Ecological Engineering 74 (2014) 187–195

Spatial and seasonal changes of phosphorus internal loading in twolakes with different trophy

Katarzyna Kowalczewska-Madura *, Ryszard Gołdyn, Martyna DeraAdam Mickiewicz University, Faculty of Biology, Department of Water Protection, Umultowska 89, 61-614 Pozna�n, Poland

A R T I C L E I N F O

Article history:Received 31 March 2014Received in revised form 2 October 2014Accepted 9 October 2014Available online xxx

Keywords:LakePhosphorus releaseBottom sediments

A B S T R A C T

Two lakes with different trophy, Strzeszy�nskie Lake and Uzarzewskie Lake (Western Poland), werestudied in ex situ experiments with the aid of intact bottom sediment cores in order to analyse theinternal loading as well as the spatial and seasonal changes of total phosphorus (TP) release from bottomsediments. It was stated that the TP release from a hypereutrophic Lake Uzarzewskie, despite its earlierrestoration using iron treatment, was over 6 times higher than in a mesotrophic one. The observed valuesin both lakes were higher in the deepest part of the lake than in the littoral zone. TP release inStrzeszy�nskie Lake was very similar in all four seasons of the year with the maximum value of2.8 mgP m�2 d�1 in winter. TP release did not exceed 1.3 mgP m�2 d�1 in the littoral zone and in winter itsaccumulation prevailed over release from bottom sediments. TP release in Uzarzewskie Lake was44.7 mgP m�2 d�1 in autumn in the profundal zone and 6.8 mgP m�2 d�1 in the littoral zone. A dominationof P accumulation in sediments over release was observed at both stations in winter (0.33 mgP m�2 d�1 inthe deepest place and 0.62 mgP m�2 d�1 in the littoral). Knowledge of the internal loading of P in lakesallows managers to compare changes in human impact on the ecosystem and to design practicaltreatments for limiting their trophy and approaches to the management of restoration measures.

ã 2014 Elsevier B.V. All rights reserved.

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1. Introduction

The estimation of total phosphorus (TP) accumulation inbottom sediments as well as its release to overlying water isone of the most important hydrobiological issues. The process ofphosphorus release plays a significant role in the process ofeutrophication of water bodies (Søndergaard et al., 2003). Lakesediments act as a temporary or permanent sink for settling P andplay a key role in the P cycle (Hupfer and Lewandowski, 2005). Alake’s bottom sediments, instead of fulfilling the role of a trap forphosphorus, often become its source (Boström et al., 1988;Golterman, 1995; Kentzer, 2001; Wang et al., 2003). Some of thephosphorus accumulated in the sediments can be mobilized to theoverlying water in algal-available form (Graneli,1999). Phosphoruscan be released from sediment depths of as great as 20 cm belowthe water-sediment interface (Søndergaard et al., 2003). A numberof factors influence the exchange of phosphorus between waterand sediments, including redox conditions, pH, Fe:P ratio andresuspension (Søndergaard et al., 2002; Kleeberg et al., 2013).

* Corresponding author. Tel.: +48 618295878.E-mail addresses: madura@amu.edu.pl (K. Kowalczewska-Madura),

rgold@amu.edu.pl (R. Gołdyn).

http://dx.doi.org/10.1016/j.ecoleng.2014.10.0330925-8574/ã 2014 Elsevier B.V. All rights reserved.

Shallow lakes are very vulnerable to internal loading becausenutrients stored in their sediments have a greater impact on waterquality than in deeper lakes (Granéli,1999). However, in deep lakesmechanisms which allow phosphorus transport from the hypo-limnion to surface water layer also exist; it may be transportedwith methane bubbles or phytoplankton vertical migration (Headet al., 1999).

Phosphorus released from bottom sediments can significantlycontribute to the lake eutrophication. The internal phosphorusloading in Swarzedzkie Lake was 26.9 mgP m�2 d�1 and it washigher in summer than the external loading (Kowalczewska-Madura and Goldyn, 2009). Sometimes internal loading could bevery high e.g. in Grosser Müggelsee was up to 100 mgP m�2 d�1, inSøbygaard Lake to 145.0 mgP m�2 d�1 (Kleeberg 1997; Søndergaardet al., 2001). The current data suggest that one of the major factorsresponsible for the load of phosphorus released from the bottomsediments is the trophic state of the lake (Kentzer, 2001). Typically,it can be explained by the anaerobic conditions in the water layerabove bottom of eutrophic lakes, causing a very low redoxpotential, which promotes the release of TP accumulated on metalcompounds, especially iron oxyhydroxides (Søndergaard et al.,2002; Kleeberg et al., 2013). However, in many cases inmesotrophic or slightly eutrophic lakes oxygen depletion in thehypolimnion and low redox potential are also observed, but the

188 K. Kowalczewska-Madura et al. / Ecological Engineering 74 (2014) 187–195

internal phosphorus loading from bottom sediments is muchlower than in hypereutrophic lakes (Psenner, 1984; Sen et al.,2007). It is typical for thermally stratified lakes that aerobicconditions vary in different seasons. It is believed that thesechanges entail a change in the intensity of phosphorus release. It isnot known whether this occurs with the same intensity inmesotrophic and hypereutrophic lakes. A high phosphorus releaseis often observed in the littoral zone of eutrophic lakes, despite thefact that the bottom is in contact with well oxygenated waters ofepilimnion (James and Barko, 1991; Andersen and Ring, 1999;Dondajewska 2008; Kowalczewska-Madura et al., 2008b). Wesuppose that greater sedimentation of organic matter to thebottom sediments results in a higher oxygen demand in sedimentsand periodic oxygen deficits. Therefore, these spatial as well asseasonal changes of phosphorus release or accumulation indifferent parts of lakes with different trophic states were one ofthe main objects of this study.

