) protocol. At each transect point we recorded the percentage of the water surface area (%WSA) in a 0.5 m x 0.5 m quadrat occupied by each plant species. We also recorded the percentage of the channel wetted cross-sectional area or volume (%CAV) in this area occupied by each species. This enabled us to calculate %WSA and %CAV metrics by individual species and as a total (Matheson et al. 2012a, Davies-Colley et al. 2012).

At each transect point we measured water depth and recorded dominant substrate type (as boulder, cobble, gravel, sand, silt or clay). We also quantified % shade using a concave spherical densiometer (Forestry Suppliers®). A total of 25 observations of water depth, substrate and shade were made at each site on each visit.


Table 2-2: Substrate size categories used in the monitoring protocol.

Substrate size categories

Boulder >25.6 cm

Cobble >6.4 to 25.6 cm

Gravel >0.2 to 6.4 cm

Sand <2 mm

Silt <63 um

Clay <3.9 um

To assess periphyton biomass (as chlorophyll a) we collected five samples at equidistant points across the central transect according to the quantitative defined area sampling protocols (1b and 3) of Biggs and Kilroy (2000). These samples were combined into a single sample for analysis. Where substrate was dominated by cobbles and/or gravels we collected whole rocks with a combined surface area of at least 10 x 10 cm at each sampling point. Where substrate was boulder (or, rarely, bedrock) we carefully selected an adjacent cobble with comparable periphyton cover. Boulders were most frequently encountered in the Ohinemuri River with snorkelling generally required to collect samples. This precluded use of the Biggs and Kilroy (2000) boulder sampler device. Where substrate was fine sediment (sand and/or silt), in either shallow or deeper water, samples to c. 2 cm depth were scooped up in a 7.5 cm x 7.5 cm fine mesh net. Where macrophytes were present at the sampling point we collected a circular defined area (10 cm diameter) on the uppermost surface of the macrophyte bed.

In the laboratory we carefully removed periphyton from the substrates in each of our samples. To remove periphyton from cobbles and gravels we used a small scrubbing brush. We carefully dislodged periphyton adhering to macrophytes and fine sediments by hand, by wiping leaf blades clean and agitating sediment, respectively. Periphyton samples with substrate removed were homogenised in a blender. Duplicate subsamples were filtered for analysis of chlorophyll a by acetone pigment extraction and spectrophotometry (APHA 1998). We measured the a, b and c axes of all gravels and cobbles. Chlorophyll a content was converted to areal units (mg/m2) by dividing by solar-exposed surface area of the stones according to Dall (1979) and/or the defined surface area sampled for fine sediments and macrophytes. Using the conversion factors in Kilroy et al. (2013) we calculated an estimate of periphyton chlorophyll a from the percentage cover data for comparison with our direct measurements of chlorophyll a.

For each site we obtained the entire flow record (5 min intervals) from 1 January 2015 to 23 March 2017 from WRC. We calculated the median flow for each site, including Waiwawa, for the 2-year period 1 February 2015 to 31 January 2017. In general, a flood of three times the median flow or greater is considered likely to scour periphyton from the river bed (Clausen and Biggs 1997). For each site and sampling date we examined the flow record to calculate the number of days since a flushing event that was greater than or equal to three times the median flow. Relationships between plant abundance and the number of days since an event that was greater than or equal to three times the median flow were examined using linear regression.


3 Results

3.1 Kauaeranga River The monitoring site was located c. 20 m downstream of the Regional Council’s flow monitoring tower (Figure 3-1). This site was monitored 13 times between 1 February 2016 and 31 March 2017 on a monthly basis as river flow conditions allowed (Figure 3-2), by wading. The only month not sampled was May.

Figure 3-1: Kauaeranga River site. View looking upstream from true left bank of most downstream transect. Photo taken in February 2016 (Photo: T. Burton).

Figure 3-2: Kauaeranga River flow from 1 January 2016 to 23 March 2017. Flow data from Waikato Regional Council. Sampling dates are marked with a red cross.


At this site river wetted width was c. 30 m. Average water depth on sampling dates ranged from 0.29 m in January to 0.62 m in October with an overall range of 0.03 m to 0.97 m (See Table 3-1 on following page). Substrate was predominantly cobble intermixed with boulders and gravel, occasional patches of sand and, rarely, silt. Average shading of the site from riparian vegetation ranged from a low of 7% in August to a high of 28% in March 2016.

Periphyton cover only once reached nuisance levels at this site (Figure 3-3), namely in December. Thin films, short filaments and medium mats (light brown) were usually most prevalent at this site. At the peak of periphyton abundance in December, thick mats (green/light brown) and long filaments (green and brown/red) presented a combined nuisance cover (i.e., LF + TM) of 58% and a weighted composite nuisance cover (i.e., LF + (TM/2)) of 34%.

Figure 3-3: Kauaeranga River periphyton cover on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in May 2016. Tfm = Thin film, Sf = Short filaments, Mmlb = Medium mat light brown, Mmbldb = Medium black dark brown, Mmgr = Medium mat green, Lfbrrd = Long filaments brown/red, Lfgr = Long filaments green, Tmgrlb = Thick mat green/light brown. See Appendix A for underlying data including standard error values.

Measured periphyton biomass (chlorophyll a) at this site was consistently low, with no obvious seasonal pattern, ranging from a minimum of 2 mg/m2 in November and March (2017), to a maximum of 26 mg/m2 in February 2017 (Table 3-1). We found that measured chlorophyll a was significantly correlated with chlorophyll a derived from periphyton cover using Kilroy et al. (2013) conversion factors (Figure 3-4).


Figure 3-4: Derived versus measured derived chlorophyll a values for Kauaeranga River site. Derived chlorophyll a values calculated from periphyton cover for the chlorophyll a sampled transect using conversion factors of Kilroy et al. (2013).

We also found that measured periphyton biomass was positively correlated to cover metrics but this relationship was only statistically significant with total periphyton cover (i.e., combined cover of short and long filaments plus medium and thick mats) (Figure 3-5).

Figure 3-5: Periphyton total cover versus measured periphyton biomass as chlorophyll a at Kauaeranga River site. Periphyton total cover = cover of long and short filaments plus medium and thick mats.

The native charophyte, Nitella aff. cristata was periodically observed in low abundance adjacent to one or two inshore transect points on the true right bank. No other macrophytes were recorded in, or adjacent to, transects.


Table 3-1: Kauaeranga River monthly summary data for shade, water depth, substrate, chlorophyll a and the number of days since an event greater than or equal to three times the median flow. Values are means with range in parentheses where applicable.

Parameter Month

Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

Shade (%) 18 (0-77)

28 (9-50)

22 (0-67)

- 22

(4-47) 12

(0-51) 7

(0-40) 9

(0-53) 8

(0-33) 13

(0-55) 13

(0-63) 13

(0-71) 13

(0-55) 11

(0-54)

Water depth (m) 0.40 (0.05-0.95)

0.53 (0.16-0.91)

0.36 (0.05-0.67)

- 0.51

(0.16-0.90)

0.55 (0.24-0.97)

0.55 (0.19-0.95)

0.43 (0.10-0.82)

0.62 (0.29-0.92)

0.40 (0.11-0.80)

0.35 (0.14-0.73)

0.31 (0.03-0.72)

0.29 (0.10-0.62)

0.46 (0.06-0.68)

Substrate C (G-B)

C (C-B)

C (G-B)

-

C (C-B)

C (S-B)

C (S-B)

C (S-B)

C (S-B)

C (T-B)

C (S-B)

C (S-B)

C (G-B)

C (S-B)

Measured chl a (mg/m2) 15 21 8 - 23 6 21 23 17 2 25 15 26 2

Derived chl a (mg/m2) 66 11 44 58 39 64 89 24 37 128 74 81 2

Ratio of derived to measured chl a 4.4 0.5 5.5 - 2.5 6.5 3.1 3.9 1.4 18.5 5.1 4.9 3.1 1.0

Days since 3Xmedian flow event

3 1 9 - 3 0 1 14 1 12 34 52 24 9

‘-‘ = missing data. B=Boulder, C=Cobble, G=Gravel, S=Sand, T=Silt.


Examining the dataset derived from individual transect points we found that nuisance cover of long filaments (>30%) and thick mats (>60%) occurred exclusively where there were low to moderate levels of shade (i.e., ≤55%) (Figure 3-6).

Figure 3-6: Periphyton cover versus shade at the Kauaeranga River site. Data shown are for individual transect points from February 2016 to March 2017. Nuisance covers of long filaments and thick mats are >30% and >60%, respectively.


For the two-year period from 1 February 2015 to 31 January 2017 median flow in the Kauaeranga river was 2.19 m3/s and thus three times the median flow was 6.57 m3/s. We found a significant relationship between the number of days since flow event greater than or equal to three times the median flow and periphyton total cover (Figure 3-7), weighted composite cover (not shown) and cover of thick mats (not shown). We did not find a significant relationship with biomass as chlorophyll a or with cover of long filaments.

Figure 3-7: Periphyton total cover versus days since an event greater than or equal to three times the median flow at the Kauaeranga River site. Data shown corresponds to each monthly sampling event. “Total cover” refers to cover of short and long filaments plus medium and thick mats.


3.2 Ohinemuri River The Ohinemuri monitoring site was located c. 50 m downstream of the Regional Council’s flow monitoring tower (Figure 3-8).

Figure 3-8: Ohinemuri River site. View looking downstream across the lower transects in February 2016 (Photo: F Matheson).

This site was monitored 11 times between 1 February 2016 and 31 March 2017 on a monthly basis as river flow conditions allowed, usually by snorkelling from a small inflatable boat (Figure 3-9). Months not sampled were June, August and January. A rare error with fieldwork scheduling was the cause of the missed sampling in January, rather than unsafe river flow conditions.

Figure 3-9: Ohinemuri River flow from 1 January 2016 to 23 March 2017. Flow data from Waikato Regional Council. Sampling dates are marked with a red cross.


River wetted width at this site was c. 36 m. Average water depth on sampling dates ranged from 0.90 m in February 2017 to 1.68 m in July with an overall range of 0.54 m to 2.35 m (See Table 3-2 on following page). Substrate was predominantly cobble or boulder. There were frequently areas of sand or gravel on the margins. Average shading of the site from riparian vegetation ranged from a low of 9% in July and September to a high of 38% in February 2016.

Periphyton cover did not reach nuisance levels at this site (Figure 3-10). Thin films were dominant at all times. Long green filaments were present at covers >10% in late summer and early autumn. Thick mats were rarely observed, although medium green mats (up to 14% cover) were recorded periodically in spring, summer and early autumn.

Figure 3-10: Ohinemuri River periphyton cover on monthly sampling dates from February 2016 to January 2017. Note that sampling did not occur in June, 2016, August 2016 and January 2017. Tfm = Thin film, Sf = Short filaments, Mmgr = Medium mat green, Lfbrrd = Long filaments brown/red, Lfgr = Long filaments green, Tmgrlb = Thick mats green light brown. See Appendix A for underlying data including standard error values.

Measured periphyton biomass (chlorophyll a) at this site was low to moderate (i.e., <100 mg/m2) with some evidence of seasonality. Minimum levels were encountered during winter and early spring (1 and 7 mg/m2, respectively) while highest values were recorded in early autumn (84 and 275 mg/m2, respectively) (Table 3-2).

Measured chlorophyll a was not significantly correlated with chlorophyll a derived from periphyton cover information using Kilroy et al. (2013) conversion factors. At this site, we also found that periphyton biomass and cover metrics were not significantly correlated.

The native charophyte, Nitella aff. cristata and the introduced (but relatively innocuous) oxygen weed, Elodea canadensis, were periodically recorded along the channel margins. At peak abundance in May, Elodea and Nitella occupied 4% and 1% of channel cross-sectional area or volume (CAV), respectively.


Table 3-2: Ohinemuri River monthly summary data for shade, water depth, substrate, chlorophyll a and number of days since an event greater than or equal to three times the median flow. Values are means with range in parentheses where applicable.

Parameter Month


Shade (%) 38 (6-93)

34 (8-90)

21 (5-68)

10 (1-33)

- 9

(4-20) -

9 (3-21)

31 (3-96)

35 (2-74)

32 (3-71)

- 24

(0-88) 27

(3-75)

Water depth (m) 1.13 (0.75-1.60)

1.45 (0.80-1.90)

1.10 (0.65-1.55)

1.17 (0.69-1.72)

- 1.68

(1.05-2.35)

- 1.20

(0.78-1.96)

1.21 (0.83-1.70)

1.00 (0.54-1.56)

1.01 (0.60-1.50)

- 0.90

(0.55-1.38)

1.04 0.64-1.50)

Substrate C (S-B)

C (S-B)

B (G-B)

C (S-B)

- C

(G-B) -

C (S-B)

C (S-B)

C (S-B)

C (S-B)

- C

(S-B) C

(S-B)

Measured Chl a (mg/m2) 30 84 50 45 - 1 - 7 36 27 78 - 67 275

Derived Chl a (mg/m2) 85 57 8 6 4 29 25 35 16 48 27

Ratio of derived to measured Chl a 2.8 0.68 0.16 0.13 - 4.0 - 4.1 0.69 1.3 0.21 - 0.72 0.10

Days since 3Xmedian flow event 3 0 11 2 - 0 - 3 11 8 30 - 87 12

‘-‘ = missing data. B=Boulder, C=Cobble, G=Gravel, S=Sand, T=Silt.


We found that nuisance cover of long green filaments (>30%) was associated with shading less than 60% (Figure 3-11). Long brown or red filaments and thick mats were detected only where shading was less than 20%.

Figure 3-11: Periphyton cover versus shade at the Ohinemuri River site. Data shown are for individual transect points from February 2016 to March 2017. Nuisance cover of long filaments is >30%.

For the period from 1 February 2015 to 31 January 2017 median flow in the river was 5.71 m3/s and thus three times the median flow was 17.1 m3/s. In the Ohinemuri River we did not find a significant relationship between the number of days that had elapsed since an event three times the median flow or higher and periphyton cover or biomass.


3.3 Waiwawa River The Waiwawa monitoring site was located 20 m upstream of the Rangihau Rd Ford (Figure 3-12). The Regional Council’s flow monitoring tower was located adjacent to the sampling reach on the true right bank.

Figure 3-12: Waiwawa River site. View looking upstream from the Ford. Photo taken in April 2016 (Photo: F. Matheson).

This site was monitored 11 times between 1 April 2016 and 31 March 2017 on a monthly basis as river flow conditions allowed (Figure 3-13), by wading. The only month not sampled was May.

Figure 3-13: Waiwawa River flow from 1 April 2016 to 23 March 2017. Flow data from Waikato Regional Council. Sampling dates are marked with a red cross.


River wetted width at this site was c. 35 m. Average water depth on sampling dates ranged from 0.65 m in April to 1.00 m in November with an overall range of 0.18 m to 1.40 m ( Table 3-3). Cobble was the dominant substrate with patches of sand and gravel, and boulders were also encountered. Average shading of the site from riparian vegetation ranged from a low of 19% in October to a high of 38% in April.

Periphyton cover did not reach nuisance levels at this site (Figure 3-14). The periphyton community was usually dominated by thin films or medium mats. Low to moderate abundance of long filaments was occasionally observed with the highest total cover (15%) in late spring (November). In early spring (September) there was a 35% cover of thick green mats. All periphyton mats and filaments were apparently scoured from the bed during the large flood that preceded sampling in late March 2017.

