Seasonal dynamics of macroinvertebrate assemblages in the benthos and associated with detritus packs in two low-order streams with different riparian vegetation

Seasonal dynamics of macroinvertebrate assemblages inthe benthos and associated with detritus packs in twolow-order streams with different riparian vegetation

JOHN F. MURPHY AND PAUL S. GILLER

Department of Zoology and Animal Ecology, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland

SUMMARY

1. The seasonal dynamics of the benthic macroinvertebrate assemblage, and the subset of

this assemblage colonising naturally formed detritus accumulations, was investigated in

two streams in south-west Ireland, one draining a conifer plantation (Streamhill West) and

the other with deciduous riparian vegetation (Glenfinish). The streams differed in the

quantity, quality and diversity of allochthonous detritus and in hydrochemistry, the

conifer stream being more acid at high discharge. We expected the macroinvertebrate

assemblage colonising detritus to differ in the two streams, due to differences in the

diversity and quantity of detrital inputs.

2. Benthic density and taxon richness did not differ between the two streams, but the

density of shredders was greater in the conifer stream, where there was a greater mass of

benthic detritus. There was a significant positive correlation between shredder density and

detritus biomass in both streams over the study period.

3. Detritus packs in the deciduous stream were colonised by a greater number of

macroinvertebrates and taxa than in the conifer stream, but packs in both streams had a

similar abundance of shredders. The relative abundance of taxa colonising detritus packs

was almost always significantly different to that found in the source pool of the benthos.

4. Correspondence analysis illustrated that there were distinct faunal differences

between the two streams overall and seasonally within each stream. Differences between

the streams were related to species tolerances to acid episodes in the conifer stream.

Canonical correspondence analysis demonstrated a distinct seasonal pattern in the detrital

composition of the packs and a corresponding seasonal pattern in the structure of the

detritus pack macroinvertebrate assemblage.

5. Within-stream seasonal variation both in benthic and detritus pack assemblages and in

detrital inputs was of similar magnitude to the between-stream variation. The conifer

stream received less and poorer quality detritus than the deciduous stream, yet it retained

more detritus and had more shredders in the benthos. This apparent contradiction may be

explained by the influence of hydrochemistry (during spate events) on the shredder

assemblage, by differences in riparian vegetation between the two streams, and possibly

by the ability of some taxa to exhibit more generalist feeding habits and thus supplement

their diets in the absence of high quality detritus.

Keywords: detritus, macroinvertebrates, shredder assemblages, streams, riparian vegetation

Introduction

The importance of riparian vegetation as a source of

allochthonous organic matter for forested streams is

Freshwater Biology (2000) 43, 617±631

ã 2000 Blackwell Science Ltd. 617

Correspondence: Paul S. Giller, Department of Zoology andAnimal Ecology, University College Cork, Lee Maltings,Prospect Row, Cork, Ireland. E-mail: [email protected]

well established (Egglishaw, 1964; Mathews &

Kowalczewski, 1969; Cummins et al., 1973; Webster

& Benfield, 1986; Cummins et al., 1989; Richardson,

1991; Wallace et al., 1997). In such streams, detritivor-

ous macroinvertebrates, be they shredders, collectors

or filter feeders, rely on plant litter as a major source

of energy. Accumulated detritus forms packs which, if

retained, are degraded by a combination of physical

processes and microbial and invertebrate activity.

Such packs are colonised by a variety of macroinver-

tebrates which use them as food, habitat or both

(Egglishaw, 1964; Reice, 1978; Richardson, 1992). The

quantity and diversity of detritus available in a stream

for macroinvertebrate consumption depends on the

type of riparian vegetation bordering the stream and

on the retentiveness of the stream (Benfield, Jones &

Patterson, 1977; Pozo, Basaguren & EloÂsegui, 1994,

Pozo et al., 1997; Benfield, 1997; Jones, 1997).

Petersen & Cummins (1974) proposed a `dietary

continuum' of decaying detritus due to different rates

of decomposition of different types. Based on pub-

lished decay rates, they classified detritus into fast,

medium and slow categories and suggested that

detritus from each reaches a maximum palatability

after a sequentially longer period in the stream. This

extends the duration of food availability for detriti-

vores well beyond autumn leaf fall, as they shift their

feeding to the most nutritious source available

(Cummins et al., 1989). The more diverse the riparian

vegetation, the greater is the range of detritus and,

hence, the longer the period of plentiful food supply.

Previous studies of the link between detrital inputs

and stream macroinvertebrates have sampled the

benthos or investigated the colonisation of prepared

leaf packs (e.g. Prochazka, Stewart & Davies, 1991;

Malmqvist & Oberle, 1995; Haapala & Muotka, 1998).

Prepared packs are almost always constructed solely

from whole leaves placed either in mesh bags or tied

together in an open pack. Few studies have regularly

sampled naturally formed detritus packs (King et al.,

1987; Casas, 1997). Patterns of macroinvertebrate

colonisation of prepared packs can only be extra-

polated to that of natural packs with caution (King

et al., 1987). Natural packs are more complex and

diverse since they contain detritus at various stages of

decay, from a range of different sources and hence

provide a greater range of resources to colonisers.

Naturally formed packs are therefore likely to attract

more diverse macroinvertebrates than are prepared

monospecific packs (Boulton & Boon, 1991; Dobson,

1991).

Over the past 40 years the rate of planting of

commercial conifer plantations in Ireland has

increased substantially, with many plantations being

established in the upper parts of catchments, often

bordering low-order streams (Giller et al., 1993). Such

streams then have riparian zones dominated by one

species of conifer, usually Sitka spruce (Picea sitchensis

(Bongard) Carriere), whose needles decay slowly.