Lake restoration processes can significantly change the con-ditions responsible for the release of phosphorus from sediments.One of such restoration method is the iron sulphate treatment,which increases iron content in the sediments, thus contributing tothe intensification of phosphorus adsorption on its oxyhydroxides(Katsev and Dittrich, 2013; Kleeberg et al., 2013). HypereutrophicLake Uzarzewskie had been restored with iron treatment two yearsprior to this study (Gołdyn et al., 2008b). Therefore, the second aimof this study was to determine the extent of a decrease ofphosphorus release from the bottom sediments of this lake as aresult of iron treatment and whether this release approached thelevel observed in the mesotrophic Strzeszy�nskie Lake.

We hypothesize that internal phosphorus loading is muchhigher in hypereutrophic lakes compared to lakes of lower trophicstate, and it is very important for the eutrophication of lakes.During summer time, phosphorus released from bottom sedimentsin the profundal zone is accumulated in the water layer near thebottom of the lake. In the littoral zone, which is in contact with thewarm epilimnetic waters in summer, phosphorus release alsodominates on its accumulation in bottom sediments. This is due tooptimal thermal and changeable oxygen conditions in thesediment-water interface and due to constant organic mattersupply of phytoplankton origin. Moreover, in lakes with a highertrophic state more pronounced seasonal variations of this processare observed than in mesotrophic lakes.

2. Materials and methods

Bottom sediments from two lakes with different trophy weretested: mesotrophic Strzeszy�nskie Lake and hypereutrophic

Fig. 1. Location of the sampling stations and the depth ranges of the profundal (

Uzarzewskie Lake. Strzeszy�nskie Lake is dominated by submergedmacrophytes, has high water transparency (over 6 m) and lowchlorophyll-a content (up to 7.5 mg l�1) (Szelag-Wasielewska,2006, 2007). Strongly eutrophicated Uzarzewskie Lake, on theother hand, is of poor water quality (SD ca. 0.5 m, chlorophyll-a upto 170 mg l�1) and there are strong cyanobacteria blooms insummer (Gołdyn et al., 2008a,b,b).

Strzeszy�nskie Lake is a natural postglacial lake which fills awide depression in the upper part of the River Bogdanka, in thenorth-western part of Pozna�n (52�27 0N,16�49 0E). Its area is 34.9 haand the mean depth is 8.2 m. The lake is divided into 2 parts: themaximum depth of the smaller part is 3.1 m and in the other partthere are two deep places – 16.9 and 17.8 m. The lake is thermallystratified and dimictic and, as regards trophy, mesotrophic(Szelag-Wasielewska, 2006).

Uzarzewskie Lake is a postglacial lake in the shape of a kettle,located in the 17th km of the course of the River Cybina, which is aneastern tributary of the River Warta (52�27 0N, 17�08 0E). Its surfacearea is 10.6 ha, maximum depth is 7.3 m and mean depth �3.4 m(Ja�nczak, 1996). It is a hypereutrophic, dimictic and bradymicticlake with short mixing periods. It was treated with iron sulphatesolution (commercial name PIX-112, containing 12% of Fe) in2006–2007. The lake was treated six times in 2006 and three timesin 2007, with doses of 60–70 kg of chemicals (380 kg and 180 kg intotal, respectively), but the lake is still highly eutrophic (Gołdynet al., 2008a,b,b).

Higher concentration of iron was stated in bottom sediments ofrestored Uzarzewskie Lake in the profundal zone at station 1(8.35 mgFe g�1DW) than in the littoral at station 2 (4.47 mgFeg�1DW). In turn, in Strzeszy�nskie Lake its content was lowerreaching 4.07 mgFe g�1DW in the profundal zone and 1.29 mgFeg�1DW in the littoral zone.

Phosphorus release and accumulation in bottom sedimentswas studied between March 2008 and February 2009. Four ex situexperiments (one in each season) were carried out with the aid ofintact bottom sediment cores from each lake. The cores forlaboratory experiments were sampled using a modified Kajak coresampler from 2 stations – from the deepest part of the lake(station 1) and from the littoral zone (at a depth of approximately2 m – station 2) (Fig. 1). Three replicate cores were collected intransparent and rigid plastic tubes (PMMA – polymethylmethacrylate), 6 cm in diameter. Each tube contained ca. 15 cmof sediment together with ca 25 cm of the above lying water. Thecores were incubated in the laboratory in darkness at constantthermal conditions similar to the temperature within the lakeduring sampling. Depending on the oxygen content of the waterabove the sediments within the lake, the cores were incubated in

I) and the littoral (II) zones (A – Uzarzewskie Lake, B – Strzeszy�nskie Lake).