Figure 3-14: Waiwawa River periphyton cover on monthly sampling dates from April 2016 to March 2017. Note that sampling did not occur in May 2016. Tfm = Thin film, Sf = Short filaments, Mmlb = Medium mat light brown, Mmgr = Medium mat green, Lfbrrd = Long filaments brown/red, Lfgr = Long filaments green, Tmgrlb = Thick mat green/light brown. See Appendix A for underlying data including standard error values.

Measured periphyton biomass (chlorophyll a) at this site ranged from a minimum of 6 mg/m2 in July to a maximum of 56 mg/m2 in January, with no obvious seasonal pattern (Table 3-3). Measured chlorophyll a was not significantly correlated with chlorophyll a derived from periphyton cover information using Kilroy et al. (2013) conversion factors. At this site, we found that periphyton biomass and cover metrics were not significantly correlated. No macrophytes were encountered on the Waiwawa transects during this study.


Table 3-3: Waiwawa River monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times the median flow. Values are means with range in parentheses where applicable.

Parameter Month

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

Shade (%) 38 (10-73)

- 36

(4-79) 33

(2-71) 26

(1-71) 27

(3-68) 19

(0-61) 24

(0-73) 21

(1-82) 27

(0-82) 26

(0-66) 24

(1-86)

Water depth (m)

0.65 (0.15-1.00)

- 0.75

(0.33-1.01) 0.89

(0.35-1.32) 0.88

(0.36-1.20) 0.78

(0.30-1.20) 0.87

(0.47-1.21) 1.00

(0.55-1.40) 0.74

(0.30-1.10) 0.67

(0.19-1.01) 0.67

(0.24-1.00) 0.69

(0.18-1.07)

Substrate C (G-B)

- C

(G-B) C

(S-B) C

(S-B) C

(S-B) C

(S-B) C

(S-B) C

(S-B) C

(S-B) C

(S-B) C

(S-B)

Measured Chl a (mg/m2)

10 - 17 6 46 17 15 19 15 56 49 1

Derived Chl a (mg/m2)

43 - 119 23 66 49 36 27 32 62 47 1

Ratio of derived to measured Chl a

4.3 - 7.0 3.8 1.4 2.9 2.4 1.4 2.1 1.1 1.0 1.0


9 3 1 1 14 2 0 19 53 22 9 9

‘-‘ = missing data. B=Boulder, C=Cobble, G=Gravel, S=Sand.


At this site, we found that nuisance (>60%) and non-nuisance covers of thick mats were associated with riparian shading less than 35-40% (Figure 3-15). Cover of long filaments >5% was rare but only occurred where shading was ≤10%.

Figure 3-15: Periphyton cover versus shade at the Waiwawa River site. Data shown are for individual transect points from April 2016 to March 2017. Nuisance cover of long filaments is >30%.

For the period from 1 February 2015 to 31 January 2017 median flow in the river was 2.84 m3/s and thus three times the median flow was 8.53 m3/s. At this site we found a significant relationship between the number of days since an event greater than or equal to three times the median flow and periphyton biomass (Figure 3-16), but not periphyton cover.


Figure 3-16: Periphyton biomass as chlorophyll a versus number of days since an event greater than or equal to three times the median flow at the Waiwawa River site. Data shown corresponds to each monthly sampling event.


3.4 Piako River – Kiwitahi The Kiwitahi monitoring site was located upstream of the Regional Council monitoring station (Figure 3-17). The most downstream transect was c. 20 m upstream of the station.

Figure 3-17: Piako River - Kiwitahi site. View looking upstream over lower-mid transects from true left bank. Photo taken in December 2016 (Photo: F. Matheson).

The Kiwitahi site was monitored 12 times between 1 February 2016 and 31 March 2017 on a monthly basis as river flow conditions allowed (Figure 3-18), usually by wading, occasionally by snorkelling. The two months not sampled were April and July.

Figure 3-18: Piako River - Kiwitahi flow from 1 February 2016 to 23 March 2017. Flow data from Waikato Regional Council. Sampling dates are marked with a red cross.


River wetted width at this site was c. 10 m. Average water depth on sampling dates ranged from 0.71 m in January to 1.25 m in August with an overall range of 0.14 m to 1.94 m ( Table 3-4). The substrate at the Kiwitahi site was highly variable. Sand was frequently encountered particularly towards the centre of the channel with silt on the margins. However, following high flow events it was clear that these fine sediments were removed with scouring exposing hard compact clay, and occasional boulders. Average shading of the site from banks and riparian vegetation was minimal and ranged from 0% in August and October to 9% in March 2016.

At this site periphyton was frequently attached to macrophytes and, occasionally, during periods of stable flow, adhering to fine bottom sediments. Nuisance cover of periphyton as long filaments (i.e., >30%) was observed in May (green), September (red), February 2017 (red) and March 2017 (red), covering at least half of the available attachment surface (Figure 3-19). Periphyton mats (green or light brown) were found only in December and January and not at nuisance levels.

Figure 3-19: Piako River - Kiwitahi periphyton cover on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in April and July 2016. No periphyton cover was observed in August and October. Tfm = Thin film, Sf = Short filaments, Mmlb = Medium mat light brown, Lfbrrd = Long filaments brown/red, Lfgr = Long filaments green. See Appendix A for underlying data including standard error values.

Measured periphyton biomass (chlorophyll a) at this site ranged from a minimum of 6 mg/m2 in August to a maximum of 486 mg/m2 in February 2017, which is regarded as a nuisance level (i.e., >200 mg/m2). There was evidence of a seasonal pattern at this site with low abundance during winter and peak abundances in late summer and autumn. Measured chlorophyll a was not significantly correlated with chlorophyll a derived from periphyton cover information using Kilroy et al. (2013) conversion factors. At this site, we did not find significant relationships between periphyton biomass and cover metrics.


Table 3-4: Piako River - Kiwitahi monthly summary data for shade, water depth, substrate, chlorophyll a and number of days since an event greater than or equal to three times median flow. Values are means with range in parentheses where applicable.

Parameter Month


Shade (%) 6 (0-46)

9 (0-32)

- 2

(0-12) 3

(0-14) -

0 (0-2)

- 0

(0-3) 2

(0-16) 2

(0-8) 3

(0-18) 1

(0-7) 2

(0-13)

Water depth (m)

0.92 (0.14-1.94)

0.85 (0.19-1.29)

- 0.87

(0.26-1.55)

0.83 (0.42-1.30)

- 1.25

(0.55-1.70)

1.05 (0.48-1.55)

1.14 (0.60-1.65)

0.89 (0.49-1.40)

0.84 (0.42-1.25)

0.71 (0.23-1.10)

0.71 (0.23-1.14)

0.85 (0.45-1.30)

Substrate S (T-C)

S (T-B)

- S

(T-C) S

(T-B) -

S (L-B)

T (L-G)

S (T-B)

G (T-B)

G (T-B)

T (L-B)

S (L-B)

S (L-B)


50 283 - 191 12 - 6 45 7 35 32 44 64 189


219 249 198 19 1 238 1 2 6 82 486 295


4.4 0.88 - 1.0 1.6 - 0.2 5.3 0.1 0.1 0.2 1.9 7.6 1.6


146 171 - 212 247 - 2 14 13 32 54 90 125 9

‘-‘ = missing data. B=Boulder, C=Cobble, G=Gravel, S=Sand, T=Silt, L=Clay.


Aquatic macrophytes occupied 50% or more of the channel cross-sectional area or volume (CAV) from February to June 2016 after which a scouring flow removed a large proportion of the biomass (Figure 3-20). At their peak in February they occupied nearly 90% of the watercolumn. After being flushed out in winter there was a steady increase in macrophyte occupation of the channel from August through to February. A flow event in early March 2017 reduced CAV in the final month of sampling. The dominant species present was the exotic submerged oxygen-weed, Egeria densa. A second submerged species, the curly-leaved pondweed, Potamogeton crispus, was also frequently present in low to moderate abundance. Emergent sprawling species, swamp willow weed (Persicaria spp.) and mercer grass (Paspalum dischitum) encroached from the margins from February to June.

Macrophytes occupied more than 50% of the channel water surface area (WSA) in February 2016 only (Figure 3-21). In February 2017 WSA was c. 30%. In other months during the main summer season (i.e., December to March) the percentage of the water surface occupied was usually c. 20-25%. From June to November this was reduced to ≤5%. Dominant species occupying the water surface were Egeria densa, Azolla spp., and/or Persicaria spp.

Figure 3-20: Piako River - Kiwitahi percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in April and July 2016. Solid bars indicate submerged species, striped bars show emergent species and dotted bars indicate floating species. Ed = Egeria densa, Le = Lemna spp., Pd = Persicaria spp., Pk = Potamogeton crispus. See Appendix A for underlying data including standard error values.


Figure 3-21: Piako River - Kiwitahi macrophyte percentage of channel water surface area (WSA) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in April and July 2016. Solid bars indicate surface-reaching submerged species, striped bars show emergent species and dotted bars indicate floating species. Ap = Azolla spp., Ed = Egeria densa, Le = Lemna spp., Pd = Persicaria spp., Pk = Potamogeton crispus, Ps = Paspalum distichum. See Appendix A for underlying data including standard error values.

We did not examine periphyton and macrophyte relationships with shade at this site as shade levels were negligible.

For the period from 1 February 2015 to 31 January 2017 median flow at this site was 0.63 m3/s and thus three times the median flow was 1.89 m3/s. We did not find any significant relationships between the number of days since a flow greater than or equal to three times the median and periphyton biomass or cover. In contrast, we found a highly significant relationship between the number of days elapsed since a three times the median flow event and the percentage of the channel cross-sectional area or volume occupied by macrophytes (i.e., macrophyte CAV) (Figure 3-22). The relationship with the percentage of the channel water surface area occupied by macrophytes was not significant (data not shown). Periphyton chlorophyll a was positively correlated with macrophyte CAV but the relationship was not statistically significant (p = 0.07) (Figure 3-23).


Figure 3-22: Percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes versus days since an event greater than or equal to three times the median flow at the Piako River - Kiwitahi site. Data shown corresponds to each monthly sampling event.

Figure 3-23: Measured periphyton biomass as chlorophyll a versus the percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes. Data shown corresponds to each monthly sampling event.


3.5 Piako River – Paeroa-Tahuna Rd The Paeroa-Tahuna Rd (PT Rd) monitoring site was located c. 20 m downstream of the Regional Council monitoring station (Figure 3-24).

Figure 3-24: Piako River - Paeroa-Tahuna Rd site. View looking downstream from position on true right bank just upstream of first transect. Photo taken in December 2016 (Photo: F. Matheson).

The PT Rd site was monitored 12 times between 1 February 2016 and 31 March 2017 on a monthly basis as river flow conditions allowed, usually by wading, but occasionally by snorkelling (Figure 3-25). The two months not sampled were May and July.

Figure 3-25: Piako River - Paeroa-Tahuna Rd flow from 1 February 2016 to 23 March 2017. Flow data from Waikato Regional Council. Sampling dates are marked with a red cross.


River wetted width at this site was c. 10 m. Average water depth on sampling dates ranged from 0.64 m in April to 1.59 m in August with an overall range of 0.09 m to 2.40 m (See Table 3-5 on following page). The substrate at the PT Rd site was predominantly sand or silt, with patches of gravel and compact clay. Average shading of the site was low and ranged from 0% in October and March 2017 to 9% in April.

At this site nuisance cover of periphyton as long filaments (i.e., >30%) was observed in March 2016 (green) and February 2017 (red), covering >50% of the available attachment surface (Figure 3-26). Non-nuisance cover of long green filaments and short filaments were present at other times in the period from February to June 2016 and in January 2017. The red algae, Compsopogon spp., was regularly encountered at this site. However, the green appearance of this algae meant that it was classified as long green filaments. During July to November, the period of extended high flows and low macrophyte abundance (see later), periphyton cover was negligible.

Figure 3-26: Piako River - Paeroa-Tahuna Rd periphyton cover on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in May and July 2016. Tfm = Thin film, Sf = Short filaments, Lfbrrd = Long filaments brown red, Lfgr = Long filaments green. See Appendix A for underlying data including standard error values.

Measured periphyton biomass at this site ranged from a low of 1 mg/m2 in June to a very high nuisance level of 775 mg/m2 in March 2016. There was evidence of a seasonal pattern at this site with low abundance during winter and peak abundances in late summer and early autumn. At this site measured chlorophyll a was significantly correlated with chlorophyll a derived from periphyton cover information using Kilroy et al. (2013) conversion factors (Figure 3-27). We also found a significant relationship between periphyton biomass and cover, which was virtually all in the form of long filaments (Figure 3-28).


Table 3-5: Piako River - Paeroa-Tahuna Rd monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times the median flow. Values are means with range in parentheses where applicable.

Parameter Month


Shade (%) 4

(0-68) 8

(0-30) 9

(0-30) -

3 (0-14)

- - - 0

(0-1) -

1 (0-7)

3 (0-18)

4 (0-32)

0 (0-2)

Water depth (m)

0.87 (0.34-1.43)

0.83 (0.43-1.17)

0.64 (0.15-1.15)

- 1.07

(0.32-1.80)

- 1.59

(0.25-2.40)

1.14 (0.09-2.02)

1.22 (0.20-2.00)

1.10 (0.50-2.00)

0.96 (0.40-1.80)

0.82 (0.40-1.19)

0.82 (0.50-1.23)

1.01 (0.50-1.50)

Substrate T

(L-S) S

(L-S) S

(L-S) -

T (L-S)

- T

(L-S) T

(L-S) T

(L-S) T

(L-S) T

(L-G) T

(L-S) S

(L-G) T

(L-S)


50 775 188 - 1 - 4 45 3 48 3 21 27 6


9 307 175 - 5 - 1 1 1 1 1 5 104 1


0.2 0.4 0.9 - 5.0 - 0.3 <0.1 0.3 <0.1 0.3 0.2 3.9 0.2


89 114 151 - 5 - 0 3 6 24 18 54 89 6

‘-‘ = missing data. G=Gravel, S=Sand, T=Silt, L=Clay


Figure 3-27: Derived versus measured periphyton biomass as chlorophyll a at the Piako River - Paeroa-Tahuna Rd site. Note log scale on x-axis. Derived chlorophyll a values were calculated from periphyton cover data using conversion factors of Kilroy et al. (2013).

Figure 3-28: Cover of long filaments vs measured periphyton biomass as chlorophyll a at the Piako River - Paeroa-Tahuna Rd site. Note log scale on x-axis. Black dots represent monthly (cover) mean values for transect where chlorophyll a was sampled.

At their peak in February 2016 and 2017, aquatic macrophytes at the PT Rd site occupied c. 50-60% of the channel cross-sectional area or volume (Figure 3-29). In January 2016 and 2017 and from March to June 2016 the CAV was c. 35-45%. In early March 2017, an unseasonal storm event reduced macrophyte CAV to c. 20%. The most abundant species at this site were the exotic submerged plants, Egeria densa, Ceratophyllum demersum and Potamogeton crispus, and the native pondweed, Potamogeton ochreatus. The sprawling emergent, Persicaria spp., was also present in moderate abundance, especially in 2016, and also the native charophyte, Nitella aff. cristata in lower amounts. A number of these species were flushed out in the winter months but a very low abundance of Persicaria spp., Egeria densa, and Potamogeton ochreatus persisted. In addition to the above species, the sprawling emergents, parrots feather (Myriophyllum aquaticum) and gypsywort (Lycopus europaeus), and the oxygen weed, Elodea canadensis, were occasionally encountered during summer and/or early autumn. We observed that parrots feather was apparently spreading throughout this area of the river during this study.