This contrasts with streams flowing through decid-

uous woodland, where a greater range of tree species

is present and leaves cover a broader range of

processing rates (Murphy & Giller, 2000). The

objectives of the present study were to investigate

the seasonal dynamics of the macroinvertebrate

assemblage associated with naturally formed detritus

accumulations in two streams differing in their

riparian vegetation and therefore, their detrital inputs.

The relationship between the assemblage colonising

the detritus packs and the surrounding benthos was

examined in both streams, and we attempted to

establish those pack attributes, e.g. mass of various

detritus categories, that best determined the assem-

blage structure in the packs. We hypothesised that the

macroinvertebrate assemblage colonising detritus

accumulations in the stream flowing through the

conifer plantation would differ from that of the mixed

deciduous stream, due to differences in the diversity

and quantity of allochthonous detritus. We predicted

that shredders would be more abundant both in the

benthos and in detritus accumulations in the decid-

uous stream.

Methods

This study was undertaken in two first order streams

in north Co. Cork, Ireland, from 31 March 1994 to 14

February 1995. Both study sites were described by

Murphy & Giller, (2000). Streamhill West (52°18'N,

08°359W) flows through a conifer plantation and has

riparian vegetation composed almost entirely of

mature Douglas Fir (Pseudotsuga menziesii (Mirabel)

Franco) with a sparse understory of bramble (Rubus

spp.) willow (Salix spp.), birch (Betula spp.) and

Rhododendron spp. Glenfinish stream (52°109N,

08°099W) flows through agricultural land with some

adjacent conifer plantations but with a substantial belt

of deciduous riparian vegetation along its length. The

618 J. F. Murphy & P. S. Giller

ã 2000 Blackwell Science Ltd, Freshwater Biology, 43, 617±631

riparian belt is more diverse and dominated by beech

(Fagus sylvatica L.), holly (Ilex aquifolium L.), oak

(Quercus petreaea L.) and hazel (Corylus avellana L.)

and also some birch, rowan (Sorbus sp.) and sycamore

(Acer spp.).

The pH was recorded in the field (Wissenschaftlich

Technische WerksaÈttten 192 pH meter) every two

weeks from 22 June 1994 to 14 February 1995.

Retentiveness was measured once a month between

July and December 1995. On each occasion, 25 plastic

`leaves' (heavy-duty plastic, cut into 6 ´ 9 cm pieces)

were released in mid-stream one at a time, at the

upstream end of a 30-m reach. The distance travelled

by each plastic leaf before being retained for greater

than 60 s was recorded. A Friedman test was carried

out on median distances, to test for significant

differences between streams and sampling occasions.

Discharge was estimated by measuring stream flow

and depth at 1, 3, 5 and 7 eighths of the wetted

channel width and summing the calculated dis-

charges of the four cross-sections in each of five

transects. Stream discharge was then estimated as the

mean of the five transects and ranged from 0.06 to

0.31 m3 s±1 at Streamhill West and 0.04±0.39 m3 s±1 at

Glenfinish over the five months.

On 17 March 1994 five detritus traps were placed

within the 30 m study site of both streams in areas of

fast flow, but not in the main flow path (thalweg). The

traps consisted of a 12 ´ 10 cm piece of plastic mesh

(1 cm mesh), secured between two narrow gauge

metal rods (40 cm long) hammered into the substra-

tum perpendicular to the direction of flow. The traps

were emptied on 31 March and every two weeks

subsequently until 20 July. Thereafter, they were set

for a two-week period every month until sampling

ended on 14 February 1995. Each trap was emptied by

placing a pond net (500 mm mesh) immediately

downstream and carefully lifting the plastic mesh

off the rods allowing the retained detritus and

associated fauna to collect in the net. Samples were

preserved in the field.

Benthic macroinvertebrates were sampled every

four weeks from 13 April 1994 to 14 February 1995 at

both study sites. Five random Surber sample

units (0.0625 m2, mesh-size 500 mm) were taken to

» 10 cm depth of substratum in areas of riffle

within the 30-m study reach and preserved in the

field. The benthos was usually sampled on the

same day that detritus traps were emptied, or

within two weeks of the collection of the detritus

samples.

In the laboratory, sediment and macroinvertebrates

were thoroughly washed from each detritus-trap

sample. Macroinvertebrates were sorted from the

remaining coarse particulate organic matter (CPOM)

and identified, to species where possible. Some

groups (e.g. Orthocladiinae, Simuliidae, early instar

Limnephilidae) were identified only as far as sub-

family or family level. The remaining components of

each detritus trap sample were separated into various

categories (e.g. oak, hazel, holly, beech, birch, willow,

Rhododendron spp. conifer needles, grasses, CPOM

> 1 mm, and FPOM > 0.25 mm). The dry mass of each

category was recorded, as described by Murphy &

Giller (2000). Macroinvertebrate abundance was

expressed per unit dry mass of the detritus pack

they had colonised (e.g. numbers g±1)

The detritus and macroinvertebrates from each

Surber sample unit were separated from the sediment

by repeated elutriation and the remaining sediment

was subsequently searched for cased caddis larvae

and gastropods. The macroinvertebrates were sorted

from the detritus and identified as described above.

Benthic detritus present in the September-February

surber samples was dried at 60 °C for 48 h and

weighed to the nearest 0.01 g. Macroinvertebrate

abundance and detrital mass were both expressed

per m2 of streambed.