K. Kowalczewska-Madura et al. / Ecological Engineering 74 (2014) 187–195 189

aerobic or anaerobic conditions. Aeration of the water overlyingthe sediments in the tubes have been used only in the case ofoxygen decreasing in order to maintain it at a level which wasstated at the beginning of the experiment.

During the experiments the concentration of TP in water wasmonitored, initially every day, and later every 3–4 days. Water foranalyses was sampled from every tube after slow and sensitivemixing and the same volume (50 ml) of water, taken for thispurpose into a separate container from the lake above thesediments, was added to the tubes. This supplementary waterwas sampled at both stations before sediment sampling. Concen-tration of phosphorus was analysed both in the water from thetubes and from supplementary water, which enabled the calcula-tion of phosphorus load in the overlying water within the tubesbefore and after sampling.

TP content in the water above the sediment core was calculatedusing the measured concentration of TP (mgP l�1) and volume ofwater (based on the length of water column and the tubediameter). From the difference of TP content in water in twosubsequent dates of analysis the load of TP released (increase ofcontent) or accumulated in the sediments (decrease of content)was estimated, and recalculated for one day (mgP d�1). Knowingthe area of the surface of the sediment in the tube the result wasconverted to obtain the value of TP released (or accumulated) perunit area of the bottom, thus giving the result in mgP m�2 d�1. Theweighted mean of the load for the whole experiment wascalculated in the final stage, taking into account the time betweensuccessive dates of TP analyses. The daily loads were summarizedand divided by the number of days of the experiment, therebyyielding a mean load of TP released or accumulated in the core.

Each experiment lasted 2 weeks. TP analyses were carried outspectrophotometrically with ascorbic acid as a reducer. Before TPanalysis samples were mineralized using the method withsulphuric acid and potassium peroxodisulphate (Elbanowskaet al., 1999). Water temperature, dissolved oxygen concentration,pH and conductivity were measured in the tubes with a WTWMulti 350i-meter before water sampling for TP.

Two zones differing in rates of TP release from bottomsediments were distinguished in the lakes. Zone I comprised thesediments in which oxygen was depleted during summer, andranged from a depth of 3.0 to 7.3 m in Uzarzewskie Lake and

Table 1Physical and chemical parameters of water of Strzeszy�nskie Lake and Uzarzewskie Lak

Variables Units Strzeszy�nskie

Transparency m Spring 3.0

Summer 2.8

Autumn 5.3

Winter 8.1

Chlorophyll-a (mean from a depth profile) mg l�1 Spring 0.27

Summer 2.77

Autumn 6.90

Winter 2.41

Oxygen – above the bottom mg l�1 O2 Spring 10.00

Summer 0.00

Autumn 0.92

Winter 0.34

Total phosphorus (mean from a depth profile) mg l�1 P Spring 0.024

Summer 0.042

Autumn 0.065

Winter 0.021

Conductivity – above the bottom mS cm�1 Spring 610

Summer 622

Autumn 608

Winter 423

10.0–17.8 m in Strzeszy�nskie Lake. Zone II denotes the littoral zone,ranging from the shoreline to a depth of 3 m in Uzarzewskie Lakeand of 0–10 m in Strzeszy�nskie Lake. The area of zone II inStrzeszy�nskie Lake covered 19.84 ha and in Uzarzewskie Lake�1.9 ha. Zone I was 15.06 ha and 8.7 ha respectively (Fig. 1). Onlythe main part of Strzeszy�nskie Lake was taken into account,designated by the bold shoreline in Fig. 1. The annual total ofinternal TP loading to the lake was calculated as the sum of theareal loading of both zones which was estimated by multiplyingthe daily mean TP release for each zone by its area and 365 days.

Additionally, the upper layer of the bottom sediments (10 cmthick) was sampled at every station for analysis of the content oforganic matter (according to My�sli�nska, 2001) as well as of thetotal phosphorus concentration of its individual extractable Pfractions based on an analytical pattern proposed by Psenner et al.(1988). The following were assayed in a volume of 1 cm3 of wetsediment:

� loosely bound phosphorus (NH4Cl-P) – extraction with 1 MNH4Cl for 2 h;

� phosphorus bound with iron (BD-P) – extraction with a mixture(1:1) of 0.11 M NaHCO3 and 0.11 M Na2S2O4 for 2 h;

� phosphorus bound with aluminium (NaOH-P) and organicmatter (NaOH-NRP) – extraction with 1.0 M NaOH for 18 h;

� phosphorus bound with calcium (HCl-P) – extraction with 0.5 MHCl for 18 h; and the residue (Res-P), being the differencebetween total P concentration and the sum of the first fourfractions. After each stage of extraction, the sample wascentrifuged and phosphorus concentration in the supernatantwas measured spectrophotometrically with ascorbic acid as areducer.

Pore waters were separated by centrifugation for 1 h at3000 rotations per minute in closed containers to preventoxidation of the samples. In the supernatant, SRP and TPconcentration were measured spectrophotometrically with ascor-bic acid as a reducer (TP after mineralization) (Elbanowska et al.,1999).

