Hydrobiologia 394: 153–161, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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Assessment of the long-term effectiveness of sediment dredging to reducebenthic phosphorus release in shallow Lake Müggelsee (Germany)

Andreas Kleeberg1,∗ & Johannes-Günter Kohl2

1Brandenburg Technological University of Cottbus, Chair of Water Conservation, Seestraße 45,D-15526 Bad Saarow, GermanyE-mail rueklee@t-online.de (∗author for correspondence)2Humboldt-University Berlin, Institute of Biology, Ecological Group, Unter den Linden 6,D-10099 Berlin, GermanyE-mail jgkohl@biologie.hu-berlin.de

Received 11 June 1998; in revised form 19 January 1999; accepted 8 February 1999

Key words:shallow lake, sediment dredging, phosphorus release, phosphorus speciation

Abstract

Two series of laboratory experiments mimicking dredging of the uppermost phosphorus (P) rich sediment layers ofthe shallow eutrophic L. Müggelsee were carried out to study the extent of P release from deeper sediment layers,and changes in P mobility by means of fractional P composition of the in 10 cm steps ‘dredged’ sediment cores.In the first run over 38 days, the aerated controls reached 55%, and the ‘dredged’ cores from 18.1% (−10 cm)down-core to 0.4% (−50 cm) of the non-aerated control (50.82 mg P m−2 d−1 = 100%). In the second run over oneyear, the fractional P composition in the revealed sediment layers changed slightly. The water-soluble P (H2O-P)increased for the respective ‘dredged’ horizon by between 1.5 and 5.6% TP. The redox-sensitive P (BD-P) increasedin each horizon from the intact core to the situation following ‘dredging’, as well as with depth in each horizonfrom 4.9% TP (−10 cm) to 11.4% TP (−40 cm). The organic bound P portion (NaOH-NRP) decreased least (1.7%TP) in the uppermost layer and most (15.6% TP) in the deepest horizon exposed to water after ‘dredging’. Basedon the changes in P pools following dredging at the future sediment–water interface, it is predictable that dredgingwithout reduction of the external loading may give only temporary improvement followed by a slow return to thepresent situation.

Introduction

Lake sediments, e.g. due to benthic release of phos-phorus (P), have a profound effect on the quality ofoverlying water. This net internal P loading in eu-trophic L. Müggelsee (Behrendt et al., 1993; Kleeberg& Dudel, 1997) would delay the effect of reductionof the external loading (Kleeberg, 1995) and is, there-fore, of major concern to water quality management.Based on scenario analysis on eutrophication controlfor L. Müggelsee, Recknagel et al. (1995) found thatthe lake demands an efficient control of benthic P re-lease, e.g. by sediment dredging. To consolidate amesotrophic steady state in a long-term perspective,the single sediment removal has to be combined with

a permanent reduction of the external P load by atleast 50%. The dredging criteria according to van derDoes et al. (1992) are accomplished (Kleeberg, 1995;Kleeberg & Kozerski, 1997).

Much knowledge has been accumulated about sed-iment dredging and its environmental effects (e.g.Bengtsson et al., 1975; Sustar et al., 1977; Ryding,1982; Björk, 1985; Klapper, 1992; Keizer & Sinke,1992). It is a lake restoration/rehabilitation techniqueto remove surface bottom layers rich in P and to con-trol its release in order to achieve P-limiting conditionsto algal growth (e.g. Moss et al., 1986; van Liereet al., 1990; Hovenkamp-Obbema & Fieggen, 1992;van der Does et al., 1992). However, dredging is nopanacea and success is still variable (Bengtsson et al.,

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1975; Björk, 1985; van Liere et al., 1990; Klapper,1992). Because of high costs of dredging measures,reasonable decisions concerning the importance of Precycling from the revealed sediment layers must bemade prior dredging. There are only a few studies onP release from dredged sediment cores (Ryding, 1982;Peters & van Liere, 1985). However, information onthe effects of dredging to the future sediment–waterinterface and especially to the question whether an in-tensive P cycle would be re-initialized after dredgingare generally lacking, and are provided in this pa-per. The dredging depth must be optimized dependingon whether strata containing less readily exchange-able P forms are reached. Consequently, it remainsto be demonstrated whether the dredging measurehas longevity in curtailing P release after skimmingoff certain sediment layers and to study changes insedimentary P forms.