Analysis

Wilcoxon paired-sample tests (Zar, 1999) were used to

test for significant differences between the two

streams in total macroinvertebrate abundance, shred-

der abundance and taxon richness in the benthos and

in detritus packs. Taxa were assigned to the shredder

functional feeding group for the purposes of this

study, based on Malmqvist, Nilsson & Svensson

(1978), Merritt & Cummins (1996) and Mihuc (1997)

(Appendix 1). We acknowledge that some of these

taxa have been reported to feed on other resources in

certain habitat types (e.g. Ledger & Hildrew, 2000) or

at certain developmental stages (e.g. Giller & Sang-

pradub, 1993). However, all assigned taxa have been

shown to include detritus as a major component of

their diets and in some cases are limited by its supply

(e.g. Gee, 1988; Dobson & Hildrew, 1992). ANOVAs

were carried out, after log transformation, to test for

Macroinvertebrates in benthos and detritus packs 619


significant seasonal variation in the abundance mea-

sures. For the taxon richness data, sampling occasions

were combined into four seasonal groups (March-

May, June-August, September-November and Decem-

ber-February), with total taxon richness on each

sampling occasion within a season considered as a

replicate. Fisher's multiple comparison test was used

following the detection of significant seasonal differ-

ences to identify which groups differed significantly.

Spearman rank correlations were carried out between

the 20 most abundant taxa in the benthos and the 20

most abundant taxa in the detritus packs on each

sampling occasion, to test for significant associations

between the two assemblages, within each stream.

Correspondence Analysis (CA) was carried out,

using the CANOCO 3.10 package (ter Braak, 1990), on

the benthic and detritus trap macroinvertebrate data

from both streams in order to ascertain the degree of

seasonal variability in assemblage composition and

the relative similarity between the four data sets. The

20 most abundant taxonomic groups over the entire

year were selected from both the benthic and detritus

trap data at each stream and a combined taxon list

was constructed, comprising 35 taxa. Abundances on

each sampling occasion were converted to relative

abundances so that all data sets (benthos ± no. m±2

and detritus traps ± no. g±1) would be in a comparable

format. CA works with relative abundances, not

actual densities, so no information was lost by this

transformation (ter Braak, 1985; Palmer, 1993).

Canonical Correspondence Analysis (CCA) was

also carried out, using CANOCO, to assess the

relationship between the colonising macroinverte-

brate assemblage and measured attributes of the

detritus packs (dry mass of each detritus category).

CCA is a direct gradient analysis that assumes a

unimodal model for the relationship between the

response of each species to environmental gradients

and it calculates ordination axes from linear combina-

tions of the environmental variables. Each axis is

responsible for a proportion of the variation in the

assemblage and has an eigenvalue that measures its

importance, i.e. the extent to which the environmental

variables aligned to that axis explain the distribution

of the taxa. A forward selection procedure was

incorporated in the analysis that ranked the environ-

mental variables in order of importance in explaining

the variation in species data and incorporated only the

significant ones (as tested through Monte Carlo

simulations within the program) in the overall

analysis.

Results

Both streams were circumneutral and had similar

current velocities at baseflow throughout the study

period, although Streamhill West, draining conifers,

tended to be more acidic at high discharge (mean and

range in Streamhill West: 6.71, 4.86±7.68; Glenfinish:

7.00, 6.26±7.5). Streamhill West (SHW) and Glenfinish

(GFH) were similarly retentive (median distance

travelled and range at SHW: 4.9, 0.4±30 m; GFH:

3.45, 0.1±30 m; S = 0.2, P < 0.655) and retentiveness

did not vary significantly between sampling occasions

within each stream (S = 6.4, P = 0.172).

Macroinvertebrate abundance and richness

Benthic macroinvertebrate density and taxon richness

did not differ significantly between Streamhill West

and Glenfinish over the study period (Wilcoxon two-

tailed paired-sample, d.f. = 12, density: T+ = 38,

P < 0.5, taxon richness: T+ = 24, P < 0.2) (Fig. 1).

However, macroinvertebrate abundance (no. g±1 of

detritus) and taxon richness in the detritus packs were

significantly higher at Glenfinish than at Streamhill

West (Wilcoxon one-tailed paired-sample, d.f. = 16,

abundance: T+ = 25, P < 0.05, taxon richness: T+ = 3,

P < 0.001) (Fig. 1). Benthic macroinvertebrate density

varied significantly across sampling occasions at

Streamhill West (F11,57 = 5.32, P < 0.001) being higher

in autumn than in spring or winter (Fig. 1a). Benthic

macroinvertebrate density also varied significantly

across sampling occasions at Glenfinish (F11,57 = 6.91,

P < 0.001) with significant peaks in early summer and

early autumn (Fig. 1a). There was no significant

seasonal variation in benthic taxon richness at

Glenfinish or Streamhill West (Fig. 1b). Macroinverte-

brate abundance (no. g±1) in detritus packs peaked

significantly in summer at both sites (SHW:

F15,75 = 6.01, P < 0.001; GFH: F15,75 = 8.16, P < 0.001)

and was lowest in winter and spring (Fig. 1c). Taxon

richness in the packs only varied significantly over the

year at Glenfinish (F3,15 = 5.13, P < 0.016), with a peak

in richness during the autumn (Fig. 1d).

The dominant shredder taxa in Streamhill West

benthos and detritus packs were Protonemura meyeri

Pictet, Amphinemura sulcicollis Stephens and Leuctra



spp., while in Glenfinish Gammarus duebeni Lilljeborg,

P. meyeri, A. sulcicollis and Leuctra spp. were most

numerous. Benthic shredder density was significantly

greater over the study period in Streamhill West than

in Glenfinish (Wilcoxon one-tailed paired-sample,

d.f. = 12, T+ = 75, P < 0.0025) (Fig. 2). No significant

difference was found over the year between the two

streams in shredder abundance in the detritus packs

(two-tailed, d.f. = 16, T+ = 104, P < 0.1) (Fig. 2). How-

ever, there was significant seasonal variation in

shredder density in the benthos of both streams

(SHW: F11,57 = 7.58, P < 0.001; GFH: F11,57 = 3.61, P <

0.001), with a significant sustained density increase

over autumn and winter in Streamhill West and a

lesser increase in density over autumn in Glenfinish

(Fig. 2). A significant seasonal change in shredder

abundance was found in the detritus packs (SHW:

F15,75 = 3.44, P < 0.001; GFH: F15,75 = 6.94, P < 0.001).