The water for chemical analyses was sampled from the deepestparts of the lakes. In Uzarzewskie Lake samples were collectedfrom the surface to a depth of 7 m at 1 m intervals and in

e at station 1 (mean from the depth profile and above the bottom water layer).

Lake Mean value(SD) Uzarzewskie Lake Mean value (SD) t-test t t-test p

4.8 (2.5) 1.3 1.0 (0.41) 3.27 0.0460.41.11.2

3.09(3.89) 13.63 31.02 (29.87) 3.93 0.00120.1157.2333.11

2.8(4.8) 3.40 2.2 (2.6) 0.24 0.8260.000.105.47

0.037(0.035) 0.096 0.165 (0.112) 5.21 0.0000.0520.2790.233

565.7(95.4) 1062 835.5 (232.2) 3.41 0.042972768540

Table 2Chemical composition of bottom sediments in Strzeszy�nskie Lake (S) and Uzarzewskie Lake (U).

Parameter Total phosphorus Organic mattermg P g�1DW %

Station Spring Summer Autumn Winter Spring Summer Autumn WinterSt. 1 S 1.08 1.41 1.22 1.54 14.2 17.6 19.0 22.0Mean value (SD)

1.31 (0.2) 18.2 (3.24)

St. 1 U 2.47 2.53 1.93 1.96 14.4 15.6 17.2 16.0Mean value (SD)

2.22 (0.32) 15.8 (1.15)t-test t = 4.23 p = 0.024 t = 1.85 p = 0.161

St. 2 S 0.39 0.46 0.22 0.37 4.3 2.2 4.9 4.9Mean value (SD)

0.36 (0.1) 4.07 (1.28)

St. 2 U 2.12 1.88 0.88 1.30 17.8 13.6 18.7 15.5Mean value (SD)

1.55 (0.56) 16.4 (2.3)t-test t = 4.93 p = 0.016 t = 15.7 p = 0.000

190 K. Kowalczewska-Madura et al. / Ecological Engineering 74 (2014) 187–195

Strzeszy�nskie Lake from the surface and from depths of 6, 10 and16 m).

TP concentration in water samples from the depth profile ofeach lake was measured after mineralization using ascorbic acid asa reducer (Elbanowska et al., 1999), while the chlorophyll-acontent was analysed using the acetone extraction method(Lorenzen, 1967). The range of transparency was measured witha Secchi disk, and a YSI 556MPS-meter was used to measuredissolved oxygen in the depth profiles of both lakes.

Statistical calculations were made with STATISTICA version8.0 software. To analyse the differences between the internalloading of phosphorus from the bottom sediments of both lakesand within two zones of the lakes, Canonical Varieties Analysis(CVA) was employed using Canoco for Windows 4.5 softwarepackage (ter Braak and Šmilauer, 2002). Phosphorus release wasused as a descriptive variable. Supplementary environmentalvariables did not influence the ordination. The significance of theparticular explanatory variables used in the model (concentrationof oxygen, temperature, conductivity and pH) was tested using aMonte Carlo permutation test set for 5000 permutations. Avariable encoding of particular sampling events was introduced tothe analysis as a grouping of covariate defining blocks of data in

Table 3The values of physical parameters (mean of tree replicates) in water overlying of sedim

Temperature (�C) (SD) Oxygen (mg l

St. 1 S Spring 4.6 (0.42) 5.6

Summer 5.1 0

Autumn 4.7 4.3

Winter 4.4 0

St. 1 U Spring 4.7 (0.45) 3.8

Summer 5.2 0

Autumn 5.0 3.5

Winter 4.6 8.7

t-test t = 2.91 p = 0.048 t = 1.56 p = 0.1

St. 2 S Spring 4.6 (7.24) 6.9

Summer 22 4.1

Autumn 4.7 6.3

Winter 4.4 6.8

St. 2 U Spring 4.7 (6.74) 5.5

Summer 21 5.4

Autumn 5.0 4.7

Winter 4.6 10.0

t-test t = 0.78 p = 0.438 t = 0.06p = 0.9

order to reduce the influence of spatio-temporal autocorrelation.Permutations were restricted to those blocks, assuming that theyform a time series.

3. Results

The physical and chemical variables of water quality indicatedthe mesotrophic state of Strzeszy�nskie Lake (Table 1). Watertransparency in winter reached 8.1 m and in summer 2.8 m(average 4.8). Chlorophyll-a concentration in the surface layerreached 12.3 mg l�1 in summer and did not exceed 7.0 mg l�1 inother seasons (average 3.09). In comparison, the average value ofwater transparency in hypereutrophic Uzarzewskie Lake was only1.0 and this was reduced to 0.4 m during the summer. Theconcentration of chlorophyll-a reached 118.0 mg l�1 (surface layer)and the average was 31.02 mg l�1 (Table 1). Thermal and oxygenstratification developed in both lakes in summer and oxygenconcentration above the bottom at station 1 was completelydepleted, whereas mean total phosphorus concentration reached0.037 mgP l�1 in Strzeszy�nskie Lake and 0.165 mgP l�1 in Uzar-zewskie Lake (Table 1). The mean value of total phosphorus nearthe bottom sediments was significantly higher in the

ent cores (the average of the experiment).