Sediment–water laboratory microcosms mimick-ing dredging in L. Müggelsee were used to:1. Evaluate the effects of dredging the uppermost

sediment layers on P release,2. Study the potential impacts of newly settled ma-

terial in re-initialization on P cycling at the futuresediment–water interface by a P speciation, and

3. Recommend the necessary dredging depth referedto the lowest P remobilization.

Material and methods

Site description

The shallow, polymictic and highly eutrophic L.Großer Müggelsee is one of the intercalated lakesof the River Spree. The average inflow of the riveramounts to 9.23 m3 s−1 (1979–1993). The amountand quality of lake water are mainly influenced bythe inflowing river water (Driescher et al., 1993). Itsmain characteristics are given in Table 1. Stratifica-tion periods in the lake occur stochastically lastingusually between one and two weeks (Dudel & Kohl,1992; Kleeberg & Dudel, 1997). Hence, the lake ingeneral is well supplied with oxygen (Behrendt et al.,1993; Kleeberg & Kozerski, 1997). Since early 1970s,L. Müggelsee changed from a macrophyte rich to aphytoplankton-dominatedstate (Dudel & Kohl, 1992).Despite efforts to restore the lake by reducing ex-ternal nutrient loading, large cyanobacterial biomassesin low diversity, i.e. Limnothrix redekei(Van Goor)Meffert,Planktothrix agardhii(GOM.) andAphanizo-

menon flos-aquae(L.) Ralfs. ex Bornet et Flah., re-appeared every summer (Kleeberg & Kozerski, 1997).The lake and its environment is described in detailby Driescher et al. (1993). The sediment quality issurveyed elsewhere (Kozerski & Kleeberg, 1998).

Sediment sampling and analysis

Intact sediment cores between 60 and 70 cm long weretaken with an Uwitec(C)-corer (flutter valve, acrylicglass tubes, i.d. = 6.0 cm) at 6 m water depth on Au-gust 6th 1993 and on August 26th 1994. The coreswere kept no longer than 2 h at 4◦C (cooling box) untilanalysis and incubation, respectively. From each corethe overlying lake water was siphoned off in two steps(20–10 cm and 10–0 cm) down-core. The sedimentwas sliced into 5 mm segments between 0–6 cm, andinto 2 cm intervals from 6–44 cm depth and the pore-water was pressed out. Interstitial ortho-phosphate(PO3−

4 ) concentrations was determined using ion chro-matography (DX-100 model, Dionex). After determ-ining dry weight (dw; 48 h, 60◦C) and loss on ignition(LOI; 3 h, 550◦C) the ash was digested (0.5 h, 105◦C, 1 M HCl) for the determination of total P (TP)and total iron (TFe) (Andersen; 1976; Nausch, 1981).After filtration (0.45µm), the P was measured assoluble reactive P (SRP; Freeman et al., 1990) andFe spectrophotometrically (Legler et al., 1986) usingflow injection analysis (FIA; EVA-Analyzer, Eppen-dorf), respectively. Dried aliquots of sediment werehomogenised and analysed for total carbon (TC) andtotal nitrogen (TN) in a C/N-Analyzer (CHNO-Rapid,Heraeus).

Laboratory microcosm experiments

For two consecutive series of experiments assessingsediment dredging in L. Müggelsee, two at a timeundisturbed cores (control) and ‘dredged’ cores wereincubated in a continuous flow system according toBoers et al. (1984) and Boers & van Hese (1988). Fordredging, from undisturbed cores the uppermost 10,20, 30, 40 and 50 cm sediment layer was skimmed by asuction pump, respectively, corresponding to the cores−10,−20,−30,−40 and−50 cm. The water drainedoff simultaneously was immediately replaced by lakewater. In both experiments the cores were incubatedin the dark atin-situ water temperature± 3 ◦C. Thethrough flow rate (10 ml d−1) was controlled by aperistaltic pump and was set to lake water residencetimes in summer between 96 and 138 days (Kleeberg& Dudel, 1997) throughout the experiments.

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Table 1. Morphometrical and limnological characteristics of L. Großer Müggelsee (lat. 58◦ 13′, long. 34◦ 08′), Germany

Morph. parameter (unit) Limnol. parameter (unit) Mean (range)

Surface area (km2) 7.18 Total phosphorus (µg l−1) 80 (35–260)

Volume (106 m3) 36.0 Total nitrogen (mg l−1) 3 (1.4–5.9)

Mean depth (m) 4.85 Secchi depth (m) 1.4 (0.4–3.7)a

Maximum depth (m) 7.5 Seston (mg d.w. l−1) 11.0 (2.3–19)

Mean residence time (d) 48 Chlorophylla (µg l−1) 46.2 (1.1–108.8)b

Drainage area (km2) 6344 pH 8.4 (7.2–9.6)

a(1979–1990; Driescher et al., 1993).b(1981–1990, April–September; Dudel & Kohl, 1992).