Macrophytes never occupied more than 50% of the channel water surface area at this site (Figure 3-30). Maximum total WSA was 33% in March 2016 comprising mostly surface-reaching Ceratophyllum demersum and the two emergent species, Persicaria spp., and Lycopus europaeus. Very low WSA was occupied by macrophytes from August to December 2016, and in March 2017 after the storm event.

Figure 3-29: Piako River - Paeroa-Tahuna Rd percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes on monthly sampling dates from Feb 2016 to Jan 2017. Note that sampling did not occur in April and July 2016. Solid bars indicate submerged species and striped bars show emergent species. Cd = Ceratophyllum demersum, Ec = Elodea canadensis, Ed = Egeria densa, Lp = Lycopus europaeus, Ma = Myriophyllum aquaticum, Nc = Nitella aff. cristata, Pd = Persicaria spp., Pk = Potamogeton crispus, Po = Potamogeton ochreatus. See Appendix A for underlying data including standard error values.

Figure 3-30: Piako River - Paeroa-Tahuna Rd percentage of channel water surface area (WSA) occupied by macrophytes on monthly sampling dates from Feb 2016 to Jan 2017. Note that sampling did not occur in May and July 2016. Solid bars indicate submerged species and striped bars show emergent species. Cd = Ceratophyllum demersum, Ec = Elodea canadensis, Ed = Egeria densa, Lp = Lycopus europaeus, Ma = Myriophyllum aquaticum, Pd = Persicaria spp., Pk = Potamogeton crispus, Po = Potamogeton ochreatus. See Appendix A for underlying data including standard error values.


We did not examine periphyton and macrophyte relationships with shade at this site due to the generally low levels of shading.

For the period from 1 February 2015 to 31 January 2017 median flow at this site was 2.03 m3/s and thus three times the median flow was 6.09 m3/s. We found a significant positive correlation between the number of days since an event greater than or equal to three times the median flow and periphyton cover (as long filaments) (Figure 3-31) but not biomass. Relationships with the two macrophyte metrics were also significant, especially with WSA (Figure 3-31).

Figure 3-31: Periphyton and macrophyte metrics versus days since an event greater than or equal to three times the median flow at the Piako River - Paeroa-Tahuna Rd site. Data shown corresponds to each monthly sampling event.


3.6 Waihou River The Waihou River monitoring site was located immediately downstream of the SH26 road bridge at Te Aroha (Figure 3-32).

Figure 3-32: Waihou River site. View looking upstream towards the upper transects from the true right bank. Photo taken in December 2016 (Photo: F. Matheson).

The Waihou River site was monitored nine times between 1 February 2016 and 31 March 2017 on a monthly basis as river flow conditions allowed (Figure 3-33) by snorkelling from a small boat. The three months not sampled were June, August and January.

Figure 3-33: Waihou River flow from 1 February 2016 to 23 March 2017. Flow data from Waikato Regional Council. Sampling dates are marked with a red cross.


River wetted width at this site was c. 35 m. Average water depth on sampling dates ranged from 1.43 m in February 2017 to 2.53 m in August with an overall range of 0.50 m to 3.50 m (See Table 3-6 on following page). The substrate at this site was predominantly sand with silt on the margins and occasional patches of clay, gravel or boulder. Average shading of the site from ranged from 2 to 8%.

At this site obvious periphyton cover was observed only in February 2016 where we recorded an average 3% cover of long filaments and 1% cover of both thick black dark brown mats and thin films. In contrast, we were able to detect periphyton chlorophyll a in our biomass samples in all months, although it was frequently at very low levels. The highest periphyton biomass (32-36 mg/m2) was recorded in February (2016 and 2017).


Table 3-6: Waihou River monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times the median flow. Values are means with range in parentheses where applicable.

Parameter Month


Shade (%) 7

(0-65) 8

(0-30) 4

(0-50) 3

(0-27) -

2 (0-28)

- 2

(0-28) 4

(0-46) 2

(0-34) 3

(0-28) -

3 (0-46)

3 (0-32)

Water depth (m)

1.66 (0.75-2.80)

1.84 (0.95-2.60)

1.50 (0.50-2.70)

1.90 (1.05- 2.75)

- 2.53

(1.12-3.50)

- 1.61

(0.75-3.15)

1.77 (0.61-3.15)

1.59 (0.75-3.20)

1.52 (0.50-3.10)

- 1.43

(0.51-3.00)

1.49 (0.79-2.80)

Substrate S

(T-G) S

(T-S) S

(T-S) S

(T-S) -

S (L-S)

- S

(T-B) S

(T-B) S

(T-B) S

(S-B) -

S (S-S)

S (S-G)


32 2 22 1 - 8 - 4 10 10 2 - 36 13


14 1 1 1 - 1 - 1 1 1 1 - 1 1


0.4 0.5 <0.1 1.0 - 0.1 - 0.3 0.1 0.1 0.5 - <0.1 0.1


157 197 33 61 - 5 - 56 17 38 60 - 117 145

‘-‘ = missing data. B = Boulder, G=Gravel, S=Sand, T=Silt, L=Clay.


Only four species of macrophyte were detected at the Waihou river site: three exotic species, Ceratophyllum demersum, Elodea canadensis and Potamogeton crispus, and the native charophyte Nitella aff. cristata. At their peak in February 2016 they occupied 16% of the channel cross-sectional area or volume (CAV) and <5% of the channel water surface area (WSA). They were barely detectable from July to December.

Figure 3-34: Waihou River percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in June and August 2016, and January 2017. Cd = Ceratophyllum demersum, Ec = Elodea canadensis, Nc = Nitella aff. cristata, Pk = Potamogeton crispus. See Appendix A for underlying data including standard error values.

Figure 3-35: Waihou River percentage of channel water surface area (WSA) by macrophytes on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in June and August 2016, and January 2017. Cd = Ceratophyllum demersum, Ec = Elodea canadensis, Nc = Nitella aff. cristata, Pk = Potamogeton crispus. See Appendix A for underlying data including standard error values.


We did not examine periphyton and macrophyte relationships with shade at this site due to the generally low levels of shading.

For the two-year period from 1 February 2015 to 31 January 2017 median flow at this site was 26.2 m3/s and thus three times the median flow was 78.7 m3/s. We found a strong positive correlation between the number of days since an event greater than or equal to three times the median flow and macrophyte CAV (Figure 3-36), but not periphyton biomass or cover, or macrophyte WSA.

Figure 3-36: Percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes versus days since an event greater than or equal to three times the median flow at the Waihou River site. Data shown corresponds to each monthly sampling event.


3.7 Waitoa River The Waitoa River monitoring site was located c. 50 m downstream of the Regional Council’s flow monitoring station (Figure 3-37).

Figure 3-37: Waitoa River site. View looking downstream from true left bank. The most upstream transect is located c. 20 m above small tree visible on true right bank. Photo taken in December 2016 (Photo: F. Matheson).

The Waitoa River site was monitored nine times between 1 February 2016 and 31 March 2017 on a monthly basis as river flow conditions allowed (Figure 3-38), by wading and, occasionally, by snorkelling. The three months not sampled were April, July and September.

Figure 3-38: Waitoa River flow from 1 February 2016 to 23 March 2017. Flow data from Waikato Regional Council. Sampling dates are marked with a red cross.


River wetted width at this site was c. 9 m. Average water depth on sampling dates ranged from 0.74 m in January to 1.12 m in October with an overall range of 0.18 m to 1.80 m (See Table 3-7 on following page). The substrate at this site was predominantly sand. Patches of compact clay, silt gravel and boulder were also often encountered. Average shading of the site from riparian vegetation was low and ranged from 0 to 9%.

At this site a nuisance cover of periphyton, as predominantly long green filaments (i.e., >30%), was observed in March 2016 and February 2017, covering >40% of the available attachment surface (Figure 3-39). Non-nuisance covers of long green and, occasionally, brown/red filaments were present at other times during the summer, autumn and early winter months. The red algae, Compsopogon spp., was regularly encountered at this site. The green appearance of this algae meant that it was classified as long green filaments.

Figure 3-39: Waitoa River periphyton cover on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in April, July and September. Tfm = Thin film, Lfbrrd = Long filaments brown/red, Lfgr = Long filaments green. See Appendix A for underlying data including standard error values.

Periphyton biomass at this site ranged from a low of 2 mg/m2 in June to a very high nuisance level of 558 mg/m2 in March 2016 (Table 3-7). There was evidence of a seasonal pattern at this site with low abundance during winter and peak abundances in early autumn. At this site, we found that measured and derived periphyton biomass were significantly correlated but derived values were always lower than measured values. We also found a significant relationship between periphyton biomass and cover, the latter occurring predominantly as long filaments (Figure 3-41).


Table 3-7: Waitoa River monthly summary data for shade, water depth, substrate, chlorophyll a and days since an event greater than or equal to three times median flow. Values are means with range in parentheses where applicable.

Parameter Month


Shade (%) 6

(0-47) 9

(0-45) -

4 (0-20)

1 (0-3)

- - - 0

(0-6) - -

2 (0-23)

0 (0-3)

0 (0-0)

Water depth (m)

0.81 (0.18-1.34)

0.79 (0.40-1.34)

- 0.83

(0.29-1.40) 1.05

(0.54-1.60) -

1.06 (0.19-1.60)

- 1.12

(0.60-1.75) 1.09

(0.70-1.60)

0.86 (0.40-1.45)

0.74 (0.34-1.48)

0.79 (0.35-1.26)

1.04 (0.55-1.80)

Substrate S

(S-S) S

(L-S) -

S (T-S)

S (T-C)

- S

(L-S) -

S (T-B)

G (S-B)

G (S-B)

T (L-S)

G (S-B)

S (L-C)


- 558 - 70 6 - 2 - 6 21 79 37 167 16


5 244 - 45 5 - 1 - 1 1 25 34 162 1


- 0.5 - 0.6 0.8 - 0.5 - 0.2 <0.1 0.3 0.9 1.0 0.2


150 175 - 227 262 - 9 - 7 26 47 83 118 4

‘-‘ = missing data, February measured chlorophyll a data missing due to laboratory analytical error B=Boulder, C=Cobble, G=Gravel, S=Sand, T=Silt, L=Clay


Figure 3-40: Derived versus measured periphyton biomass as chlorophyll a at the Waitoa River site.

Figure 3-41: Cover of long filaments versus measured periphyton biomass as chlorophyll a at the Waitoa River site. Black dots represent monthly (cover) mean values for the transect where biomass (chlorophyll a) was sampled.

At their peak in February 2016, aquatic macrophytes occupied c. 60% of the river channel cross-sectional area or volume (Figure 3-42) and c. 40% of the water surface area (Figure 3-43). Through to June, macrophyte CAV and WSA remained relatively high; however by August CAV had been reduced to <10% and WSA to <5%. In summer 2017 the peak abundance was lower than the previous year (28% CAV, 10% WSA). This is linked to a higher frequency of flushing flows in summer 2017 (see Figure 3-44 below). The dominant species at all times were the marginal emergent species, reed canary grass (Phalaris arundinacea) and the submerged species, Egeria densa and Potamogeton crispus. Other species that were periodically present at moderate abundance were hornwort (Ceratophyllum demersum), duckweed (Lemna spp.,) and water pepper (Persicaria spp).


Figure 3-42: Waitoa River percentage of channel cross-sectional area or volume (CAV) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in April, July and September 2016. Cd = Ceratophyllum demersum, Ed = Egeria densa, Nc = Nitella aff. cristata, Pa = Phalaris arundinacea, Pd = Persicaria spp., Pk = Potamogeton crispus, Ps = Paspalum dischitum. See Appendix A for underlying data including standard error values.

Figure 3-43: Waitoa River percentage of channel water surface area (WSA) occupied by macrophytes on monthly sampling dates from February 2016 to March 2017. Note that sampling did not occur in April, July and September 2016. Cd = Ceratophyllum demersum, Ed = Egeria densa, Le = Lemna spp., Nc = Nitella aff. cristata, Pa = Phalaris arundinacea, Pd = Persicaria spp., Pk = Potamogeton crispus, Ps = Paspalum dischitum. See Appendix A for underlying data including standard error values.


We did not examine periphyton and macrophyte relationships with shade at this site as shading of the channel was minimal.

For the two-year period from 1 February 2015 to 31 January 2017 median flow at this site was 2.23 m3/s and thus three times the median flow was 6.69 m3/s. We found a strong positive correlation between the number of days since an event greater than or equal to three times the median flow and macrophyte CAV and WSA, (Figure 3-44) but not periphyton biomass or cover.

Figure 3-44: Percentage of channel cross-sectional area or volume (CAV) or water surface area (WSA) occupied by macrophytes versus days since an event greater than or equal to three times the median flow at the Waitoa River site. Data shown corresponds to each monthly sampling event.


4 Discussion

4.1 Provisional NOF periphyton attribute states The National Objectives Framework requires monthly periphyton biomass data (as chlorophyll a) at a site for a 3-year period before an attribute state can be defined (MfE 2015). This longer-term monitoring requirement allows for expected inter-annual variation in river flows, because river-flow variability, and in particular flood frequency, is one of the major determinants of periphyton standing crop at any particular time. This study has monitored periphyton at seven Hauraki-Coromandel sites for a period of only just over one year, hence only a provisional attribute state can be assigned at this stage. We have done this on the basis of the first 12 months of data collected for each site.

All of the sites in this study, except Piako - PT Road, sit within the ‘default’ river classes of the periphyton attribute so to be assigned to a band the site must not exceed the numeric criteria for this band more than 8% of the time. In the ‘productive’ river classes (i.e., River Environment Classification WDSS, CDSS, WDVA, CDVA, WDVB and CDVB, where WD = Warm Dry, CD = Cool Dry, SS = Soft Sedimentary, VA = Volcanic Acidic and VB = Volcanic Basic) this frequency of exceedance criterion relaxes to 17%. Our 14-month dataset had some missing values, particularly during the autumn and winter months when flow conditions made it difficult to find a window of opportunity for sampling, so we used interpolation between measured data points to fill these gaps in a systematic manner (Table 4-1).

Table 4-1: Periphyton biomass data for Hauraki-Coromandel rivers showing interpolated missing values. Interpolated values are shown in parentheses.

Site 2016 2017


Kauaeranga 15 21 8 (16) 23 6 21 23 17 2 25 15 26 2

Ohinemuri 30 84 50 45 (23) 1 4 7 36 27 78 (54) 67 275

Waiwawa - - 10 (14) 17 6 46 17 15 19 15 56 49 1

Kiwitahi 50 283 (237) 191 12 9 6 45 7 35 32 44 64 189

PT Rd 50 775 188 (95) 1 (3) 4 45 3 48 3 21 27 6

Waihou 32 2 22 1 (5) 8 (6) 4 10 10 2 (17) 36 13

Waitoa - 558 (314) 70 6 (4) 2 (4) 6 21 79 37 167 16

The provisional periphyton attribute states assigned to the rivers show that two sites (Piako River at Kiwitahi, and Waitoa River) currently lie within the D band, i.e., below the national bottom line for this attribute (Table 4-2). There is some uncertainty about these assignments as one of the two high chlorophyll a values that placed these sites in the D band, as opposed to the C band, have been interpolated. The Piako-PT Rd site currently sits within the B band. As the only site with a productive river class assignment, there would need to be three monthly samples exceeding the 120 mg/m2 chlorophyll a threshold to place the Piako PT Rd site within the C band. The Waihou River site sits within the A band but this result likely reflects light limitation and a lack of suitable attachment surface for periphyton at this site, rather than negligible nutrient enrichment. This site had deep, usually turbid, water, a predominantly mobile sand bed, and macrophytes restricted to a narrow band on channel margins.