�1 O2) (SD) pH Conductivity (mS cm�1) (SD)

(2.84) 7.78–8.30 634 (65.95)7.01–7.50 7007.01–7.60 6807.20–7.50 735

(3.45) 7.74–7.96 948 (46.45)7.40–7.60 10417.39–8.26 8787.56–8.37 937

23 t = 28.41 p = 0.000

(1.54) 8.03–8.64 628 (43.99)7.19–7.50 6437.81–8.00 6207.80–8.10 644

(2.74) 8.04–8.15 922 (112.75)7.50–8.30 7797.70–8.26 6907.71–8.46 931

54 t = 8.94 p = 0.000

Table 4Changes of the percentage share of extractable fractions of TP in particular seasons in the bottom sediments of Strzeszy�nskie Lake (S) and Uzarzewskie Lake (U). Explanations:NH4Cl-P (phosphorus loosely adsorbed on the surface of sediment particles), BD-P (iron-bound fraction), NaOH-P and NaOH-NRP (phosphorus bound with aluminium andcontained in organic matter), HCl-P (phosphorus compounds with calcium) and Res-P (insoluble inorganic and organic compounds).

Spring Summer Autumn Winter Spring Summer Autumn WinterSt. 1 S St. 2 S

NH4Cl-P 5.29 4.24 6.86 7.08 3.02 3.03 3.08 1.88BD-P 6.59 6.57 8.73 8.70 3.02 4.55 1.76 2.15NaOH-P 6.31 0.28 0.92 1.48 2.27 3.03 0.00 0.35NaOH-NRP 22.17 18.45 24.24 25.19 33.50 8.87 15.86 10.48HCl-P 15.58 12.00 10.51 8.70 27.20 29.00 20.70 25.79Res-P 44.06 58.45 48.74 49.35 30.98 51.52 58.59 59.36

St. 1 U St. 2 UNH4Cl-P 0.84 0.24 0.83 1.78 0.90 1.22 0.49 1.92BD-P 33.17 4.94 10.19 16.69 24.07 12.71 0.56 7.90NaOH-P 7.14 13.08 0.98 4.16 4.35 1.80 9.20 1.92NaOH-NRP 7.79 16.64 17.22 20.66 18.68 26.85 12.13 26.17HCl-P 13.50 11.30 11.01 24.02 19.39 14.78 16.43 20.95Res-P 37.55 53.79 59.77 32.68 32.62 42.64 61.20 41.14

Autumn

Winter

Sprin

g

Summer

Autumn

Winter

Sprin

g

Summer

Autumn

Winter

S st.1U st.2U

ttom sediments in Strzeszy�nskie Lake (S) and Uzarzewskie Lake (U) at both stations.

K. Kowalczewska-Madura et al. / Ecological Engineering 74 (2014) 187–195 191

hypereutrophic lake than in the mesotrophic one (0.21 mgP l�1 and0.07 mgP l�1, respectively). Similarly, the mean value of conduc-tivity in the same zone was 835.5 mS cm�1 and 565.7 mS cm�1,respectively.

Total phosphorus content in bottom sediments was also higherin Uzarzewskie Lake (mean 2.22 mgP g�1DW in the profundal and1.55 mgP g�1DW in the littoral zone). It reached 2.53 mgP g�1DW atstation 1 in summer (Table 2). TP concentration in the sediments ofStrzeszy�nskie Lake was lower in both zones (1.31 mgP g�1DW and0.36 mgP g�1DW, respectively). The highest value was stated atstation 1 in winter at 1.54 mgP g�1DW. The content of organicmatter was higher at the deeper station in Strzeszy�nskie Lake(mean 18.2%), while it was higher at the littoral station inUzarzewskie Lake (mean 16.4%). This content in Uzarzewskie Lakewas very similar in both zones, while in Strzeszy�nskie Lake it was4-times lower in the littoral zone (mean 4.07%) compared to theprofundal zone (Table 2).

The dominant phosphorus fraction in both lakes was theresidual (Res-P) fraction (over 40%), i.e. phosphorus that ispractically biologically inaccessible. The total share of thefractions with the highest biological accessibility (loosely bound

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

Sprin

g

Summer

Autumn

Winter

Sprin

g

Summer

st.1S st.2

mgP

l-1

SRP

TP

Fig. 2. Seasonal changes of SRP and TP concentration in pore waters of bo

NH4Cl-P, redox sensitive BD-P and NaOH-P bound to metaloxides) was somewhat higher in Uzarzewskie Lake (23.5% -station 1 and 16.8% station 2) than in Strzeszy�nskie Lake (15.8%and 7.1%, respectively) (Table 4). A greater share of the NH4Cl-Pfraction was observed in Strzeszy�nskie Lake while the BD-P andNaOH-P fractions were higher in Uzarzewskie Lake at bothsampling stations.

Soluble reactive phosphorus (SRP) and TP concentrations inpore waters of bottom sediments were higher in Uzarzewskie Lakethan in Strzeszy�nskie Lake, both in the profundal and in the littoralsamples. This concentration was higher in the profundal station ofboth lakes. SRP concentration in the profundal zone in hyper-eutrophic Uzarzewskie Lake reached up to 5.55 mgP l�1 and TPconcentration up to 6.43 mgP l�1 in summer. In mesotrophicStrzeszy�nskie Lake lower values were recorded: up to 1.66 and2.26 mgP l�1 respectively (Fig. 2).