Shredder densities in Glenfinish packs peaked sig-

nificantly in September and then declined steadily

over autumn and winter. In Streamhill West there was

less difference between spring-summer and autumn-

winter densities but the autumn-winter peak occurred

a month later and was sustained for longer (Fig. 2).

Detritus

Benthic detritus was significantly more abundant at

Streamhill West than Glenfinish over the six-month

period (September±February) (Wilcoxon one-tailed

paired-sample, d.f. = 6, T+ = 18, P < 0.05) (Fig. 3).

Within both streams, benthic detritus varied signifi-

cantly over time (SHW: F5,29 = 3.7, P < 0.013; GFH:

F5,27 = 2.73, P < 0.046). At Streamhill West it was most

plentiful in September (136 g m±2) and decreased

steadily to a minimum of 30 g m±2 in February. At

Glenfinish detritus standing crop peaked a month

later, in October (72 g m±2) and again decreased

during the winter to a minimum of 22 g m±2 in

February. There was a significant positive correlation

between log-transformed benthic shredder density

and benthic detritus density in both streams over the

study period (SHW: d.f. = 28, r = 0.598, P < 0.001;

GFH: d.f. = 26, r = 0.588, P < 0.001).

Composition of the macroinvertebrate assemblage

The resultant biplot from the Correspondence Analy-

sis (Fig. 4) shows the relative positions in ordination

space of the benthic and detritus pack macroinverte-

Fig. 1 Seasonal changes in (a) mean benthic macroinvertebrate density; (b) benthic taxon richness; (c) mean macroinvertebrate

abundance per mass of detritus pack and (d) taxon richness in the detritus packs. Error bars = 1 SE.



brate assemblages (Streamhill West and Glenfinish

benthos and Streamhill West and Glenfinish detritus

assemblages). Each sample is located at the centroid of

the taxa that occur there. The abundance or prob-

ability of occurrence of a taxon in a sample tends to

decrease with its distance from the taxon's location in

the diagram (ter Braak & Prentice, 1988) and therefore

samples that contain a certain taxon are scattered

around the point for that taxon (ter Braak &

Verdonschot, 1995). The CA distinctly separates the

four assemblages based on their taxonomic composi-

tion and it expresses the seasonal variation within

each assemblage. The Glenfinish benthos is relatively

distinct from the other three. Its direction of max-

imum variation is aligned perpendicular to that of

both Streamhill West assemblages and the Glenfinish

detritus assemblage (Fig. 4a). Axis 1 separates the

samples primarily on the basis of season, and Axis 2

separates them on the basis of stream and sample

type. Each of the four groups has an intrinsic seasonal

cycle of variation in taxonomic composition in the CA

ordination. In Glenfinish, the benthos and detritus

assemblages have markedly different seasonal pat-

terns (Fig. 4a), and differed in the ranked abundance

of their dominant taxa throughout the year (rs varies

from ± 0.326±0.351; P > 0.05). In Streamhill West, the

two assemblages varied similarly over time although

again the ranked abundance of the dominant taxa

differed on almost all sampling occasions throughout

the year (rs = 0.005±0.371; P > 0.05). Only in August

did the two assemblages correlate significantly

(rs = 0.458±0.500; P < 0.05).

Fig. 2 Seasonal changes (� 1 SE) in (a) benthic density of

shredders and (b) density of shredders per unit mass of detritus

pack.

Fig. 3 Variation in the quantity of benthic detritus (� 1 SE) in Streamhill West (squares) and Glenfinish (triangles) from 28 September

1994 to 14 February 1995.



The Streamhill West benthos was characterised by

higher than average relative abundances of Siphono-

perla torrentium, Isoperla grammatica, Oulimnius sp.

(adults and larvae), Wiedemannia sp. and Leuctra spp.,

especially in the winter/spring samples. The sum-

mer/autumn assemblage tended to have relatively

more Ephemerella ignita, Helodes sp., and Orthocladii-

nae (Fig. 4b). The Streamhill West detritus assemblage

in winter/spring had higher than average relative

abundances of A. sulcicollis. In the summer/autumn

samples E. ignita, Orthocladiinae, Dixidae and Rhya-

cophila spp. were at above average abundances. The

Streamhill West summer/autumn samples were dis-

tinct from those in winter/spring (Fig. 4a).

Fig. 4 Correspondence analysis ordination plot

of sampling occasions (a) and taxa (b) for the

assemblages in Streamhill West and Glenfinish.

The sampling occasions for each assemblage

are outlined by a dotted line. Axes 1 and 2 had

eigenvalues of 0.483 and 0.392, respectively.