Table 4-2: Provisional periphyton attribute states assigned to Hauraki-Coromandel rivers.

Site REC

climate/geology

Provisional periphyton attribute state

Band Numeric state Narrative state

Kauaeranga at Smiths Cableway

WW/VA A ≤50 mg/m2, exceeded no more than 8% of time

Rare blooms reflecting negligible nutrient enrichment and/or alteration of the natural flow regime or habitat.

Ohinemuri at Karangahake

WW/VA B >50 and ≤120 mg/m2, exceeded no more than 8% of time

Occasional blooms reflecting low nutrient enrichment and/or alteration of the natural flow regime or habitat.

Waiwawa at Rangihau Rd

WW/VA A ≤50 mg/m2, exceeded no more than 8% of time


Piako at Kiwitahi WW/VA D >200 mg/m2, exceeded more than 8% of time

Regular and/or extended-duration nuisance blooms reflecting high nutrient enrichment and/or significant alteration of the natural flow regime or habitat.

Piako at Paeroa-Tahuna Rd

WD/VA B >50 and ≤120 mg/m2, exceeded no more than 16% of time

Occasional blooms reflecting low nutrient enrichment and/or alteration of the natural flow regime or habitat.

Waihou at Te Aroha

WW/SS A ≤50 mg/m2, exceeded no more than 8% of time


Waitoa at Mellon Rd

WW/SS D >200 mg/m2, exceeded more than 8% of time

Regular and/or extended-duration nuisance blooms reflecting high nutrient enrichment and/or significant alteration of the natural flow regime or habitat.


4.2 Substituting cover for biomass The NOF requires that periphyton is monitored as biomass (chlorophyll a). However, the periphyton monitoring programmes of a number of Regional Councils, including Waikato, have formerly focused on visual assessments of periphyton cover. We have measured these two metrics simultaneously in this study to determine whether a significant correlation exists and, consequently, whether it might be possible to convert periphyton cover to biomass in a robust manner. We have also evaluated the cover to chlorophyll a conversion factors derived from Canterbury waterways by Kilroy et al. (2013) for potential application in Hauraki-Coromandel rivers.

At the three hard-bottom river sites in this study (Kauaeranga, Ohinemuri and Waiwawa), we did not find a significant relationship between measured biomass and cover, and between measured and derived biomass, likely because periphyton biomass and cover levels at these sites were usually low. At the four soft-bottom sites, with a broader range of periphyton abundance, both as biomass and cover, we did find significant relationships. Combining the data from all seven sites, periphyton biomass was most strongly (and significantly) related to the weighted composite cover of nuisance periphyton (i.e., cover of long filaments plus [cover of thick mats/2]), and secondly, with the cover of long filaments (Figure 4-1, top two panels). We also found a significant relationship between measured and derived biomass (Figure 4-1, lower panel). These relationships could potentially be used to estimate periphyton biomass from visual assessment data in the Hauraki-Coromandel rivers. However, we caution that there was considerable scatter in these datasets, and that these relationships were driven by data from soft-bottom sites where a much broader range of periphyton abundance (cover and biomass) occurred. It is therefore likely that these relationships would only provide a crude approximation of biomass at quite a coarse scale.

Furthermore, undertaking periphyton cover assessments across five transects would likely take longer than collecting biomass samples from a single transect. Therefore it may be more cost-effective to collect biomass samples only. Sampling for this study took c. 1.5 to 2 h per site, sometimes a little longer under challenging flow conditions, but this also included visual assessment of macrophytes. We estimate that sampling for periphyton biomass on a single transect alone would take c. 0.5 h per site, and undertaking visual assessments of periphyton cover alone at five transects would take 0.5 to 1 h per site. The downside of the former approach is that biomass measurements alone provide no information about the types of periphyton that are present (see Kilroy et al. 2013 for further discussion of this point). Moreover, collecting periphyton biomass samples over multiple transects would be preferable to collecting samples from a single transect only (C. Kilroy, pers. comm.). Visual assessment of macrophytes is also very informative, particularly in soft-bottom rivers where they form the primary attachment surface for periphyton, can be a nuisance in their own right, but can also provide important habitat at low to moderate abundance.


Figure 4-1: Relationships between measured periphyton biomass and cover, and measured and derived periphyton biomass, in Hauraki-Coromandel rivers. Axes are log scale. The dashed line and equation indicates the linear regression for all data points. The red line shows the ideal 1:1 relationship between measured and derived chlorophyll a.


4.3 Options to manage D band sites The NOF requires that rivers with a periphyton attribute state below the national bottom line require remedial action. Primary drivers of periphyton abundance in rivers are flow, light, nutrients, temperature and invertebrate grazing. Larger (and more frequent) flood events and grazing increase periphyton losses, while increased light, nutrient availability and temperature favour periphyton accrual. In general, it is unlikely that invertebrate grazing could be feasibly manipulated to control periphyton in these, or other, river systems. In these un-regulated Hauraki-Coromandel rivers it is also unlikely that flow could be manipulated to “flush out” periphyton at high-risk periods.

The two sites most at risk of being assigned a D band periphyton attribute state, Piako at Kiwitahi and Waitoa at Mellon Rd are amongst those that are most nutrient-enriched (Vant 2016). This suggests that reducing nutrient concentrations in these rivers could reduce periphyton abundance and thus lift these sites above the national bottom line. However, it would likely be very difficult to reduce water column nutrients in these rivers to levels which would be expected to limit periphyton growth, i.e., <40-100 μg/L DIN and <15-30 μg/L DRP in general terms (MfE 1992) or at least <295 μg/L DIN and <26 μg/L DRP where days of accrual are less than 20 (see MfE 2000 for [lower] nutrient limiting criteria associated with longer accrual period scenarios). These two rivers lie in catchments that are highly modified by productive land use.

The most feasible option to bring about a reduction in nuisance periphyton abundance at the three high risk sites identified in this study is riparian shading. At present all of these sites have negligible shade. Results from the three hard-bottom sites, all of which had a substantial riparian canopy, clearly showed that nuisance periphyton (i.e., long filaments and thick mats) only occurred where shading was less than c. 60%. At these sites, where the typical channel width was greater than 30 m, high levels of shade were prevalent along the margins, particularly on the most northerly bank, due to the influence of the riparian canopy.

At the Piako and Waitoa river sites channel width was only c. 9-10 m so it is possible that a high degree of shading could be achieved across a moderate proportion of the channel width at these sites. Modelling with sWAIORA suggests that with active native tree planting, streams of 6.6 and 14 m width could ultimately achieve shading >90% and >60%, respectively (Davies-Colley et al. 2009). The planting of riparian margins to create shade also brings a number of potential co-benefits including lower water temperatures, provision of more diverse stream habitat and carbon resource to support the food-web through enhanced leaf and wood inputs, improved aesthetic appeal of waterways. Enhanced interception and trapping of sediment and nutrient contaminants from adjacent land by riparian trees and shade-tolerant understorey, might also be possible. A recent synthesis of existing information also suggests that riparian shading >70% would likely be effective at reducing the abundance of instream macrophytes to levels likely to improve the dissolved oxygen status of impacted lowland NZ rivers (Matheson et al. 2017).

A potential downside of riparian shading to reduce periphyton (and macrophyte) abundance is less nutrient uptake from river water during the spring and summer growing season (Quinn et al. 1997, Matheson et al. 2011, 2012b, Howard-Williams & Pickmere 1999, 2010). However, instream uptake by periphyton and macrophytes may exert no influence where nutrient concentrations are high (O’Brien et al. 2014). Moreover, this could be offset by enhanced interception and transformation of nutrients from land runoff by the reinstated riparian vegetation (McKergow et al. 2016) used to create shade and, potentially, by reduced release of phosphorus from river sediments. The latter is hypothesised to occur under anoxic conditions and high pH associated with prolific growth of


macrophytes in unshaded channels (Matheson et al. 2017). Furthermore, nutrient uptake by periphyton (and macrophytes) is a generally regarded as a temporary sink for nutrients, as seasonal dieback of plants re-releases the assimilated nutrients, although a small proportion may be converted into refractory (i.e., biologically unavailable) forms.


5 Conclusions Periphyton and macrophyte monitoring in seven Hauraki-Coromandel rivers for a period of 14 months has shown the following:

Two of the four soft-bottom river sites monitored (Piako – Kiwitahi and Waitoa) have a provisional NOF periphyton attribute state below the national bottom line and a third river site (Piako – Paeroa-Tahuna Rd) has a B band assignment.

These sites also have nuisance macrophytes occupying more than 50% of the channel cross-sectional area or volume (and up to 90%) during the summer-autumn period which exceeds provisional guidelines to protect aesthetic, angling, ecological condition and flow conveyance values (Matheson et al. 2012a, 2016).

At these soft-bottom sites, macrophyte abundance was strongly correlated with the number of days since a flushing flow event equivalent to three times the median flow or higher, so the risk of nuisance abundance will be elevated in dry summers.

The most promising approach to reduce periphyton biomass and nuisance macrophyte abundance at these sites is riparian planting to produce shading >60% across a large proportion of the channel. There are also a number of significant co-benefits of this approach.

The three hard-bottom river sites monitored (Kauaeranga, Ohinemuri and Waiwawa) and the fourth soft-bottom river site (Waihou) have provisional NOF periphyton attribute states in the A and B bands.

While, overall, we found that periphyton biomass was significantly correlated with several periphyton cover metrics and with biomass estimates derived from cover data using conversion factors derived for Canterbury streams, considerable scatter in the dataset and the strong influence of wide-ranging data from the soft-bottom sites, means it is likely that these relationships would only provide a crude approximation of biomass at quite a coarse scale.


6 Acknowledgements The authors thank Aleki Taumoepeau, Mike Martin, Aslan Wright-Stow, Josh Smith, Amber Rowan-Sanders, Tracey Burton, Peter Williams, Asaeli Tulagi (WRC) and Chris McKinnon (WRC) for field support, Glenys Croker for preparing periphyton samples for analysis and data entry, and the NIWA Hamilton Water Quality Laboratory for chlorophyll a analyses. We also thank Debbie Eastwood (WRC) for providing site flow records. NIWA’s Strategic Science Investment Fund (SMARTer Riparian and Wetland Strategies Research Project) supported collection of additional data from sites in February and March 2017.


7 Glossary of abbreviations and terms Attribute According to the NPS-FM this is defined as:

a measurable characteristic of fresh water, including physical, chemical and biological properties, which supports particular value.

Macrophyte A macrophyte is an aquatic plant that grows in or near water and is either emergent, submerged, or floating.

CAV Channel cross-sectional area or volume (as in the percentage of CAV occupied by macrophytes).

NOF National Objectives Framework.

NPS-FM National Policy Statement for Freshwater Management (2014).

Periphyton Periphyton refers to communities of algae in aquatic systems that are attached to the sediment surface or to aquatic, macrophyte vegetation.

WRC Waikato Regional Council.

WSA Channel water surface area (as in the percentage of WSA occupied by macrophytes).


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Appendix A Periphyton and macrophyte summary data

Table A-1: Kauaeranga River periphyton cover summary data. Values are means with standard errors in parentheses.

Periphyton type Feb 16 Mar 16 Apr 16 May 16 Jun 16 Jul 16 Aug 16 Sep 16 Oct 16 Nov 16 Dec 16 Jan 16 Feb 17 Mar 17

Bare 33 (5) 53 (6) 10 (3) - 19 (3) 70 (5) 25 (4) 18 (3) 27 (5) 18 (3) 19 (3) 27 (4) 12 (2) 89 (2)

Thin film 29 (6) 36 (5) 66 (5) - 40 (7) 12 (3) 35 (5) 28 (6) 62 (4) 56 (6) 0 (0) 0 (0) 27 (6) 11 (2)

Short filaments 0 (0) 0 (0) 1 (0) - 2 (1) 0 (0) 0 (0) 3 (1) 0 (0) 9 (3) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat light brown 32 (5) 4 (1) 22 (4) - 26 (7) 18 (6) 34 (4) 50 (6) 0 (0) 0 (0) 23 (6) 51 (6) 44 (7) 0 (0)

Medium mat dark brown 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 (1) 0 (0)

Medium mat green 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 10 (3) 15 (4) 0 (0) 0 (0) 1 (1) 0 (0)

Long filaments brown red 1 (1) 0 (0) 0 (0) - 0 (0) 0 (0) 3 (1) 0 (0) 0 (0) 0 (0) 3 (3) 0 (0) 0 (0) 0 (0)

Long filaments green 6 (4) 6 (3) 0 (0) - 1 (0) 0 (0) 1 (0) 1 (0) 0 (0) 2 (1) 8 (3) 0 (0) 5 (3) 0 (0)

Thick mat black dark brown 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Thick mat green light brown 0 (0) 0 (0) 0 (0) - 11 (5) 0 (0) 3 (3) 1 (0) 0 (0) 0 (0) 48 (8) 22 (6) 10 (4) 0 (0)

Sludge 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Submerged bryophytes 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Iron bacteria 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Table A-2: Ohinemuri River periphyton cover summary data. Values are means with standard errors in parentheses.


Bare 30 (5) 61 (7) 25 (3) 32 (6) - 60 (8) - 18 (4) 27 (6) 16 (5) 21 (4) - 14 (3) 11 (3)

Thin film 48 (6) 24 (8) 73 (3) 67 (6) - 40 (8) - 70 (6) 58 (7) 78 (5) 72 (4) - 73 (4) 55 (6)

Short filaments 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 2 (1) 0 (0) 6 (2) 3 (1) - 0 (0) 0 (0)

Medium mat light brown 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Medium mat dark brown 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Medium mat green 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 4 (4) 14 (7) 0 (0) 3 (1) - 0 (0) 16 (4)

Long filaments brown red 0 (0) 0 (0) 2 (1) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Long filaments green 23 (6) 15 (4) 0 (0) 1 (1) - 0 (0) - 6 (3) 1 (0) 0 (0) 0 (0) - 11 (2) 5 (2)

Thick mat black dark brown 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Thick mat green light brown 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 2 (2) 0 (0)

Sludge 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 1 (1) 13 (3)

Submerged bryophytes 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Iron bacteria 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)


Table A-3: Waiwawa River periphyton cover summary data. Values are means with standard errors in parentheses.


Bare - - 9 (2) - 3 (1) 53 (9) 35 (6) 29 (6) 23 (4) 24 (6) 19 (6) 40 (7) 10 (3) 85 (2)

Thin film - - 58 (8) - 7 (4) 24 (7) 19 (5) 15 (6) 50 (6) 61 (6) 34 (6) 5 (4) 47 (6) 15 (2)

Short filaments - - 2 (1) - 5 (4) 0 (0) 0 (0) 2 (1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat light brown - - 31 (9) - 82 (6) 16 (6) 39 (7) 19 (5) 0 (0) 0 (0) 0 (0) 56 (8) 37 (7) 0 (0)

Medium mat dark brown - - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat green - - 0 (0) - 0 (0) 7 (5) 4 (3) 0 (0) 27 (7) 0 (0) 46 (8) 0 (0) 0 (0) 0 (0)

Long filaments brown red - - 0 (0) - 2 (1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Long filaments green - - 0 (0) - 1 (0) 0 (0) 0 (0) 0 (0) 1 (0) 15 (6) 0 (0) 0 (0) 0 (0) 0 (0)

Thick mat black dark brown - - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Thick mat green light brown - - 0 (0) - 0 (0) 0 (0) 3 (3) 35 (7) 0 (0) 0 (0) 0 (0) 0 (0) 5 (2) 0 (0)

Sludge - - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 (1) 0 (0)

Submerged bryophytes - - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Iron bacteria - - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Table A-4: Piako River - Kiwitahi periphyton cover summary data. Values are means with standard errors in parentheses.