The average percentage of SRP in the interstitial waters of thestudied lakes was higher in the hypertrophic lake, amounting82.8% in the profundal zone and 72.2% in the littoral zone. Lowervalues but more varied in different zones of the lake were found inStrzeszy�nskie Lake (75.6% and 43%, respectively).

Table 5Seasonal changes of domination of phosphorus release or accumulation in bottom sediments of Strzeszy�nskie Lake (S) and Uzarzewskie Lake (U) at both stations (St.1, St.2)and total phosphorus loading from the both zones of the lakes.

Station Season P release or accumulation Mean P release or accumulation (SD) P loading Mean P loading from the zonesmg P m�2 d�1 mg Pm�2 d�1 kg P season�1 kg Pa�1

St.1 S Spring 2.69 2.41 (0.49) 37.3 132.7Summer 2.43 33.3Autumn 1.72 23.6Winter 2.81 38.5

St.2 S Spring 0.24 0.41 (0.61) 4.4 29.8Summer 1.28 23.1Autumn 0.26 4.7Winter -0.13 -2.4

St.1 U Spring 0.84 14.55 (20.99) 6.7 457.3Summer 12.59 99.6Autumn 44.7 353.6Winter -0.33 -2.6

St.2 U Spring 0.89 2.58 (3.23) 1.5 17.8Summer 3.28 5.6Autumn 6.78 11.6Winter -0.62 -1.1

1.5

Profu ndal

192 K. Kowalczewska-Madura et al. / Ecological Engineering 74 (2014) 187–195

The results of the experiments proved that in both lakes highervalues of TP release from bottom sediments occurred in theprofundal zone rather than in the littoral zone (Table 5). The valuesof the physical features of water above the cores of sediments arepresented in Table 3.

TP release in the profundal zone with station 1 in Strzeszy�nskieLake was quite similar in each of the seasons. The maximum wasobserved in winter – 2.81 mgP m�2 d�1. Somewhat lower valueswere observed in spring and summer and the lowest in autumn –

1.72 mgP m�2 d�1. The highest TP release in the littoral zone of thislake was observed in summer – 1.28 mgP m�2 d�1 and it was verysimilar in spring and autumn – 0.24 and 0.26 mgP m�2 d�1,respectively. However, in winter phosphorus accumulation inbottom sediments prevailed over the release and was equal to0.13 mgP m�2 d�1 (Table 5).

TP release was significantly higher in Uzarzewskie Lake than inStrzeszy�nskie Lake. The highest release was observed in theprofundal zone of the lake (station 1) in autumn – 44.7 mgP m�2

d�1. A lower value was stated in summer – 12.6 mgP m�2 d�1, andthe lowest in spring – 0.84 mgP m�2 d�1. A domination of theprocess of phosphorus accumulation over its release was observedin winter – 0.33 mgP m�2 d�1. TP release from bottom sedimentswas lower in the littoral (station 2) where it reached only6.8 mgP m�2 d�1 in autumn. In winter, phosphorus accumulationprevailed – 0.62 mgP m�2 d�1 (Table 5)

Table 6Annual phosphorus release from bottom sediments of Uzarzewskie Lake before(2005), during (2006; 2007) and after (2008) iron treatment.

Year Zone Mean annual P release SDkg Pa�1

2005 I 789.7 177.3II 162.4 172.1

2006 I 1040.8 367.1II 86.9 80.5

2007 I 519.9 318.1II 40.2 85.7

2008 I 458.4 666.0II 17.8 22.2

Comparing the mean values of TP release from the bottomsediments of both lakes in different zones it can be stated thathigher values were observed in Uzarzewskie Lake. Values fromstation 2 and station 1 were 6.3 and 6 times higher, respectively(Table 5).

The yearly phosphorus internal loading from bottom sedimentswas calculated on the basis of seasonal results. Higher values inboth lakes were observed for the profundal zone (zone I). Theywere over three times higher in Uzarzewskie Lake than inStrzeszy�nskie Lake (457.3 kgP a�1 and 132.7 kgP a�1, respectively),although its area was 3 times smaller. In the littoral zones of thelakes (zone II) a slightly higher value was stated in Strzeszy�nskieLake than in Uzarzewskie Lake (29.8 kgP a�1 and 17.8 kgP a�1,respectively) due to the larger surface area of this zone in theformer lake (Table 6).

Application of the data recalculated per unit area of the lakesand per day allows for more objective comparisons. The obtaineddata showed that the release was six times higher in Uzarzewskie

-1.5 2. 5

-1.5

Strzeszy nskie

Uzarzewskie

Littoral

TP

Temp

Con ductivity

pH

Oxy gen

Fig. 3. Canonical variates analysis diagram showing differences in phosphorusrelease from bottom sediments between two zones in Strzeszy�nskie Lake andUzarzewskie Lake (dashed lines – parameters that do not differ significantly;triangles – groups of lakes and zones in lakes under comparison; circles –

Uzarzewskie Lake; x-marks – Strzeszy�nskie Lake, TP – phosphorus release).