In the first run from August 6th 1993 to September13th 1993 (38 days) the water overlying the undis-turbed and ‘dredged’ cores were replaced by lakewater coming from a non-aerated reservoir. In two at atime of the ‘dredged’ cores the overlying water wasreplaced by aqua destillata. The aerobic cores weredirectly bubbled with air giving a well oxygenated wa-ter overlying the sediment. The anaerobic ‘dredged’cores, as well as those treated by aqua destillata werenot aerated, which resulted in hypoxic conditions. Theaerobic and anaerobic P release rates were determinedusing the P mass balance of Boers & van Hese (1988).

In the second run from August 6th 1993 to Au-gust 26th 1994 (385 days), the water in the reservoirwas aerated and replaced beweekly by lake water inorder to be closest to the in-situ conditions. Freeze-dried phytoplankton detritus (10 mg seston dw l−1)containing about 80% Aphanizomenon flos-aquae(L.)Ralfs ex Bornet et Flah. and 20%Microcystis aeru-ginosa(Kütz.) Kütz. (7.4 mg P g dw−1), respectively,was added every two months in order to avoid thatthe newly exposed sediment–water interface becomespoor in organic matter. The P input (19.31 mg P m−2

d−1) due to the detritus additions was considered inthe P balance.

Finally, 1–1.5 g fresh sediment (fs) of each 2 cmlayer 10 cm down-core were successively extractedin duplicate following the sequential P fractionationgiven in Table 2, and pore-water as well as TC and TNwere analysed as described above. Differences in thepercentage of the P fractions on TP before and after‘dredging’ were tested by at-test.

Results

The original sediment cores ‘dredged’, and layers usedin the laboratory incubations, respectively, are char-acterized in Figures 1 and 2. The portion of dw and

organic matter (LOI) increases with depth. The ver-tical profile of the atomic Fe:P ratio increases with twodistinct peaks down-core due to minima in the verticalTFe (Figure 2) and TP distribution (Figure 4).

The O2 saturation during the course of experi-ments were always around 90% in the aerated coresand around 40% saturation in the non-aerated ones.The NO−3 concentration avaeraged 2.6± 1.3 NO−3 mgl−1. Figure 3 shows the mean P release rates from in-tact and ‘dredged’ sediment cores. Provided that, theunaerated controls showed the maximum of P releasepossible (= 100%) the aerated control reached 55%,and the ‘dredged’ cores from 18.1% (−10 cm) to 0.4%(−50 cm). The P release rates measured in the coreswith aqua destillata were slightly higher, ranging from25.2% of the uppermost layer ‘dredged’ down to 0.1%in the deepest sediment horizon exposed to the watercolumn after dredging (Figure 3).

The P fractions before ‘dredging’ are compared tothose of the sediment after ‘dredging’ and one yearincubation in Figure 4. The residual P fraction (Pr,not considered in Figure 4) of all sediment horizonsamounted on average 8.8± 7.2% TP (n = 60). Itspercentage for the sediment layers ‘dredged’ (n = 10)increased with depth from to 7.3± 5.0% TP in the up-permost layer to 14.0± 0.2% TP in the deepest hori-zon (−50 cm), with one execption being 19.4± 2.5%TP (−20 cm horizon). Hence, the recovery of P inthe H2O-, BD-, NaOH- and HCl extractants varies ontotal between 80.6 and 91.2% TP. The water-solubleP (H2O-P) – representing mainly the pore-water P orloosely adsorbed to surfaces (Psenner et al., 1984)– increased significantly on average for the respect-ive ‘dredged’ horizon by between 1.5 and 5.6% TPwithout any major trend over depth (Figure 4). Nochanges were found in the uppermost layers of the in-tact sediment core in the fraction of the redox-sensitiveP (BD-P, Table 2). Whereas, its portion increased in

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Table 2. Sequential phosphorus fractionation scheme slightly modified according to Psenner et al. (1984). Total phosphorus (TP) of eachfraction was determined according to Andersen (1976). The non-reactive P (NRP) was derived from the difference between TP and SRP ofeach fraction