Bare 75 (7) 40 (9) - 49 (7) 83 (5) - 100 (0) 47 (8) 100 (0) 95 (4) 58 (8) 82 (6) 35 (7) 25 (6)

Thin film 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 4 (4) 20 (7) 0 (0) 1 (1) 2 (1)

Short filaments 0 (0) 60 (9) - 0 (0) 17 (5) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 3 (3)

Medium mat light brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 14 (6) 0 (0) 0 (0)

Medium mat dark brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat green 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 7 (5) 0 (0) 0 (0) 0 (0)

Long filaments brown red 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 53 (8) 0 (0) 0 (0) 1 (0) 0 (0) 53 (7) 67 (8)

Long filaments green 25 (7) 0 (0) - 51 (7) 0 (0) - 0 (0) 0 (0) 0 (0) 1 (0) 15 (6) 4 (3) 1 (1) 0 (0)

Thick mat black dark brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Thick mat green light brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Sludge 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 10 (5) 3 (3)

Submerged bryophytes 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Iron bacteria 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Table A-5: Piako River - Kiwitahi macrophyte CAV summary data. Values are means with standard errors in parentheses.

Macrophyte species Feb 16 Mar 16 Apr 16 May 16 Jun 16 Jul 16 Aug 16 Sep 16 Oct 16 Nov 16 Dec 16 Jan 16 Feb 17 Mar 17

Azolla spp. 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 2 (1) 0 (0)

Egeria densa 74 (7) 71 (6) - 73 (6) 44 (6) - 4 (1) 6 (3) 8 (4) 14 (5) 22 (7) 38 (9) 49 (7) 30 (7)

Lemna spp. 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 (0) 0 (0)

Persicaria spp. 11 (6) 5 (4) - 6 (4) 6 (4) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Potamogeton crispus 4 (2) 1 (1) - 3 (2) 4 (3) - 0 (0) 0 (0) 1 (1) 5 (2) 7 (3) 9 (4) 3 (1) 1 (1)

Paspalum distichum 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Table A-6: Piako River - Kiwitahi macrophyte WSA summary data. Values are means with standard errors in parentheses.


Azolla spp. 1 (1) 4 (3) - 16 (6) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 15 (4) 0 (0)

Egeria densa 38 (8) 7 (5) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 11 (6) 18 (7) 15 (4) 6 (4)

Lemna spp. 1 (1) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Persicaria spp. 11 (6) 9 (5) - 7 (4) 4 (3) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Potamogeton crispus 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 7 (3) 8 (5) 1 (0) 0 (0)

Paspalum distichum 0 (0) 1 (1) - 0 (0) 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Table A-7: Piako River - Paeroa Tahuna Rd periphyton cover summary data. Values are means with standard errors in parentheses.


Bare 96 (2) 29 (8) 67 (7) - 98 (1) - 100 (0) 100 (0) 100 (0) 100 (0) 98 (2) 99 (1) 39 (8) 100 (0)

Thin film 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 2 (2) 0 (0) 0 (0) 0 (0)

Short filaments 0 (0) 0 (0) 9 (5) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 2 (2) 0 (0)

Medium mat light brown 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat dark brown 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat green 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Long filaments brown red 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 54 (8) 0 (0)

Long filaments green 4 (2) 71 (8) 23 (6) - 2 (1) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 (1) 4 (4) 0 (0)

Thick mat black dark brown 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Thick mat green light brown 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Sludge 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Submerged bryophytes 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Iron bacteria 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Table A-8: Piako River - Paeroa-Tahuna Rd macrophyte CAV summary data. Values are means with standard errors in parentheses.


Ceratophyllum demersum 12 (5) 17 (6) 5 (3) - 2 (1) - 0 (0) 0 (0) 1 (0) 0 (0) 2 (1) 9 (4) 16 (6) 10 (5)

Elodea canadensis 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Egeria densa 21 (6) 11 (3) 3 (1) - 4 (2) - 1 (0) 1 (1) 3 (2) 1 (0) 2 (1) 7 (3) 5 (2) 5 (2)

Lycopus europaeus 4 (2) 2 (1) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Myriophyllum aquaticum 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 2 (2) 1 (1)

Nitella aff. cristata 0 (0) 1 (1) 1 (1) - 1 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 2 (1) 2 (1) 1 (1) 0 (0)

Persicaria spp. 19 (7) 9 (4) 19 (7) - 19 (7) - 5 (2) 9 (5) 5 (3) 2 (1) 0 (0) 0 (0) 0 (0) 0 (0)

Potamogeton crispus 0 (0) 0 (0) 2 (1) - 6 (3) - 0 (0) 0 (0) 0 (0) 0 (0) 2 (1) 16 (6) 18 (4) 4 (2)

Potamogeton ochreatus 0 (0) 5 (4) 6 (3) - 5 (2) - 0 (0) 1 (1) 0 (0) 1 (0) 2 (1) 3 (2) 5 (2) 1 (0)

Table A-9: Piako River - Paeroa-Tahuna Rd macrophyte WSA summary data. Values are means with standard errors in parentheses.


Ceratophyllum demersum 8 (4) 10 (5) 4 (3) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 10 (5) 10 (5) 0 (0)

Elodea canadensis 0 (0) 0 (0) 1 (1) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Egeria densa 1 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Lycopus europaeus 4 (4) 5 (4) 1 (1) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Myriophyllum aquaticum 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 4 (3) 1 (1)

Nitella aff. cristata 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Persicaria spp. 19 (7) 18 (7) 23 (8) - 12 (5) - 0 (0) 2 (2) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Potamogeton crispus 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 8 (4) 12 (5) 0 (0)

Potamogeton ochreatus 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Table A-10: Waihou River periphyton cover summary data. Values are means with standard errors in parentheses.


Bare 95 (3) 100 (0) 100 (0) 100 (0) - 100 (0) - 100 (0) 100 (0) 100 (0) 100 (0) - 100 (0) 100 (0)

Thin film 1 (1) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Short filaments 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Medium mat light brown 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Medium mat dark brown 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Medium mat green 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Long filaments brown red 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Long filaments green 3 (1) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Thick mat black dark brown 1 (1) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Thick mat green light brown 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Sludge 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Submerged bryophytes 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Iron bacteria 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)


Table A-11: Waihou River macrophyte CAV summary data. Values are means with standard errors in parentheses.


Ceratophyllum demersum 6 (4) 8 (4) 6 (3) 2 (2) - 0 (0) - 1 (1) 0 (0) 0 (0) 0 (0) - 5 (3) 2 (2)

Elodea canadensis 9 (4) 3 (1) 4 (2) 2 (1) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Nitella aff. cristata 0 (0) 1 (0) 2 (1) 2 (1) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Potamogeton crispus 1 (1) 1 (1) 1 (1) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Table A-12: Waihou River macrophyte WSA summary data. Values are means with standard errors in parentheses.


Ceratophyllum demersum 4 (4) 0 (0) 2 (2) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 2 (2)

Elodea canadensis 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Nitella aff. cristata 0 (0) 0 (0) 1 (1) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)

Potamogeton crispus 0 (0) 0 (0) 1 (1) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) - 0 (0) 0 (0)


Table A-13: Waitoa River periphyton cover summary data. Values are means with standard errors in parentheses.


Bare 97 (1) 55 (10) - 88 (4) 95 (2) - 100 - 100 100 79 (7) 87 (4) 48 (5) 100

Thin film 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 17 (7) 0 (0) 2 (2) 0 (0)

Short filaments 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat light brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat dark brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Medium mat green 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 7 (3) 0 (0)

Long filaments brown red 0 (0) 1 (1) - 8 (3) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Long filaments green 3 (1) 44 (9) - 3 (2) 5 (2) - 0 (0) - 0 (0) 0 (0) 4 (1) 12 (3) 43 (6) 0 (0)

Thick mat black dark brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Thick mat green light brown 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Sludge 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Submerged bryophytes 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Iron bacteria 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Table A-14: Waitoa River macrophyte CAV summary data. Values are means with standard errors in parentheses.


Ceratophyllum demersum 3 (3) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 2 (1) 0 (0)

Elodea canadensis 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Egeria densa 16 (5) 20 (6) - 15 (4) 5 (2) - 2 (1) - 1 (0) 1 (0) 3 (1) 7 (3) 12 (4) 6 (2)

Lemna spp. 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Nitella aff. cristata 0 (0) 0 (0) - 1 (1) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Phalaris arundinacea 33 (9) 33 (9) - 26 (8) 29 (8) - 3 (1) - 7 (4) 6 (3) 5 (3) 4 (2) 2 (2) 14 (5)

Persicaria spp. 4 (4) 1 (0) - 2 (2) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Potamogeton crispus 3 (2) 1 (1) - 6 (3) 10 (3) - 1 (0) - 0 (0) 0 (0) 1 (1) 11 (4) 12 (3) 2 (1)

Paspalum distichum 0 (0) 0 (0) - 0 (0) 0 (0) - 1 (1) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Table A-15: Waitoa River macrophyte WSA summary data. Values are means with standard errors in parentheses.


Ceratophyllum demersum 3 (3) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 3 (2) 0 (0) 0 (0)

Elodea canadensis 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Egeria densa 0 (0) 0 (0) - 0 (0) 1 (1) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Lemna spp. 0 (0) 3 (3) - 1 (0) 4 (3) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Nitella aff. cristata 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Phalaris arundinacea 33 (9) 37 (9) - 38 (9) 28 (8) - 2 (2) - 0 (0) 0 (0) 4 (3) 10 (4) 9 (6) 9 (4)

Persicaria spp. 4 (4) 1 (1) - 2 (2) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Potamogeton crispus 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 1 (1) 0 (0)

Paspalum distichum 0 (0) 0 (0) - 0 (0) 0 (0) - 0 (0) - 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)


Waikato Regional Council Technical Report 2014/50

Dissolved oxygen and temperature dynamics in the Piako catchment www.waikatoregion.govt.nz ISSN 2230-4355 (Print) ISSN 2230-4363 (Online

Prepared by: Paul Franklin National Institute of Water & Atmospheric Research Ltd For: Waikato Regional Council Private Bag 3038 Waikato Mail Centre HAMILTON 3240 May 2014 Document #: 3070263

Doc # 3070263

Peer reviewed by: Date August 2016 Edmund Brown

Approved for release by: Date August 2016 Tracey May

Disclaimer

This technical report has been prepared for the use of Waikato Regional Council as a reference document and as such does not constitute Council’s policy. Council requests that if excerpts or inferences are drawn from this document for further use by individuals or organisations, due care should be taken to ensure that the appropriate context has been preserved, and is accurately reflected and referenced in any subsequent spoken or written communication. While Waikato Regional Council has exercised all reasonable skill and care in controlling the contents of this report, Council accepts no liability in contract, tort or otherwise, for any loss, damage, injury or expense (whether direct, indirect or consequential) arising out of the provision of this information or its use by you or any other party.

Doc # 3070263

16 September 2014 3.20 p.m.

Dissolved oxygen and temperature

dynamics in the Piako catchment

Prepared for Waikato Regional Council

May 2014

© All rights reserved. This publication may not be reproduced or copied in any form without the permission of

the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s

contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of

information retrieval system.

Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this document is

accurate, NIWA does not give any express or implied warranty as to the completeness of the information

contained herein, or that it will be suitable for any purpose(s) other than those specifically contemplated

during the Project or agreed by NIWA and the Client.

Authors/Contributors:

Paul Franklin

For any information regarding this report please contact:

Dr Paul Franklin

Scientist

Freshwater Ecology

+64-7-859 1882

[email protected]

National Institute of Water & Atmospheric Research Ltd

Gate 10, Silverdale Road

Hillcrest, Hamilton 3216

PO Box 11115, Hillcrest

Hamilton 3251

New Zealand

Phone +64-7-856 7026

Fax +64-7-856 0151

NIWA Client Report No: HAM2014-016

Report date: May 2014

NIWA Project: EVW13212

Dissolved oxygen and temperature dynamics in the Piako catchment

Contents

Executive summary ............................................................................................................. 6

1 Introduction .............................................................................................................. 8

1.1 Background ............................................................................................................... 8

1.2 Project scope ............................................................................................................ 8

2 Methodology ............................................................................................................. 9

3 Results .................................................................................................................... 12

3.1 Flow conditions during sampling ............................................................................ 12

3.2 Water temperature dynamics ................................................................................ 13

3.3 Dissolved oxygen dynamics .................................................................................... 22

4 Discussion ............................................................................................................... 42

4.1 Dissolved oxygen and flow ..................................................................................... 42

4.2 Lower Piako dissolved oxygen dynamics ................................................................ 43

5 Conclusions ............................................................................................................. 45

6 Acknowledgements ................................................................................................. 45

7 References ............................................................................................................... 46

Appendix A Proposed National Objectives Framework ........................................ 48

Tables

Table 1-1: Flows that approximate to the proposed NOF dissolved oxygen attribute

state class boundaries for ecosystem health. 6

Table 2-1: Location of dissolved oxygen and temperature monitoring sites in summer

2012 and 2013. 9

Table 2-2: Proposed dissolved oxygen levels for protection of New Zealand freshwater

fish communities (Franklin 2013). 9

Table 3-1: Preferred water temperatures of selected fish species typical of the Piako

River system. 13

Table 3-2: Statistical significance of monthly correlations between maximum water

temperature and mean daily flow for the 2012-13 monitoring period. 18

Table 3-3: Flow and tide conditions for the four longitudinal profiles. 37

Table 4-1: Flows that approximate to the proposed NOF dissolved oxygen attribute

state class boundaries for ecosystem health. 43

Table A-1: Proposed NOF limits for dissolved oxygen concentrations for ecosystem health

in rivers. 48

Dissolved oxygen and temperature dynamics in the Piako catchment

Figures

Figure 2-1: Location of dissolved oxygen monitoring sites in the Piako catchment. 10

Figure 3-1: Flow time series for the Piako River at the Paeroa-Tahuna Road gauging

site showing the dissolved oxygen sampling periods. 12

Figure 3-2: Water temperature time series from October 2011 to April 2013 for the

monitoring site on the Piako River at the Paeroa-Tahuna Road bridge. 14

Figure 3-3: Water temperature time series from the 2011-12 and 2012-13 sampling

periods for the monitoring site on the Piako River at the Kiwitahi. 15

Figure 3-4: Water temperature time series for the 2012-13 low flow period for the

Piakonui Stream and the Waitoa at Puketutu Road. 16

Figure 3-5: Water temperature time series for the 2012-13 low flow period for the

Waitoa at Waharoa and Mellon Road. 17

Figure 3-6: Scatter plots of maximum daily water temperature against mean daily

flow for the Piako River at Kiwitahi and the Paeroa-Tahuna Road bridge. 19


flow for the Piakonui Stream and the Waitoa River at Puketutu Road. 20


flow for the Waitoa River at Waharoa and Mellon Road. 21

Figure 3-9: Dissolved oxygen time series for the Piako River at the Paeroa-Tahuna

Road bridge. 23

Figure 3-10: Dissolved oxygen time series for the Piako River at Kiwitahi. 24

Figure 3-11: Dissolved oxygen time series for the Piakonui Stream and the Waitoa at

Puketutu Road. 25

Figure 3-12: Dissolved oxygen time series for the Waitoa at Waharoa and Mellon

Road. 26

Figure 3-13: Scatter plots of daily minimum, 7-day mean daily minimum and 7-day

mean daily dissolved oxygen concentrations against mean daily flow. 28











Figure 3-19: Scatter plots of the estimated reaeration coefficient (k2) and river flow at

the Kiwitahi and Paeroa-Tahuna Road sampling sites on the Piako River. 35

Figure 3-20: Scatter plots of the estimated reaeration coefficient (k2) and river flow at

the Puketutu Road and Waharoa sampling sites on the Waitoa River. 36

Figure 3-21: Sites on the lower Piako River where water quality was surveyed during

2011-12. 37

Figure 3-22: Long profiles of water temperature, salinity, dissolved oxygen and turbidity

in the lower Piako. 39

Figure 3-23: Provisional results from a survey of environmental conditions in the lower

Piako River over a 3-week period in April-May 2013. 41

Dissolved oxygen and temperature dynamics in the Piako catchment 5

Reviewed by Approved for release by

B. Wilcock D. Rowe

Formatting checked by


Executive summary

The Waikato Regional Council (WRC) is responsible for managing the status of water resources in the

Waikato Region. WRC is preparing for a scheduled review of flow and allocation limits in the Piako

catchment. The aim of this project was to characterise the dissolved oxygen and water temperature

dynamics in the Piako catchment during summer low flow conditions with a view to identifying

potential limits on the assimilative capacity of the waterway.