K. Kowalczewska-Madura et al. / Ecological Engineering 74 (2014) 187–195 193

Lake (8.56 mgP m�2 d�1) than in the case of Strzeszy�nskie Lake(1.41 mgP m�2 d�1).

Differences of phosphorus release from sediments weresignificant, both between the analysed stations (profundal andlittoral) and between the lakes. A higher degree of variability in TPrelease was observed in Uzarzewskie Lake, especially in autumn.Higher TP loading, particularly from the profundal zone, depen-dent on conductivity values, was also noted in Uzarzewskie Lake.Oxygen concentrations and pH values in the same lake werenegatively correlated with TP release. However, only phosphorusrelease (TP) significantly differed between the sampling stationsand the lakes studied (p < 0.001, F = 4.88) (Fig. 3).

4. Discussion

The internal loading of TP from bottom sediments in the studiedlakes allowed both the temporal and spatial variability of thisprocess to be analysed. It has been shown that this processdepends on the trophic state of the water body. The hypereutrophicUzarzewskie Lake was characterized by lower transparency, higherconcentration of chlorophyll-a, total phosphorus and conductivity.The observed phosphorus release was lower in both zones ofStrzeszy�nskie Lake. Uzarzewskie Lake was characterized by asizeable anaerobic zone during the summer and a high concentra-tion of nutrients and water blooms, known in this lake for manyyears (Gołdyn et al., 2008a,b). Mesotrophic Strzeszy�nskie Lake wasoxygenated in summer to a depth of 10 m and primary productionwas very low. Influenced by low nutrient loading, its trophiccondition generated different phytoplankton composition andlower biomass compared to Uzarzewskie Lake. It seems probablethat differences in the sedimentation rate of organic matter wasthe primary reason for such dissimilarity of results of TP releasefrom bottom sediments of these lakes. As stated by Katsev andDittrich (2013) in Lake Sempach, intensive phosphorus releasefollowed a higher sedimentation of organic matter. The betteroxygenation of Strzeszy�nskie Lake was also related to its trophicstate, especially in the cold seasons of the year. The lower oxygendemand of bottom sediments in this lake influenced longer lastingaerobic conditions in the hypolimnion during the stratificationperiod. Due to the short duration of the anaerobic zone in summerin the deepest part of the lake, phosphorus release from bottomsediments did not rise drastically. This was generally similarthroughout all seasons. Bottom sediments in the littoral zone werewell oxygenated during the whole year, so the release of thiselement was lower than in the deepest part of the lake, and inwinter, as a result of low temperatures, its accumulation insediments prevailed over the release. More intensive organicmatter mineralization due to higher temperatures in other seasonscaused TP release to prevail over its sorption and accumulation.Lavoire and Auclair (2012) suggest that an enhanced phosphorusrelease from the littoral zone sediments is due to an organic carbonimpoverishment, limiting microbial oxide reduction and allowingphosphorus to be rapidly recycled instead of being slowlyincorporated into humic material. There are a number of reportswhich confirm the domination of phosphorus release from bottomsediments in aerobic conditions (James and Barko, 1991; Andersenand Ring, 1999; Dondajewska, 2008; Kowalczewska-Madura et al.,2008b, 2009). Mean annual TP release from the whole lake bottomwas estimated as only 1.41 mgP m�2 d�1. In other weakly eutro-phicated lakes TP release was also quite low, for instance inPiburger Lake 0.57 mgP m�2 d�1 (Psenner, 1984), in Beaver Reser-voir 0.37 mgP m�2 d�1 (Sen et al., 2007) and in Lusowskie Lake2.6 mgP m�2 d�1 (Kowalczewska-Madura et al., 2009).

Small changes in TP concentration in bottom sediments inStrzeszy�nskie Lake were observed at both sampling stations. Alower concentration in autumn was the result of P release from

sediments in the summer. A similar phenomenon was observed inspring as a result of phosphorus release in winter.

In Uzarzewskie Lake TP release values significantly exceededthose observed in Strzeszy�nskie Lake. A wide anaerobic zonecomprising almost 68% of the bottom area contributed to a high TPrelease in summer. However, the highest phosphorus release wasobserved in autumn. As Golosov and Ignatieva (1999) state,intensive phosphorus exchange between sediments and wateroccurs in autumn, when the water temperature falls faster than thetemperature of sediments. Together with the thermal convectionof interstitial water, phosphorus transport from bottom sedimentsis very intensive. This process had already been observed in thislake in previous years (Kowalczewska-Madura et al., 2008a, 2010).The process was less intense in the littoral zone which can becontributed to favourable aerobic conditions stimulating Paccumulation in sediments enriched with iron compounds as aresult of the restoration measures in previous years. Goodoxygenation of water and low temperature in the whole lake inspring led to a predominance of phosphorus accumulationprocesses in bottom sediments over its release at both stations.Comparing the internal loading from bottom sediments of bothzones, 96% of phosphorus reaches the water column fromsediments in the profundal zone. In Strzeszy�nskie Lake, 82% ofthe released P originates from the profundal and 18% from thelittoral zone. The maximum value of phosphorus release inUzarzewskie Lake (44.7 mgP m�2 d�1) was higher than inSwarzedzkie Lake (26.9 mgP m�2 d�1), located downstream onthe same river (Kowalczewska-Madura et. al., 2009), and similar tothose observed in Rusałka Lake (48.6 mgP m�2 d�1) (Kowalczew-ska-Madura et al., 2011). Internal phosphorus loading in Uzar-zewskie Lake has already been studied in previous years(Kowalczewska-Madura et al., 2008a, 2010). As a result ofrestoration processes, a reduction of TP internal loading wasobserved at both sampling stations. A comparison of the resultsfrom 2008 with the data from previous years shows a furtherreduction (Table 6). Nevertheless, this decrease was much less thanexpected and still well above the internal loading found in themesotrophic Strzeszy�nskie Lake.