Months with two sampling occasions are

further labelled a and b. Key to taxa

abbreviations: Bmuti- Baetis muticus L., Brhod-

Baetis rhodani Pictet, Eigni- Ephemerella ignita

Poda, Rsemi- Rhithrogena semicolorata Curtis,

Asulc- Amphinemura sulcicollis, Brisi-

Brachyptera risi Morton, Storr- Siphonoperla

torrentium Pictet, Igram- Isoperla grammatica

Poda, Leuct- Leuctra spp., Pmeye- Protonemura

meyeri, Pcing- Potamophylax cingulatus

Stephens, Leins- early instar Limnephilidae,

Hinst- Hydropsyche instabilis, Rdors- Rhyacophila

dorsalis Curtis, Rhyac- early instar Rhyacophila

spp., Eaena- Elmis aenea (larva) MuÈ ller, Epara-

Esolus parallelepipedus (larva), Lvolk- Limnius

volkmari (larva) Panzer, Oulima- Oulimnius sp.

(adult), Ouliml- Oulimnius sp. (larva), Helod-

Helodes sp., Helop- Helophorus sp., Hgrac-

Hydraena gracilis Germer, Chiro- Chironominae,

Dicro- Dicranota sp., Dixid- Dixidae, Ortho-

Orthocladiinae, Simul- Simuliidae, Wiedm-

Wiedemannia sp., Enchy- Enchytaeidae, Tubif-

Tubificidae, Nemat- Nematoda, Gdueb-

Gammarus duebeni, Hydra- Hydracarina. There

were two sampling occasions each month from

April to August, indicated by an additional `a'

and `b'.



The Glenfinish benthos was characterised by

Rhithrogena semicolorata, G. duebeni, Baetis muticus,

Hydropsyche instabilis Curtis, Agapetus sp., Tubificidae,

Baetis rhodani, Hydraena gracilis, and Nematoda during

spring, autumn and winter (Fig. 4b). The June/July

assemblage tends to be more similar to that of the

other three assemblages, with numerous Orthocladii-

nae and early instar Limnephilidae. The Glenfinish

detritus assemblage between March and October had

a high relative abundance of Orthocladiinae, Rhyaco-

phila spp. and Dixidae. The November to February

detrital assemblages tend more towards the winter

taxa of Streamhill West assemblages (in the same

direction but not to the same extent) (Fig. 4a). Taxa

such as Rhyacophila dorsalis, Brachyptera risi, Wiede-

mannia sp. and Enchytraeidae are situated roughly

equidistant from each assemblage suggesting they

were equally well represented in the benthic and

detritus pack samples of both streams (Fig. 4b).

Macroinvertebrate-environmental variable relationships

Canonical Correspondence Analysis was carried out

on the detritus pack assemblages of both streams.

Eigenvalues greater than 0.3 indicate a very strong

gradient along the corresponding axis (ter Braak &

Verdonschot, 1995). The environmental variables

selected in the analysis are represented in the biplot

by arrows, which point in the direction of maximum

change in the value of the associated variable. The

arrowhead co-ordinates on each axis are essentially

the correlations of the environmental variable with

that ordination axis (arrowhead co-ordinates can only

vary from 0 to 1 on each axis). The length of the arrow

is therefore proportional to the maximum rate of

change for the variable, so those variables with

relatively short arrows do not vary much across the

ordination plot (ter Braak & Verdonschot, 1995).

At Streamhill West, clear seasonal changes in the

detritus pack assemblage is again apparent (Fig. 5).

March±May was characterised by high relative abun-

dance of larvae of Simuliidae, A. sulcicollis, Oulimnius

sp. larvae and Helodes sp. Birch leaves were relatively

rare, but particulate organic matter > 1 mm was

prominent in the packs. From June to August the

assemblage contained a higher than average relative

Fig. 5 Canonical correspondence analysis

ordination plot of taxa (crosses), detritus

pack sampling occasions (dark squares)

and foward-selected detritus pack

components (arrows) in Streamhill West.

Axes 1 and 2 have eigenvalues of 0.302

and 0.092, respectively. See Fig. 4 legend

for key to taxa abbreviations.



abundance of Hydracarina, Chironominae, B. rhodani,

E. ignita and Helophorus sp. Willow leaves and CPOM

> 1 mm were then scarce. In September and October,

Orthocladiinae, Dixidae, Rhyacophila spp. and Wiede-

mannia sp. were numerous and both birch and willow

leaves increased their contribution to the packs, with

birch reaching its maximum abundance. From

November to February P. meyeri, B. risi, early instar

Limnephilidae, I. grammatica and Leuctra spp. all

reached maximum relative abundance corresponding

to the peaks in the mass of willow and CPOM

> 1 mm. CPOM > 1 mm was very strongly positively

correlated with Axis 1 which had an eigenvalue

(0.303) indicative of a strong gradient between the

summer and winter samples.

At Glenfinish a seasonal pattern was also apparent

though less well defined than at Streamhill West. The

late spring, summer and autumn samples are more

clumped (Fig. 6) leaving the winter samples more

distinct than at Streamhill West. The gradient along

Axis 1 is not as strong as in Streamhill West, having an

eigenvalue of 0.202. The analysis selected grasses

(positively correlated with Axis 1), hazel (positively

correlated with Axis 2) and woody fragments as the

detritus pack components that best explained the

variation in the assemblage between samples (Fig. 6).

The spring/summer samples generally had a higher

relative abundance of Orthocladiinae, E. ignita, R.

dorsalis, Wiedemannia sp., B. rhodani, Tubificidae and

Rhyacophila spp. Woody fragments were then scarce.

From October to November the packs contained more

Potamophylax cingulatus, early instar Limnephilidae, B.

muticus, Dixidae, Helodes sp. larvae and Chironominae

than average. They also contained a higher mass of

hazel leaves than at other times of the year. By

December the contribution from hazel had fallen to

Fig. 6 Canonical correspondence analysis

ordination plot of taxa (crosses), detritus

pack sampling occasions (dark squares)

and forward-selected detritus pack

components (arrows) in Glenfinish. Axes 1

and 2 have eigenvalues of 0.202 and 0.063,

respectively. See Fig. 4 legend for key to

taxa abbreviations.



the summer value. P. meyeri, Leuctra spp., B. risi, A.

sulcicollis, I. grammatica, G. duebeni, Hydracarina and

Simuliidae were relatively numerous from December

to February and grasses were prominent in the packs

(Fig. 6).