Dissolved oxygen time series collected from six sites around the Piako catchment showed that in the

Waitoa River sub-catchment, dissolved oxygen levels were generally maintained at or above the

recommended protection levels for maintaining ecosystem health. However, in the Piako River at

Kiwitahi and at the Paeroa-Tahuna Road site, dissolved oxygen concentrations fell below

recommended protection levels for several weeks during the summer of 2012-13.

Water temperatures reached levels that were above the preferred temperatures of inanga and smelt

at all sites except the Piakonui Stream. No robust relationships were identified between maximum

water temperatures and flow at any of the sites, but there was some evidence of negative

associations at some sites for some months of the year that may merit further investigation for the

limit setting process.

Threshold responses were observed between dissolved oxygen concentrations and flow at four of

the six sites. These relationships were used to identify the flows that approximate to the lower class

boundaries for the proposed National Objectives Framework (NOF) limits for dissolved oxygen (Table

1-1). Based on the observed correlations between flow and dissolved oxygen concentrations, it is

recommended that minimum flow limits be set at a level sufficient to avoid dissolved oxygen

concentrations falling below the recommended protection levels for ecosystem health.

Table 1-1: Flows that approximate to the proposed NOF dissolved oxygen attribute state class boundaries

for ecosystem health. Flow values are for the corresponding on-site hydrological gauging station with the

exception of the Waitoa at Puketutu Road where the flow values are those recorded at Waharoa, the nearest

gauging station.

Site

Proposed NOF class – 7-day mean minimum dissolved oxygen

Class C

(5.0 mg L-1)

Class B

(7.0 mg L-1)

Class A

(8.0 mg L-1)

Piako @ Paeroa-Tahuna Road >0.5 m3 s-1 >0.8 m3 s-1 >1.0 m3 s-1

Piako @ Kiwitahi >0.3 m3 s-1 >0.4 m3 s-1 >0.5 m3 s-1

Waitoa @ Puketutu Road >0.2 m3 s-1 >0.5 m3 s-1 >1.0 m3 s-1

Waitoa @ Waharoa >0.25 m3 s-1 >0.5 m3 s-1 >1.0 m3 s-1

It is emphasised that these flow thresholds are based on monitoring results from only one or two

summer periods. It is recommended that continuous dissolved oxygen and temperature monitoring

be implemented in at least one location on the Waitoa River and one on the Piako River to better

refine these thresholds over multiple years and differing flow conditions. This will be required to

increase WRC’s ability to confidently define limits that meet the likely NOF dissolved oxygen and

temperature requirements for protecting ecosystem health.


In the lower river, downstream of the confluence with the Waitoa River, a zone of dissolved oxygen

depletion has been identified where dissolved oxygen concentrations drop below recommended

protection levels for aquatic ecosystem health. A similar phenomenon has been observed in the

lower Waihou River. From the data available, the magnitude and spatial extent of the zone of

depletion appears to be greatest during summer and associated with the development of a zone of

high turbidity under summer low flows. This may act as a constraint on upstream water resource use

and it is recommended that more detailed monitoring of this feature be undertaken to better

elucidate the temporal and spatial extent of the oxygen depletion zone. Enhanced understanding of

the processes driving the observed patterns will allow WRC to make more informed and transparent

decisions about the management of upstream water resources.


1 Introduction

1.1 Background

The Waikato Regional Council (WRC) is responsible for managing the status of water resources in the

Waikato Region. WRC’s approach to the protection, allocation and use of water resources is set out

in the Waikato Regional Plan: Variation No. 6 – Water Allocation, which became operative on 10 April

2012 (Waikato Regional Council 2012). As required by the National Policy Statement for Freshwater

Management (MfE 2011), the Plan defines minimum flows and allocation limits for all catchments in

the region (Table 3-5, Waikato Regional Council 2012).

A review of the flow and allocation limits in the Piako catchment is scheduled for July 2014 (Table 3-

4A, Waikato Regional Council 2012). In preparation for this, WRC have initiated a number of

investigations to support and inform the review process. The aim of this project was to characterise

the dissolved oxygen and water temperature dynamics in the Piako catchment during summer low

flow conditions with a view to identifying potential limits on the assimilative capacity of the

waterway.

1.2 Project scope

The aim of this project was to empirically evaluate the interrelationships between flow and dissolved

oxygen and water temperature dynamics in the Piako catchment. This was to be achieved through

the following programme of work:

� Characterisation of the temporal dynamics of dissolved oxygen and water temperature

at a range of sites in the Piako catchment during summer low flow conditions.

� Analysis and interpretation of the results with respect to flow and the known

tolerances of aquatic organisms.

During the course of the project, WRC carried out longitudinal surveys of dissolved oxygen

concentrations in the lower reaches of the Piako River (Vant 2013). WRC subsequently requested

that the results of those surveys be added to this report (Ed Brown, WRC, pers. com.).


2 Methodology

D-Opto loggers calibrated and deployed by WRC were used to collect continuous dissolved oxygen

and water temperature data at six sites around the Piako catchment during the summer low flow

period of 2013 (Table 2-1). The location of the six monitoring sites are shown in Figure 2-1. The

loggers were also deployed during the summer of 2012 at two of the sites. Four of the sites are also

flow gauging sites and flow data for the relevant monitoring periods were provided by WRC (Table

2-1).

Table 2-1: Location of dissolved oxygen and temperature monitoring sites in summer 2012 and 2013.

Site Easting Northing 2012 2013 Flow

Piako@Paeroa-Tahuna Road 1821514 5845214 � � �

Piako@Kiwitahi 1829552 5824019 � � �

Piakonui@Piakonui Road 1832535 5814545 �

Waitoa@Mellon Road 1832321 5843131 � �

Waitoa@Waharoa 1841868 5817036 � �

Waitoa@Puketutu Road 1839787 5804427 �

Following data checking, time series were plotted and summary statistics derived describing the

dissolved oxygen dynamics at each site. Summary statistics were compared against the limits for

ecosystem protection proposed by Franklin (2013) (Table 2-2). All summary statistics were calculated

from continuous dissolved oxygen measurements. The 7-day mean dissolved oxygen concentration

was calculated as a rolling 7-day average of the mean daily dissolved oxygen concentrations. The 7-

day mean daily minimum dissolved oxygen concentration was calculated as a rolling 7-day average of

the daily minimum dissolved oxygen concentration and the instantaneous minimum is equivalent to

the daily minimum dissolved oxygen concentration.

Table 2-2: Proposed dissolved oxygen levels for protection of New Zealand freshwater fish communities

(Franklin 2013). Imperative protection level is the minimum recommended protection level for adult fish.

Guideline protection level should be the target protection level or minimum for salmonids and the early life

stages of all species. For salmonid spawning redds or embryonic stages of fish, this should be the interstitial

concentration.

Summary statistic Protection level Dissolved oxygen

concentration (mg L-1)

7-day mean

(mg L-1)

Guideline 8.0

Imperative 7.0

7-day mean daily minimum

(mg L-1)

Guideline 6.0

Imperative 5.0

Instantaneous minimum

(mg L-1)

Guideline 5.0

Imperative 3.5

10 Dissolved oxygen and temperature dynamics in the Piako catchment

Figure 2-1: Location of dissolved oxygen monitoring sites in the Piako catchment.


Flow dependent relationships with dissolved oxygen were analysed using linear regression and

Pearson correlation coefficients. The concentration of dissolved oxygen in water has also been linked

to the influence of different hydraulic properties on the dissolved oxygen reaeration coefficient (k2)

(e.g., O'Connor & Dobbins 1958). A range of methods are available to estimate the value of k2

(Aristegi et al. 2009, Chapra & Di Toro 1991, Hornberger & Kelly 1975, Thyssen & Erlandsen 1987),

but the relative performance of different methods when transferred between systems is variable

(Aristegi et al. 2009, Thyssen & Erlandsen 1987). Both Aristegi et al. (2009) and Young, in Wilcock et

al. (2011), recommended the night-time regression method of Hornberger and Kelly (1975) as being

suitable for estimating the value of k2 from continuous dissolved oxygen measurements under stable

flow conditions. This method relies on the fact that plant photosynthesis stops at night and so any

changes in oxygen concentration are due to either uptake by respiration within the river or exchange

of oxygen with the atmosphere at the river surface. This relationship is represented in the following

equation:

��

��= −� + �

Where O is the dissolved oxygen concentration, t is the time of day, R is the rate of oxygen uptake by

respiration, k2 is the stream reaeration coefficient and D is the dissolved oxygen deficit such that

D=Osat – O where Osat is the saturation concentration of dissolved oxygen at the monitoring site. The

rate of change of the oxygen concentration (dO/dt) and D are known from the oxygen record. The

slope of the regression line on those data collected at night is used to estimate k2 and the intercept

to estimate mean R. The r2 value of the regression line gives an indication of the confidence in the

estimation, with little confidence given to values when r2 is <0.4 (Aristegi et al. 2009, Wilcock et al.

2011).

The value of k2 was estimated for each site using the night-time regression method. Only regressions

with an r2 ≥ 0.4 were used in the analysis of the relationship between k2 and flow.


3 Results

3.1 Flow conditions during sampling

River flows over the sampling period were characterised by a spring time recession of the

hydrograph interspersed with two high flow events in late October 2012 and early December 2012,

followed by an extended period of base flow where flows were at or below Q5 until mid-April 2013

(Figure 3-1). Dissolved oxygen loggers were also deployed at Kiwitahi and the Paeroa-Tahuna Road

bridge for the 2011-12 summer period (Figure 3-1). River flow during this period was characterised

by frequent summer flood events, including two large floods in late December 2011 and early

January 2012. Flow did not fall below Q5 for the whole of this period. Monitoring at the Paeroa-

Tahuna Road bridge site was also continued throughout the winter period of 2012 (Figure 3-1).

Figure 3-1: Flow time series for the Piako River at the Paeroa-Tahuna Road gauging site showing the

dissolved oxygen sampling periods. The solid shaded areas represent the duration of the 2011-12 and 2012-

13 monitoring periods. Monitoring also continued over the winter period (striped shading) at the Paeroa-

Tahuna Road bridge monitoring site. Horizontal dashed line is the estimated Q5 flow.


3.2 Water temperature dynamics

3.2.1 Water temperature time series

Water temperature time series for each of the six monitoring sites are shown in Figure 3-2 to Figure

3-5 and are strongly correlated with ambient air temperature. Water temperature was recorded in

the Piako River at the Paeroa-Tahuna Road bridge from October 2011 to April 2013 (Figure 3-2).

Summer maxima were around 22°C and 24°C for the 2011-12 and 2012-13 summer periods

respectively. The lowest recorded temperature was 6°C in May 2012. During summer a diel variation

in water temperatures was observed of around 2°C. Summer water temperatures at this site exceed

the thermal preferences of both inanga and smelt (Table 3-1), both of which would be expected to

occur in this part of the Piako River. Summer water temperatures in the Piako River at Kiwitahi were

very similar to those recorded at the Paeroa-Tahuna Road bridge and again exceed the preferred

temperature of both inanga and smelt (Figure 3-3). Summer water temperatures in the Piakonui

Stream generally did not exceed 17°C during 2012-13 (Figure 3-4). This is within the temperature

preference range for the main fish species (longfin and shortfin eel, Cran’s bully and banded kokopu)

that occur in this stream (Franklin & Bartels 2012). The lower water temperatures at this site reflect

its location higher in the catchment and the greater proportion of stream shading in many of the

nearby tributaries.

Water temperatures in the three Waitoa River monitoring sites are relatively similar with summer

maxima during 2012-13 of around 24°C (Figure 3-4 & Figure 3-5). The main difference is that diel

variability is slightly higher (c. 3°C) at the Puketutu Road site, compared to c. 2°C at the other two

sites. At all three sites summer water temperatures (December to February) are generally in excess

of those preferred by both smelt and inanga (Table 3-1). Both species would be expected to occur at

these sites based on their location in the catchment and the habitat available (Franklin & Bartels

2012).

Table 3-1: Preferred water temperatures of selected fish species typical of the Piako River system. Source:

Richardson et al. (1994).

Species Life stage Preferred temperature and

quartiles (°C)

Shortfin eel (Anguilla australis) Elver 26.9 (25.6-28.5)

Longfin eel (Anguilla dieffenbachii) Elver 24.4 (22.6-26.2)

Cran’s bully (Gobiomorphus basalis) Mixed 21.0 (19.6-22.1)

Common bully (Gobiomorphus cotidianus) Mixed 20.2 (18.7-21.8)

Inanga (Galaxias maculatus) Whitebait 18.8 (18.0-19.8)

Juvenile 18.7 (17.3-20.0)

Adult 18.1 (17.2-19.1)

Banded kokopu (Galaxias fasciatus) Whitebait 16.1 (14.8-17.7)

Adult 17.4 (16.3-18.3)

Common smelt (Retropinna retropinna) Adult 16.1 (15.1-17.4)


Figure 3-2: Water temperature time series from October 2011 to April 2013 for the monitoring site on the Piako River at the Paeroa-Tahuna Road bridge.


Figure 3-3: Water temperature time series from the 2011-12 and 2012-13 sampling periods for the monitoring site on the Piako River at the Kiwitahi.


Figure 3-4: Water temperature time series for the 2012-13 low flow period for the Piakonui Stream and the Waitoa at Puketutu Road. The large diel variations in water

temperature at the Puketutu Road site during November 2012 (blue box) are thought to be a result of the logger being exposed rather than real water temperatures and should be

treated with caution (Mark Hamer, WRC, pers. com.).


Figure 3-5: Water temperature time series for the 2012-13 low flow period for the Waitoa at Waharoa and Mellon Road.