TP concentration in bottom sediments was lower in autumnand winter at both sampling stations as a result of P release insummer and autumn. In spring an increase in concentration wasobserved as a result of P accumulation in winter. High primaryproduction in the lake and phosphorus-rich organic mattersedimentation affect the increase of TP content in bottomsediments. Similar results were observed in Lake Go�scia _z (Kentzer,2001). Fluctuations in the content of P in the hypereutrophic lakewere higher than in the mesotrophic lake, which was probablyconnected with seasonal variability of organic matter productionand sedimentation (Katsev and Dittrich, 2013).

The intensity of phosphorus loading from bottom sediments isalso determined by the contribution of particular P fractions tothe TP content (Søndergaard et al., 1996; Noges and Kisand, 1999;Selig, 2003). The analysis of the numerical ratio of fractionsdiffering in biological accessibility is a significant source ofinformation on the stability of P accumulation in sediments andits potential paths of release to the water column (Kentzer, 2001;Kaiserli et al., 2002; Pardo et al., 2003). HCl-P and Res-P fractions,which represent biologically inaccessible phosphorus (Kentzer,2001), made a greater contribution to the amount of TP inStrzeszy�nskie Lake than in Uzarzewskie Lake. A higher share ofbiologically active fractions was noted in the hypereutrophicUzarzewskie Lake which suggests an increased ability to release Pfrom the sediments. Additionally, a greater contribution of theredox-sensitive BD-P fraction to the total phosphorus amountwas stated in the bottom sediments of this Lake as a result ofrestoration measures with the use of iron treatment.

194 K. Kowalczewska-Madura et al. / Ecological Engineering 74 (2014) 187–195

Phosphorus concentrations in pore waters and above sedimentsare commonly regarded as indices of the intensity of its transportacross the sediment-water interface (Kentzer, 2001; Komatsu et al.,2006). In hypereutrophic Uzarzewskie Lake, where high phospho-rus release from bottom sediments was observed, higher SRP andTP concentrations in pore waters were recorded, especially in theprofundal zone. This was similar to the conditions observed inSwarzedzkie Lake where SRP in pore waters reached up to 7.7mgP l�1, while TP was up to 8.1 mgP l�1 (Kowalczewska-Maduraand Gołdyn, 2012). In mesotrophic Strzeszy�nskie Lake TPconcentration in pore water was lower – up to 2.26 mgP l�1.Hence, the restoration of Uzarzewskie Lake in previous years wasnot sufficient, suggesting that the use of much higher doses of ironcompounds (Kleeberg et al., 2013) or Phoslock (Meis et al., 2012) isnecessary. It has been confirmed that too short or incompleterestoration measures do not bring long-term improvements inwater quality (Kagalou et al., 2008).

Statistical analyses indicated that P release from bottomsediments significantly varied between lakes of different trophicstatus as well as between bottom zones in the same lake. Moreover,the internal loading process is differentiated by seasons to a muchgreater degree in a hypereutrophic lake than in a mesotrophic one.

5. Conclusions

The comparative analysis of internal phosphorus loading frombottom sediments of two lakes with different trophic statesshowed that it is significantly lower in mesotrophic Strzeszy�nskieLake than in hypereutrophic Uzarzewskie Lake, despite itsrestoration. TP release from bottom sediments in the stronglyeutrophicated lake was six times higher than in the mesotrophicone. The comparison of the intensity of this process in two zones ofdifferent depth showed that phosphorus loading from a deeperarea (the profundal zone) was significantly higher than from ashallower one (the littoral zone). More diverse seasonal releasewas observed in the lake of higher trophy than in the mesotrophiclake. Comparison of phosphorus release from the bottom sedi-ments of Uzarzewskie Lake in 2008 with data from previous yearsshowed a reduction of TP internal loading as a result of earlierrestoration processes but it still did not reach the values stated inthe mesotrophic lake. The probable reason is still high biomass ofphytoplankton in Uzarzewskie Lake (10-fold higher than in themesotrophic lake), despite the restoration process. This results inan increased sedimentation of organic matter to the bottomsediments and a greater oxygen demand, causing periodic oxygendepletion in sediments and release of phosphorus to the watercolumn. This is evidence that the restoration was not intenseenough or too short. To achieve a greater reduction of phosphorusinternal loading restoration treatments should be intensified,which would lead to a reduction of the amount of phytoplankton inthe water column.

Acknowledgements

This study was partly supported by the Ministry of Science andHigher Education, Grant no. N 305 108 31/3632. Special thanks areextended to Bartłomiej Gołdyn, Ph.D., for his help in CVA analyses.

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