In summary, for both streams the CCA demon-

strates the strong seasonal pattern both in the

composition of the colonising macroinvertebrates

and of the detritus packs. The two streams differ in

the relative location of some taxa in the ordination

space, e.g. R. dorsalis and Chironominae, but the more

strictly seasonal taxa, such as E. ignita and B. risi were

strongly associated with the summer and winter

packs, respectively, in both streams. Variation in the

macroinvertebrate assemblages was significantly cor-

related with autumn leaf fall of birch and willow in

Streamhill West and hazel in Glenfinish.

Discussion

This study has revealed differences in the inverte-

brates associated with detritus in Streamhill West, a

conifer stream, and Glenfinish, a stream with decid-

uous trees in the riparian zone. Detritus packs at

Glenfinish were generally larger, more diverse than at

Streamhill West (Murphy & Giller, 2000) and gen-

erally consisted of detritus types of higher quality, i.e.

known from literature to be highly palatable and to

have rapid breakdown rates (Webster & Benfield,

1986; Giller & Malmqvist, 1998). The Glenfinish packs

were colonised by a greater number of individuals

and taxa but had a similar number of shredders as

packs in Streamhill West. The detritus assemblage

was a subset of the benthos but within each stream, on

most occasions, the relative abundances of the

dominant taxa differed between the two assemblages.

Benthos

The two streams did not differ significantly in the

density or taxon richness of total macroinvertebrates

in the substratum. However, they did differ signifi-

cantly in the density of shredders in the benthos and

the correspondence analysis clearly shows that they

differed in their general community structure, i.e.

taxonomic composition and relative abundances. The

most notable differences between the two benthic

assemblages were the complete absence of G. duebeni

and the very low numbers of R. semicolorata, H.

instabilis and B. muticus at Streamhill West compared

to Glenfinish. Conversely, E. ignita, A. sulcicollis,

Chloroperla torrentium, Esolus parallelepipedus MuÈ ller

and Oulimnius sp. were considerably more abundant

at Streamhill West than at Glenfinish. Gammaridae,

Heptageniidae and B. muticus are known to be very

sensitive to even moderately low pH through direct

toxic effects and also indirectly by alteration of the

quality of their food source (Sutcliffe & Hildrew, 1989;

Wade et al., 1989; Dangles & GueÂrold, 2000). At

Streamhill West the pH usually exceeded 6.0 but

was subject to acid episodes at high discharge. The

only sampling occasion when R. semicolorata was

found at Streamhill West (October) followed two

months of circumneutral pH. Some taxa, known to be

somewhat less sensitive to acid pulses, were present

but less abundant at Streamhill West (H. instabilis, B.

rhodani). The more dominant taxa at Streamhill West

must therefore be able to endure the temporary acidic

conditions. E. ignita, A. sulcicollis, C. torrentium and

Ouliminius sp., taxa that are abundant in Streamhill

West, have been shown to be tolerant of pH < 5.5

(Willoughby & Mappin, 1988; Wade et al., 1989).

However, Streamhill West should not be considered

an acid stream per se as it does not possess the

depauperate fauna characteristic of such streams (e.g.

Groom & Hildrew, 1989; Ledger & Hildrew, 2000).

Detritus pack assemblages

The detritus pack assemblages of the two streams

were distinctly separated by the correspondence

analysis on the basis of their taxonomic composition.

Taxa such as A. sulcicollis, C. torrentium and Leuctra

spp. characterised the Streamhill West packs while

Orthocladiinae, G. duebeni and early instar Limnephi-

lidae were more common at Glenfinish. The most

numerous taxa in detritus packs at both streams were

Orthocladiinae and Simuliidae, with the filter-feeding

Simuliidae generally dominating at Streamhill West

and the deposit-feeding Orthocladiinae at Glenfinish.

Indeed, for most of the year, over 70% of individuals

in the Glenfinish packs were Orthocladiinae. Other

studies of the macroinvertebrates associated with

discrete detritus accumulations have also found that

Orthocladiinae tend to dominate detritus packs

numerically (King et al., 1987; Dobson, 1991; Mal-

mqvist & Oberle, 1995; Casas, 1997). G. duebeni is a

prominent, large shredder that was almost absent



from the Streamhill West packs. The one occasion

when it was found in the packs was in September, the

period of maximum leaf (willow and birch) input to

the stream (Murphy & Giller, 2000) and during an

extended period of circumneutral pH. Generally

conditions at the site do not support a large G. duebeni

population however, and as well as acid episodes, it

may require a better quality supply of well-condi-

tioned large particulate detritus, as shown by G. pulex,

which can suffer seasonal detrital food limitation

(Gee, 1988). Streamhill West does not appear to

provide such a food source due to its rather limited

range of riparian vegetation (Murphy & Giller, 2000).

Our results compare reasonably well with those of

other studies that have investigated naturally formed

detritus packs in streams. Malmqvist & Oberle (1995)

found that Chironomidae, Baetidae, Simuliidae,

Hydropsychidae, and I. grammatica were the most

abundant taxa associated with accumulations of

detritus (mostly alder and birch leaves) in a north

Swedish lake outlet stream, and the taxa were

generally at similar densities to those found at

Glenfinish at the same time of year. In a low-order

Austrian mountain stream, Casas (1997) found that

Orthocladiinae, Simuliidae, small nymphs of Baeti-

dae/Ephemerellidae, Protonemura spp. and Amphine-

mura spp. were the dominant taxa in natural leaf

packs. Again, they were found in similar densities in

the present study except that small nymphs were

considerably more abundant than in our packs.