3.2.2 Interaction between water temperature and flow

The scatter plots of mean daily water temperature against flow showed some evidence of a slight

negative correlation between the variables (Figure 3-6 to Figure 3-8). However, this correlation is

confounded by the co-variation between flow and water temperature with season (i.e., higher water

temperatures and lower flows both occur in summer). To account for the potential temporal co-

variation, regression relationships between maximum daily water temperature and mean daily flow

were calculated on a monthly basis for each site (based on the assumption that within each month

average temperature and flow are similar).

At all sites, for the majority of months there was a negative correlation between maximum water

temperature and mean daily flow. However, in most cases this correlation was not statistically

significant (p>0.05; Table 3-2). At the Piakonui site, there were no statistically significant correlations

in any month. Statistically significant negative correlations were most common in November 2012,

but this also coincides with the steepest recession of the hydrograph and increase in water

temperatures associated with the end of spring and thus is likely to be an artefact of the natural

seasonal changes during this month. The sites on the Waitoa River at Waharoa and Mellon Road

have the greatest frequency of significant relationships between flow and water temperature.

However, the slope of the statistically significant correlations ranges from -14.9 to 11.9 at Waharoa

and -9.8 to 3.2 at Mellon Road. This lack of consistency makes it difficult to provide any robust

characterisation of any potential causal relationship between water temperature and flow.

Table 3-2: Statistical significance of monthly correlations between maximum water temperature and mean

daily flow for the 2012-13 monitoring period. All significant correlations were negative except where

indicated (+ive). NS = Not significant; * = p<0.05; ** = p<0.01; *** = p<0.001.

Site October November December January February March April May

Kiwitahi NS *** ** NS * (+ive) NS NS NS

Paeroa-Tahuna

Road NS ** NS NS NS NS NS NS

Piakonui NS NS NS NS NS NS NS NS

Puketutu Road NS *** NS NS NS ** NS NS

Waharoa NS *** ** NS ** (+ive) ** NS NS

Mellon Road NS ** * NS ** (+ive) *** NS ***


Figure 3-6: Scatter plots of maximum daily water temperature against mean daily flow for the Piako River at Kiwitahi and the Paeroa-Tahuna Road bridge. Circles represent

data from 2011-12 and crosses 2012-13 sampling periods. Note that the x-axis is a logarithmic scale.


Figure 3-7: Scatter plots of maximum daily water temperature against mean daily flow for the Piakonui Stream and the Waitoa River at Puketutu Road. Note that the x-axis is

a logarithmic scale.


Figure 3-8: Scatter plots of maximum daily water temperature against mean daily flow for the Waitoa River at Waharoa and Mellon Road. Note that the x-axis is a logarithmic

scale.


3.3 Dissolved oxygen dynamics

3.3.1 Dissolved oxygen time series

The dissolved oxygen time series for the Piako and Waitoa Rivers are shown relative to the

imperative dissolved oxygen limits for ecosystem protection proposed by Franklin (2013) in Figure

3-9 to Figure 3-12. The longer time series recorded in the Piako River at the Paeroa-Tahuna Road

bridge shows that both mean and minimum dissolved oxygen concentrations were higher than the

proposed ecosystem protection levels throughout the summer of 2011-12 and winter 2012 (Figure

3-9). However, as flows dropped during the summer of 2012-13, dissolved oxygen concentrations fell

below the recommended thresholds for all three measures of dissolved oxygen (7-day mean daily

concentration, 7-day mean daily minimum concentration and daily minimum concentration) for

several weeks. The dissolved oxygen minima recorded during this period at this site exceeded acute

lethal levels for some species of fish (Dean & Richardson 1999) and thus, in combination with the

duration of low dissolved oxygen concentrations, there is a high risk of ecological impairment at this

site.

At the Kiwitahi monitoring site, the recommended ecosystem protection levels were not met for

parts of both the 2011-12 and 2012-13 summer periods (Figure 3-10). In 2011-12, the period where

limits were exceeded occurred during December, prior to the summer flood events, but were still of

sufficient duration to increase the risk to aquatic ecosystem values. During the summer of 2012-13

when sustained low flows occurred, dissolved oxygen concentrations were below recommended

protection levels from mid-December until beyond the middle of February when the logger failed

(Figure 3-10). The risk of ecosystem impairment at this site is likely to be high as a result of this

prolonged period of low dissolved oxygen concentrations, particularly in combination with additional

stressors such as high water temperature and low flows.

Dissolved oxygen concentrations in the Piakonui Stream remained well above the recommended

ecosystem protection levels throughout the monitoring period (Figure 3-11). The cooler water

temperatures and riffle located upstream of this site will contribute significantly to this.

At all three monitoring sites on the Waitoa River, dissolved oxygen concentrations generally

remained at or above the recommended limits for ecosystem protection for the entire monitoring

period (Figure 3-11 & Figure 3-12). However, particularly at Puketutu Road and Waharoa, the 7-day

mean daily and 7-day mean daily minimum dissolved oxygen concentrations frequently approached

their respective limits during the January to March 2013 low flow period. At each of these sites, the

diel variation in dissolved oxygen concentrations between January and March was typically very high,

with dissolved oxygen concentration frequently varying by 5-10 mg L-1 between the daily minimum

and maximum values. This is likely to be a consequence of high macrophyte abundance in these

reaches.

At all six sites, the effect of the first significant rainfall event in early April 2013 is apparent in the

dissolved oxygen time series. In all cases, the elevated flows result in an improvement in dissolved

oxygen concentrations. This further emphasises the importance of flushing flows for maintaining

dissolved oxygen concentrations, which was also observed in the summer of 2011-12.


Figure 3-9: Dissolved oxygen time series for the Piako River at the Paeroa-Tahuna Road bridge. Dissolved oxygen limits indicated by solid horizontal lines are those proposed by

Franklin (2013) for the protection of aquatic ecosystem values.


Figure 3-10: Dissolved oxygen time series for the Piako River at Kiwitahi. Dissolved oxygen limits indicated by solid horizontal lines are those proposed by Franklin (2013) for the

protection of aquatic ecosystem values.


Figure 3-11: Dissolved oxygen time series for the Piakonui Stream and the Waitoa at Puketutu Road. Dissolved oxygen limits indicated by solid horizontal lines are those

proposed by Franklin (2013) for the protection of aquatic ecosystem values. None of the limits are visible for the Piakonui site as they are below the range of dissolved oxygen

concentrations observed at the site.


Figure 3-12: Dissolved oxygen time series for the Waitoa at Waharoa and Mellon Road. Dissolved oxygen limits indicated by solid horizontal lines are those proposed by

Franklin (2013) for the protection of aquatic ecosystem values.


3.3.2 Interactions between dissolved oxygen and flow

A non-linear threshold response was identified between dissolved oxygen concentrations and mean

daily flow at the Paeroa-Tahuna Road, Kiwitahi, Puketutu Road and Waharoa monitoring sites (Figure

3-13 to Figure 3-18). At the Piakonui and Mellon Road sites, no significant effect of flow on dissolved

oxygen concentrations was identified.

At the Paeroa-Tahuna Road bridge site as flow drops below 1 m3 s-1, dissolved oxygen concentrations

begin to decline (Figure 3-13). This decline accelerates rapidly as flow falls below 0.6 m3 s-1 such that

at flows below 0.5 m3 s-1 the imperative dissolved oxygen limits for all three measures of dissolved

oxygen are breached.

At Kiwitahi as flows fall below 0.5 m3 s-1, dissolved oxygen concentrations begin to decline. The flow

threshold below which dissolved oxygen concentrations exceed the recommended imperative

ecosystem protection limits is 0.3 m3 s-1 (Figure 3-14).

In the Piakonui Stream, no significant thresholds or correlations between flow and dissolved oxygen

were observed (Figure 3-15).

In the Waitoa River at Puketutu Road the daily dissolved oxygen minima and 7-day mean daily

minimum dissolved oxygen concentrations declined as flow fell below 1 m3 s-1 (Figure 3-16).

However, the 7-day mean daily dissolved oxygen concentration remained stable until flow fell below

0.3 m3 s-1. The imperative ecosystem protection limits for the 7-day mean and 7-day mean minimum

dissolved oxygen concentrations were exceeded as flow fell below 0.20 m3 s-1.

As flow falls below 1 m3 s-1 at the Waharoa site on the Waitoa River both the daily minimum and 7-

day mean daily minimum dissolved oxygen concentrations decline (Figure 3-17). As flow declines

below 0.25 m3 s-1 the recommended imperative protection levels for these measures of dissolved

oxygen concentration are exceeded. For the 7-day mean daily dissolved oxygen concentration, no

correlation is observed with flow.

Over the range of flows observed, no correlation was identified between flow and any of the

measures of dissolved oxygen concentrations at the Mellon Road sampling site on the Waitoa River

(Figure 3-18). However, dissolved oxygen did fall below the imperative 7-day mean limit on a number

of days.


Figure 3-13: Scatter plots of daily minimum, 7-day mean daily minimum and 7-day mean daily dissolved oxygen concentrations against mean daily flow. Data are for the Piako

at the Paeroa-Tahuna Bridge. Circles represent data from 2011-12 and crosses 2012-13 sampling periods. Note that the x-axis is a logarithmic scale. The recommended imperative

ecosystem protection levels for each dissolved oxygen statistic are shown as solid horizontal lines.


Figure 3-14: Scatter plots of daily minimum, 7-day mean daily minimum and 7-day mean daily dissolved oxygen concentrations against mean daily flow. Data are for the Piako

at Kiwitahi. Circles represent data from 2011-12 and crosses 2012-13 sampling periods. Note that the x-axis is a logarithmic scale. The recommended imperative ecosystem

protection levels for each dissolved oxygen statistic are shown as solid horizontal lines.


Figure 3-15: Scatter plots of daily minimum, 7-day mean daily minimum and 7-day mean daily dissolved oxygen concentrations against mean daily flow. Data are for the

Piakonui Stream. Note that the x-axis is a logarithmic scale. The recommended imperative ecosystem protection levels for each dissolved oxygen statistic are shown as solid

horizontal lines.



Waitoa Stream at Puketutu Road. Note that the x-axis is a logarithmic scale. The recommended imperative ecosystem protection levels for each dissolved oxygen statistic are

shown as solid horizontal lines.



Waitoa Stream at Waharoa. Note that the x-axis is a logarithmic scale. The recommended imperative ecosystem protection levels for each dissolved oxygen statistic are shown as

solid horizontal lines.



Waitoa Stream at Mellon Road. Note that the x-axis is a logarithmic scale. The recommended imperative ecosystem protection levels for each dissolved oxygen statistic are shown

as solid horizontal lines.


3.3.3 Reaeration rates

Using the night-time regression method, robust estimates of the dissolved oxygen reaeration rate k2

were calculated for the sites at Kiwitahi, Pareoa-Tahuna Road, Puketutu Road and Waharoa (Figure

3-19 & Figure 3-20). At all four sites the relationship between the estimate of k2 and mean daily flow

was heteroscedastic (i.e., as flow increased, the variance in the estimate of k2 also increased).

However, unlike the direct correlations between dissolved oxygen concentration and mean daily

flow, the response was more linear and did not display the same threshold type responses. At all

sites a downward trend in the average reaeration coefficient with decreasing flow was present. The

implication of this is that as flow reduces, there is less capacity for dissolved oxygen concentrations

to recover from night-time depletion of dissolved oxygen caused by respiration. It is likely this is an

effect of reduced turbulence and possibly surface area associated with lower flows reducing the

diffusion potential of oxygen across the air-water interface.


Figure 3-19: Scatter plots of the estimated reaeration coefficient (k2) and river flow at the Kiwitahi and Paeroa-Tahuna Road sampling sites on the Piako River.


Figure 3-20: Scatter plots of the estimated reaeration coefficient (k2) and river flow at the Puketutu Road and Waharoa sampling sites on the Waitoa River.


3.3.4 Dissolved oxygen long profiles

WRC undertook an investigation of water quality in the estuarine section of the Piako River between

September 2011 and June 2012. Longitudinal profiles of dissolved oxygen concentrations, water

temperature and salinity were measured in the Piako River between the mouth of the river near

Pipiroa and the confluence of the Piako and Waitoa Rivers near Maukoro Landing (Figure 3-21). A

total of four profiles were completed, with all surveys beginning at the river mouth near the time of

high tide. Surveys encompassed a range of tide and flow conditions (Table 3-3).

Table 3-3: Flow and tide conditions for the four longitudinal profiles. Tide height is the high tide limit

predicted by the NIWA Tide Forecaster for the mouth of the Piako River (175 29 5 2E, 37 11 13 S). Flows are

estimated mean daily flow based on addition of measured flows in the Piako at the Paeroa-Tahuna Road and

the Waitoa River at Mellon Road.

Date Tide height (m) Flow (m3 s-1)

29/09/2011 1.90 7.4

13/12/2011 1.49 3.1

12/03/2012 1.87 3.5

25/06/2012 1.27 6.9

Figure 3-21: Sites on the lower Piako River where water quality was surveyed during 2011-12. The star

shows the approximate upstream limit of salt water intrusion observed during this study (March 2012). Source:

Vant (2013).


The long profiles of water temperature, salinity, dissolved oxygen concentration and turbidity

collected from the lower Piako are shown in Figure 3-22. Water temperature was relatively

consistent along the extent of the long profile and, as would be expected, varied with season. Mean

temperature for the September 2011 profile was 13.6 °C, increasing to 20.5 and 19.2 °C for the

December 2011 and March 2012 profiles respectively, and then lowering again to 11.1 °C for the

June 2012 survey. This variation in water temperature had a strong influence on the mean dissolved

oxygen concentration observed during each survey, with a negative correlation (r2=0.84, p=0.05)

between the two variables. It appears from the salinity profiles that saline intrusion into the

freshwater zone is influenced by both tide height and flow (Figure 3-22). The tides on the September

2011 and March 2012 surveys were very similar (1.90 and 1.87 m respectively), but flow was more

than double that in the March 2012 survey during the September 2011 survey (Table 3-3).

Correspondingly, the limit of saline influence is 10 km further inland during the low flow conditions

of March 2012 (Figure 3-22). A similar pattern is observed for the two surveys carried out at lower

tide heights.

The dissolved oxygen profiles indicate the progressive development of a dissolved oxygen sag in the

lower river from early spring through to the end of summer (Figure 3-22). However, this has

disappeared by the time of the June 2012 survey. A similar dissolved oxygen sag has been recorded

during summer in the lower Waihou River (Franklin & Smith 2014, Vant 2011). In the Waihou

catchment, the location of the dissolved oxygen minima was strongly correlated with tide height over

the range of flows that observations were made (Franklin & Smith 2014). However, in the lower

Piako, this correlation does not exist. It is suggested that this may be a consequence of the broader

range of flow and temperature conditions captured in the Piako surveys, thus reflecting their

additional influence on dissolved oxygen concentrations. It was hypothesised that this would also be

the case in the Waihou River if a wider range of flows were included in the surveys (Franklin & Smith

2014).

In the lower Piako, the location and magnitude of the dissolved oxygen minima during spring and

summer appear to be associated with the occurrence of a turbidity maxima (Figure 3-22). Vant

(2013) suggested that this turbidity maxima is probably due to a naturally occurring estuarine

circulation cell operating in the low salinity zone of the lower river. The turbidity maxima appears to

be most developed during summer low flow conditions and the location of the maxima moves up

and down the river channel as tide height varies. Higher freshwater flows in spring and winter seem

to be associated with a reduction in the spatial extent of the turbidity maxima from approximately 18

km during summer to about 5 km in spring and winter. This area is also associated with inputs of

organic material from the Kopouatai wetland (Vant 2013). It is likely that oxidation of organic matter

trapped within the turbidity maxima accounts for much of the observed oxygen depletion in this

reach of the Piako. The higher rates of depletion that occur in summer compared to winter supports

this hypothesis, with the decay rate for organic matter being temperature dependent. Lower

temperatures reduce the decay rate of organic matter (Chapra 1997), thus reducing oxygen demand

and therefore reducing the impact of the organic matter on dissolved oxygen concentrations.