However, a 45-mm mesh size net was used in Casas's

study, compared to the 500-mm mesh in the present

study.

Benthic and detritus pack shredder assemblages

Macroinvertebrate assemblages in detritus packs in

both streams were generally structured differently to

the respective benthic assemblages from which they

are drawn. This difference was greater at Glenfinish

than at Streamhill West. In Streamhill West, seasonal

differences within both assemblages were more

pronounced than differences between the two assem-

blages at any given time. In contrast, the Glenfinish

benthic and detritus assemblages were not so closely

positioned in the CA biplot. The detritus pack

samples showed relatively little seasonal variation

due to the numerical dominance of Orthocladiinae in

the assemblage, while the benthic samples showed a

similar amount of seasonal variation to the Streamhill

West assemblages.

Detrital inputs at Streamhill West were predomi-

nantly twigs and conifer needles that are relatively

resistant to breakdown even in circumneutral streams

while, at Glenfinish, deciduous leaves were a major

component of allochthonous detritus (Murphy &

Giller, 2000. While Streamhill West had a significantly

greater density of shredders in the benthos, both

streams had similar abundances of shredders per unit

mass of detritus pack. This suggests that the packs at

Glenfinish were being colonised relatively more

strongly by shredders than at Streamhill West and

supports the contention that detritus packs at Glen-

finish provided a more attractive resource for shred-

ders. When this finding is considered along with the

lower availability of detritus at Glenfinish, it is

tempting to speculate that the shredders in Glenfinish

are resource limited, as has been found in other

streams (Richardson, 1991; Dobson & Hildrew, 1992).

In fact the relatively stable densities of shredders over

the year at Glenfinish may reflect the steady, year-

round supply of quality detritus to the benthos,

identified from the detrital inputs study (Murphy &

Giller, 2000). At Streamhill West, however, the low

diversity and quantity of detrital inputs, along with

the pronounced increase in shredder density in the

benthos in autumn, coinciding with willow and birch

leaf-fall, seems to imply that well-conditioned and

nutritious detritus maybe limiting in this stream.

Streamhill West had greater quantities of benthic

detritus than Glenfinish from September to February,

despite the fact that Glenfinish received far more

detrital inputs than Streamhill West during that

period (Murphy & Giller, 2000), that the two streams

are similarly retentive, and that Streamhill West had a

higher shredder density at the time. It has been

suggested previously that acidic streams can have a

greater build-up of detritus than circumneutral

streams because the decomposition process is

retarded by the low pH and associated hydrochemical

changes (Groom & Hildrew, 1989; Griffith & Perry,

1993). Acidic conditions could affect detritus break-

down at a number of levels; through reduced

heterotrophic microbial fauna affecting the condition-

ing of detritus, through altered macroinvertebrate

detritivore fauna or through a combination of both

(Dangles & GueÂrold, 2000). As Streamhill West is only

occasionally acidic, it is unlikely that these pulses are



sufficient to explain the surplus of benthic detritus.

Therefore, it seems more plausible to suggest that the

greater quantity of benthic detritus at Streamhill West

is related more to differences in the nature of the

detrital inputs than to the influence of the stream's

hydrochemistry. Even if microbial conditioning of

detritus was unaffected by acidic pulses, and was

similar in both streams, there could still be a build up

of detritus at Streamhill West as conifer needles take

longer to become palatable to detritivorous macro-

fauna than deciduous leaves, and hence may remain

in the stream longer (Townsend & Hildrew, 1984).

The pattern of shredder benthic density and

abundance in detritus packs over the year in the

present study is consistent with that found in other

studies (Townsend & Hildrew, 1984; Groom &

Hildrew, 1989; Dobson & Hildrew, 1992; Griffith &

Perry, 1993). For example, Griffith & Perry (1993)

found greater numbers of shredders in acidic stream

leaf packs than packs in a circumneutral stream,

although they found that packs in the latter had

greater shredder biomass. As in our study, the

invertebrate assemblage in the more acidic stream

was dominated by small leuctrids and nemourids,

whereas the neutral stream had bigger shredders, e.g.

Gammaridae, Tipula sp., and large cased-caddis.

When comparing number of individuals between the

two streams in the present study it is important to

consider that a single large G. duebeni can probably

consume much more detritus than a single late instar

leuctrid or nemourid. Therefore the lower benthic

density of detritus at Glenfinish relative to Streamhill

West may also have been due in part to the greater

rate of colonisation and consumption of detritus, both

benthic and in packs, by shredders in the deciduous

stream.

Macroinvertebrate colonisation in relation to detritus

attributes

The canonical correspondence analysis showed that

the dominant pattern in detritus pack composition in

both streams was the autumn leaf fall of birch and

willow at Streamhill West and hazel at Glenfinish.

After deciduous leaf fall, the annual variation in

CPOM > 1 mm at Streamhill West, and woody

fragments and grasses at Glenfinish, were the most

closely correlated variables with changes in faunal

composition over the year. From this study we cannot

ascertain whether macroinvertebrates were choosing

packs based on their composition. It is more probable

that species life cycles, particularly those of detriti-

vores, have evolved to coincide with autumn leaf fall

so as to benefit from the predictable increase in

detrital inputs (Cummins et al., 1989). Any active

behavioural selection process taking place between

packs is not discernible at the scale of our sampling

and is being swamped by the seasonal variation. A

more rigorous experimental arrangement of packs of

various compositions, offered to the macroinverte-

brates at the one time, would be needed to discover

the extent to which individuals actively move

between packs.

In conclusion, this study has clearly demonstrated

the distinct features of the benthic and detritus pack

assemblages in two streams differing in their detrital

inputs. It has confirmed that, at both sites, within-

stream seasonal variation both in stream faunal

composition and detrital inputs are of similar magni-

tude to the between-stream variation. While seasonal

variation in the structure of assemblages can be

attributed to macroinvertebrate life cycles, differences

between the streams are more complicated. There

appear to be a number of contradictory elements to

the findings of the present study. Streamhill West has

previously been shown to receive lower quantities

and quality of and less diverse detritus than Glenfin-

ish (Murphy & Giller, 2000), yet it had a greater

benthic density of shredders than the deciduous

stream. Benthic shredder densities in Glenfinish

varied relatively less over the year than those in

Streamhill West, which has a pronounced autumn/

winter increase in numbers. This was despite the fact

that during autumn leaf fall, Glenfinish received up to

10 times more detritus than Streamhill West. Also

there was little difference between the two streams in

shredder abundance in detritus packs during the year.

This again was despite perceived differences in the

diversity and quality of detritus in the packs.

There are a number of factors however, that have

not been accounted for in the present study, which

may explain the apparent `shredder conundrum'. The

degree to which detritivorous macroinvertebrates are

able to expand the range of food items and the

circumstances under which they may do so has been

the subject of a number of recent studies which have

shown that some detritivore-shredders can show a

degree of omnivory (Friberg & Jacobsen, 1994; Mihuc,



1997; Ledger & Hildrew, 2000). It is possible that in

the face of an insufficient quantity and diversity of

detritus, shredders in Streamhill West (such as

nemourids) supplement their diet by feeding on fine

particulate organic matter or even epilithic algae.

Also, it is probable that despite the greater density of

shredders in the benthos in Streamhill West, Glenfin-

ish has a greater biomass of shredders than the conifer

stream, due to occurrence of several large shredder

taxa. Finally, the confounding influence of stream

hydrochemistry makes it difficult to confirm the

degree to which the less diverse conifer-dominated

riparian vegetation is having a direct influence on the

stream community. Further studies on a wider range

of streams with varying riparian vegetation and on

the full extent of omnivory in detritivore taxa are

necessary to ascertain the relative influences of the

nature of allochthonous detritus and hydrochemistry

on macroinvertebrate dynamics and community

structure in forested streams.

Acknowledgments

We are grateful to Dr Colin Smith for his advice on

ordination techniques and thank Alan Hildrew and

two anonymous referees for their helpful comments

and suggestions on an earlier manuscript.

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Appendix 1 Assignment of taxa to functional feeding groups based on Malmqvist et al. (1978), Merritt & Cummins (1996) and Mihuc

(1997)

Taxa Funcional feeding group Taxa Functional feeding group

Baetis muticus Scraper Hydraena gracilis Deposit feederBaetis rhodani Scraper Agabus sp. PredatorEcdyonurus sp. Scraper Helodes sp. ScraperEphemerella ignita Deposit feeder Helophorus sp. ShredderRhithrogena semicolorata Scraper Hydrophilidae larva PredatorParaleptophlebia sp. Deposit feeder Limnebius truncatellus Thunberg ScraperAmphinemura sulcicollis Shredder Oreodytes sanmarki Sahlberg PredatorBrachyptera risi Scraper Simuliidae Filter feederSiphonoperla torrentium Predator Ceratopogonidae Deposit feederChloroperla tripunctata Scopoli Predator Dixidae Filter feederIsoperla grammatica Predator Orthocladiinae Deposit feederLeuctra spp. Shredder Tanypodinae PredatorNemoura sp. Shredder Chironominae Deposit feederProtonemura meyeri Shredder Pericoma sp. PredatorAgapetus sp. Scraper Pedicia sp. PredatorGlossosoma sp. Scraper Dicranota sp. PredatorDrusus annulatus Stephens Shredder Tipula sp. ShredderPotamophylax cingulatus Shredder Limnophila sp. PredatorPotamophylax latipennis Curtis Shredder Hexatoma sp. PredatorChaetopteryx villosa Fabricius Shredder Wiedemannia sp. PredatorHalesus radiatus Curtis Shredder Chelifera sp. PredatorSericostoma personatum Spence Shredder Ancylus fluviatilis MuÈ ller ScraperOdonotcerum albicorne Scopoli Shredder Pisidium sp. Filter feederSilo pallipes Fabricius Scraper Sphaerium sp. Filter feederCrunoecia irrorata Curtis Shredder Potamopyrgus jenkinsi Smith ScraperHydropsyche instabilis Filter feeder Enchytraeidae Deposit feederHydropsyche siltalai DoÈhler Filter feeder Lumbricidae Deposit feederRhyacophila dorsalis Predator Lumbriculidae Deposit feederRhyacophila munda McLachlan Predator Tubificidae Deposit feederPolycentropus spp. Predator Erpobdellidae PredatorPlectronemia spp. Predator Gammarus duebeni ShredderPhilopotamus montanus Donovan Filter feeder Asellus aquaticus L. ShredderWormaldia sp. Filter feeder Hydracarina PredatorLimnephilidae (early instar) Deposit feeder Turbellaria PredatorElmis aenea Scraper Neuroptera PredatorEsolus parallelepipedus Scraper Velia caprai Tamanini PredatorLimnius volkmari Scraper Nematoda Deposit feederOulimnius sp. Scraper



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Seasonal dynamics of macroinvertebrate assemblages in the benthos and associated with detritus packs in two low-order streams with different riparian vegetation