Dissolved oxygen concentrations during the two summer long profile surveys were below

recommended imperative 7-day daily mean (7.0 mg L-1) and 7-day mean daily minimum (5.0 mg L-1)

protection limits for aquatic ecology through most of the reach and would not meet the proposed

NOF bottom lines for dissolved oxygen, i.e., they would fall into Band D.


Figure 3-22: Long profiles of water temperature, salinity, dissolved oxygen and turbidity in the lower Piako.

The maximum detection limit for turbidity was 1000 NTU.


In late summer 2013, WRC deployed a water quality data sonde (YSI model 660) for 3 weeks in the

lower Piako River at Ngatea, approximately 13 km upstream from the river mouth. Conductivity,

turbidity and dissolved oxygen were measured at 20 minute intervals throughout the deployment

and results are presented in Figure 3-23.

The aim of the deployment was to provide supplementary high temporal resolution data on

dissolved oxygen dynamics in one location coinciding with the approximate high tide location of the

dissolved oxygen minima identified in the long profile surveys. Flow in the river at the time of the

measurements was generally low, with two small flushing flow events towards the end of the first

and third weeks of the deployment (Figure 3-23A). Tidal range varied from neap tides (small tides) in

the first week, through spring tides (large tides) in the second week and back to neap tides in the

third week of measurements (Figure 3-23B).

Conductivity was lowest during neap low tides and peaked during spring high tides, reflecting the

influx of saline water during the tidal cycle (Figure 3-23C). It is interesting to note that the increases

in conductivity associated with high tide were suppressed during the short periods of elevated

freshwater flows (25-26 April) indicating the importance of freshwater flows in driving the

hydrodynamics and associated water quality characteristics of the lower river.

Turbidity displayed more complex dynamics, with peaks occurring at the time of both high and low

tide, and minima occurring at mid-tide, with the greatest reduction as the tide was going out (Figure

3-23D). Superimposed on this pattern, maximum turbidity was influenced by the shift between neap

and spring tides, with the highest values of turbidity recorded under spring tide conditions. Elevated

freshwater flows in also appear to reduce the magnitude of the turbidity maximum.

Dissolved oxygen dynamics were influenced by both salinity and the shift from neap to spring tides

(Figure 3-23E). Dissolved oxygen saturation reduces as salinity increases with the incoming tide. It

then recovers as the tide goes out and salinity reduces again. Additionally, daily mean oxygen

saturation was lower during spring tides (c. 49% saturation), compared to neap tide conditions (c.

66% saturation). Assuming water temperatures were similar to those recorded during the March

2012 longitudinal surveys (19 °C), this equates to dissolved oxygen concentrations of approximately

4.5 and 6.1 mg L-1, respectively. During spring tides, the dissolved oxygen concentrations therefore

fall below the proposed NOF bottom lines for ecosystem protection. The highest dissolved oxygen

saturation (c. 80%) occurred during the elevated flows in the last week of the measurements. It is

likely this reflects increased reaeration associated with higher flows and the flushing of oxygen-

demanding organic matter downstream (Figure 3-23E).

The fact that lower mean dissolved oxygen concentrations occur during the spring tide phase, when

turbidity is also greatest, reinforces the observations from the longitudinal profiles that dissolved

oxygen depletion appears to be coupled with the magnitude and spatial extent of the turbidity

maxima. Elevated freshwater flows also coincide with reductions in the magnitude of the turbidity

maximum and increases in dissolved oxygen concentration. This may have implications for how WRC

wish to manage flows in the Piako, particularly given that currently dissolved oxygen levels in the

lower river do not meet imperative protection levels for aquatic ecology and will not meet the

proposed NOF bottom lines for dissolved oxygen.


Figure 3-23: Provisional results from a survey of environmental conditions in the lower Piako River over a 3-

week period in April-May 2013. A, combined flow of the Piako and Waitoa Rivers (as measured at Paeroa-

Tahuna Rd and Mellon Rd, respectively); B, tide height in the Southern Firth of Thames (at Tararu); and C,

conductivity; D, turbidity and E, dissolved oxygen as recorded by a datasonde deployed in the river at the SH2

bridge in Ngatea. Source: Vant (2013).

0

2

4

6

19 Apr 26 Apr 3 May 10 May

A, River flow (m3/s)

-2

-1

0

1

2


B, Tide height (m)

0

1

2


C, Conductivity (mS/cm)

0

500

1000


D, Turbidity (NTU)

0

20

40

60

80


E, Dissolved oxygen (% saturation)


4 Discussion

4.1 Dissolved oxygen and flow

The 2011-12 summer sampling period was characterised by two relatively high summer rainfall

events and regular summer flushing flows. In contrast the 2012-13 summer sampling period was

characterised by a prolonged period of low base flows.

Summer water temperatures at five of the six sites exceeded the preferred water temperatures of

several key fish species that are expected to occur at these sites. However, they did not exceed the

lethal threshold for any fish species over the monitoring period. There was some evidence of a

potential negative correlation between maximum water temperature and mean daily flow in some

months at some sites, but the relationship between the two variables was inconsistent both between

sites and between months within sites. It was therefore not possible to make any recommendations

with respect to setting water resource use limits in relation to water temperatures.

At both sites where monitoring occurred during both summers (Paeroa-Tahuna Road & Kiwitahi),

dissolved oxygen concentrations were noticeably lower during the low, stable flow conditions

experienced in 2012-13 than in the higher flows that occurred in 2011-12. At four of the six

monitoring sites, distinct threshold responses were identified between mean daily flow and dissolved

oxygen concentrations over the range of flows observed during 2012-13. It is likely that this is related

to a reduction in the dissolved oxygen reaeration coefficient (k2) as flow declines at these sites.

In the recently proposed amendments to the National Policy Statement for Freshwater Management

2011 (MfE 2013), National Objective Framework (NOF) limits for maintaining ecosystem health were

proposed for dissolved oxygen concentrations in rivers (Appendix A). Whilst it has been proposed

that these limits are to apply only downstream of point sources (MfE 2013), the supporting

documentation used to derive these limits recommended that the same limits are appropriate for

and should apply to all locations on the river network (Davies-Colley et al. 2013).

Presently, the appropriate statistical measure for defining whether an attribute state (i.e., Class) is

achieved or not has not been agreed (i.e., does the threshold have to be met, for example, 100% of

the time or is 90% of the time adequate). An indication of the flows that approximate to the attribute

state class lower boundaries for the proposed NOF are presented in Table 4-1 for those sites where

clear threshold responses occur. These values are approximations made by eye from Figure 3-13 to

Figure 3-18. More detailed statistical analyses based on longer time series are required to better

characterise the flow thresholds that correspond to differing protection levels, but these flow

thresholds can be used as a guide to the range of flows that must be considered in setting minimum

flow limits for maintaining dissolved oxygen concentration at these sites.

For all sites it appears that the time between flushing flow events may have an important role in

determining summer minimum dissolved oxygen concentrations. In the Piako catchment it appears

that as the duration of low flows increases, the dissolved oxygen minima decreases. However,

following flushing flow events (e.g., December 2011 and April 2013) the dissolved oxygen

concentrations recovered significantly. Long-term monitoring of dissolved oxygen would be required

to gain a better understanding of the role of flushing flows and low flow duration in controlling

dissolved oxygen concentrations. For example, summer floods have also been observed to result in

significant reductions in dissolved oxygen in some locations in the Piako River (Bob Wilcock, NIWA,

pers. com.).


Table 4-1: Flows that approximate to the proposed NOF dissolved oxygen attribute state class boundaries

for ecosystem health. Flow values are for the corresponding on-site hydrological gauging station with the

exception of the Waitoa at Puketutu Road where the flow values are those recorded at Waharoa, the nearest

gauging station.

Site

Proposed NOF class – 7-day mean minimum dissolved oxygen

Class C

(5.0 mg L-1)

Class B

(7.0 mg L-1)

Class A

(8.0 mg L-1)

Piako @ Paeroa-Tahuna Road >0.50 m3 s-1 >0.80 m3 s-1 >1.00 m3 s-1

Piako @ Kiwitahi >0.30 m3 s-1 >0.40 m3 s-1 >0.50 m3 s-1

Waitoa @ Puketutu Road >0.20 m3 s-1 >0.50 m3 s-1 >1.00 m3 s-1

Waitoa @ Waharoa >0.25 m3 s-1 >0.50 m3 s-1 >1.00 m3 s-1

4.2 Lower Piako dissolved oxygen dynamics

The work carried out by WRC has demonstrated the existence of a dissolved oxygen sag in the lower

reaches of the Piako River. This has also been observed for the Waihou River (Franklin & Smith 2014,

Vant 2011). From the data available it appears that the dissolved oxygen sag progressively develops

through spring to reach a maximum in late summer/early autumn. Whilst based only on spot

measurements, during summer dissolved oxygen concentrations in the lower part of the river were

below the 5.0 mg L-1 (7-day mean daily minimum) threshold set as the bottom line for the proposed

NOF limits for up to 20 km. This implies a high risk of ecosystem health being impaired by dissolved

oxygen in the lower river. The extent of the depletion is similar to that observed in the Waihou, but

the minimum dissolved oxygen concentrations are lower than those seen in the Waihou (Franklin &

Smith 2014, Vant 2011).

The evidence suggests that the development of the dissolved oxygen sag in the lower river may be

associated with the concurrent development of a zone of high turbidity, or turbidity maxima, in the

lower river. The likely mechanism for the depletion of oxygen is the oxidation of organic matter

associated with the turbidity maxima. The extent and magnitude of the turbidity maxima was

greatest during summer flows, and much reduced during higher freshwater flows in spring and

winter. This suggests that freshwater flows may be an important control on this phenomenon, and

therefore dissolved oxygen depletion, in the lower river. However, more data are required to confirm

this association.

The high resolution temporal data collected at Ngatea in 2013 demonstrate the complexity of the

interactions between freshwater inflows, tides, salinity, turbidity and dissolved oxygen in the lower

river. Broadly speaking, neap tide conditions were associated with lower turbidity maxima and higher

dissolved oxygen concentrations at Ngatea, whilst during spring tides turbidity was high and

dissolved oxygen concentrations lower. Superimposed on this were the effects of freshwater inflows,

with elevated flows apparently reducing turbidity and increasing dissolved oxygen.

The patterns observed in the high resolution data largely reinforce those identified in the long profile

measurements and are similar to those observed in the Waihou (Franklin & Smith 2014) and other

systems internationally (Mitchell et al. 1999, Uncles et al. 1998). Overall, it is clear that dissolved

oxygen concentrations become depleted over an extended reach (c. 20 km) of the lower Piako,

particularly during summer, and that they fall below recommended protection levels for aquatic

ecosystems. It appears that the magnitude and extent of the oxygen depletion zone is greatest


during the spring tide cycle and under low freshwater inflows during summer. This suggests that, as

with the Waihou River, water quality in the lower reaches of the Piako may be a bottleneck for the

management of upstream water resources. Consequently, it is recommended that WRC invest effort

in better understanding the mechanisms and drivers of water quality in the lower reaches of the

Piako (and Waihou) in order to provide transparency and confidence in setting constraints on

upstream water resource use. This should include collecting data to understand the spatial and

temporal variability in the extent, duration and magnitude of the oxygen depletion zone both

seasonally and across the full tidal cycle. Higher resolution monitoring through the deployment of

data sondes at fixed locations through the lower reach of the river is also recommended to help

elucidate the mechanisms associated with oxygen depletion. These measurements should include

water temperature, salinity, turbidity and dissolved oxygen as a minimum. Understanding the

interactions between tides, freshwater inflows, the extent of the turbidity maxima and oxygen

depletion is critical to being able to develop robust water resource use limits for the lower river.


5 Conclusions

Dissolved oxygen is an important water quality variable for maintaining ecosystem health.

Concentrations of dissolved oxygen at Kiwitahi and the Paeroa-Tahuna Road on the Piako River fell

below recommended imperative ecosystem protection levels for several weeks during the low flow

summer of 2012-13. In combination with the high water temperatures, there is an elevated risk of

ecosystem impairment at both of these sites. At all three monitoring sites in the Waitoa River,

dissolved oxygen concentrations generally remained just above the recommended threshold for

ecosystem protection. Only at the Piakonui Stream sampling site did dissolved oxygen concentrations

remain well above the recommended thresholds. In the lower river, downstream of the confluence

with the Waitoa River, there is an extended zone of dissolved oxygen depletion where

concentrations are below recommended protection levels for ecosystem health.

Based on the observed correlations between flow and dissolved oxygen concentrations, it is

recommended that minimum flow limits be set at a level sufficient to avoid dissolved oxygen

concentrations falling below the recommended protection levels for ecosystem health. Indicative

flow thresholds for differing protection levels were presented in Table 4-1 within the framework of

the proposed NOF. It must be emphasised that these thresholds are based on monitoring results

from only one or two summer periods and estimates of thresholds that are not statistically derived.

Slow flowing rivers with high macrophyte cover like the Piako are complex systems making

identification of spatially and temporally consistent relationships between flow, water temperature

and dissolved oxygen challenging. It is recommended that continuous dissolved oxygen monitoring

be implemented in at least one location on the Waitoa River and one on the Piako River to better

refine these thresholds over multiple years and differing flow conditions. This will be required to

increase WRC’s ability to confidently define limits that meet the likely NOF requirements for

protecting ecosystem health.

In the lower river, there are also indications that low flows may contribute to the development of a

turbidity maxima and associated zone of oxygen depletion. However, further work is required in both

the lower Waihou and Piako to elucidate the details of the complex interactions between flows, tide,

temperature, salinity, turbidity and dissolved oxygen dynamics operating in this zone. It is possible

WRC could decide that the low dissolved oxygen levels observed in these reaches may be a

constraint on upstream water resource use due to the potential risk to aquatic ecosystem health.

It was not possible to identify any clear relationships between water temperature and flow. It is

recommended that long term continuous monitoring of water temperature also be implemented in

the catchment so that potential correlations over multiple years can be identified and characterised.

Elevated water temperatures are a known stressor for many aquatic organisms (Olsen et al. 2012).

Higher water temperatures also reduce the dissolved oxygen saturation potential of water and can

therefore limit dissolved oxygen concentrations. Management of water temperatures may therefore

be valuable for managing the dissolved oxygen concentrations in the catchment, potentially

increasing the availability of water for out-of-stream use.

6 Acknowledgements

I would like to acknowledge WRC staff, particularly Mark Hamer, for maintaining and deploying the

dissolved oxygen loggers and providing the data as required. Flow data were also provided by WRC

and Hauraki District Council.


7 References

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Appendix A Proposed National Objectives Framework

Table A-1: Proposed NOF limits for dissolved oxygen concentrations for ecosystem health in rivers. Note:

MfE (2013) proposes that these limits should apply only downstream of point sources.

Documents

NIWA Client report · 2020-06-05 · three times the median flow at the Kauaeranga River site. 16 Figure 3-8: Ohinemuri River site. 17 Figure 3-9: Ohinemuri River flow from 1 January