Term Extractant Time Expected form

H2O-P Deionized water 0.5 h SRP/ Pore-water P, loosely adsorbed to

NRP surfaces (e.g. of Fe and CaCO3)

immediately available P

BD-P 0.11 mol l−1 1 h SRP Redox-sensitive P, mainly bound to

bicarbonate/dithionite Fe-hydroxides, Mn-compounds

NRP Organic P

NaOH-P 1 mol l−1 NaOH 16 h SRP P exchangeable against OH−, bound

to metal oxides, mainly Al and Fe;

inorganic P compounds, which are

alkaline soluble

NRP P in microbes including poly-P,

organic P compounds in detritus,

P bound in humic compounds

HCl-P 0.5 mol l−1 HCl 16 h SRP P bound to carbonates and apatite-P,

traces of hydrolysed organic P

NRP Organic P compounds

Pr 0.5 mol l−1 HCl, 105◦C 0.5 h TP Organic and other refractory P

the respective horizon from the intact core to the situ-ation following ‘dredging’ as well as with depth ineach horizon from 4.9% TP (−10 cm) to 11.4% TP(−40 cm). The increase in the−50 cm horizon wasstatistically not significant. Major changes becamealso visible in the NaOH-NRP fraction which containsorganic bound P. The decrease in this fraction was low-est in the uppermost layer ‘dredged’ with 1.7% TP andhighest in the deepest horizon ‘dredged’ with 15.6%TP (Figure 4). In the control cores (‘undredged’) itsportion amounted to 3.0% TP. The changes within thepool of HCl-P (P bound to carbonates or as apatite-P)varied from 0 to 9.5% TP, however, were within thelarger variance in this fraction.

The meanin-situ interstitial PO3−4 concentration

(August 26th 1994) in the uppermost 30 cm sedimentis with 2.9 mg PO3−4 l−1 about two times higher thanthat of the control core after one year incubation (1.3mg PO3−

4 l−1) (Figure 5). The mean PO3−4 concen-trations for the ‘dredged’ horizons were also lowerranging from 0.8 (−10 cm) over 0.3 (−20 cm), 0.2(−30 cm) and 0.08 (−40 cm) to 0.08 mg PO3−4 l−1.This last is a 36 times lower concentration than theactualin-situPO3−

4 concentration (Figure 5).

Discussion

Dredging is undisputed an useful technology contrib-uting to an exoneration in the benthic P ‘surplus’ inL. Müggelsee too (Kleeberg & Kozerski, 1997). How-ever, the question whether an intensive P cycle wouldbe re-initialized after dredging was not answered yet.The high content of organic material in the sediments(Figure 1), and its re-initialized decomposition afterexposition to the overlying water might contribute to astimulation of P mobilization. Whereas, the increasingFe:P ratio with depth (Figure 2), may favour the Pimmobilization under oxic conditions (e.g. Jensen etal., 1992). Hence, the pilot experiments carried outshould evaluate the future extent of P mobilizationafter exposing deeper sediments. However, it is dif-ficult to mimic environmental conditions in laboratoryexperiments closely. A limitation of the experimentswas the lack of a continuous input of labile organicmatter. In the lake, the rain of seston provides easilydecomposable organic matter that not only providesthe substrate for such processes as iron reduction, butalso causes O2 depletion within millimetres below thesediment surface (e.g. Carlton et al., 1989; Urban &Brezonik, 1993). Possibly the microbial activity wasreduced and, as a result, the O2 penetration into thesediment was more extensive than would occur in the

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Figure 1. Vertical profile of dry weight (dw; fs = fresh sediment) and of loss on ignition (LOI) of the sediment cores of L. Müggelsee fromAugust 6th 1993 used for the microcosm experiments. The data given within the figure show the exposed sediment surface after ‘dredging’ theoverlying sediment layer of the cores used in the experiments. The different symbols (see also Figure 2) illustrate that different cores were used.

Figure 2. Vertical profile of total iron (TFe) and of the atomic Fe: P ratio of the sediment cores of L. Müggelsee from August 6th 1993 used forthe microcosm experiments. The data given within the figure show the exposed sediment surface after ‘dredging’ the overlying sediment layerof the cores used in the experiments. The different symbols (see also Figure 1) illustrate that different cores were used.

Figure 3. Mean phosphorus release rates (39 days) of undisturbed core not aerated (A) and aerated (B) and of ‘dredged’ cores overlying bylake water (C, D) and by destilled water (E) in microcosm experiments.

lake affecting both the rate of ferric iron reduction andthe ferrous iron oxidation and the pathways for P dia-

genesis. Several attempts were undertaken to take thislimitation into account. First, the P release rates are

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Figure 4. Vertical profile of total P (TP) and of percentages of different P fractions according to Psenner et al. (1984) – water soluble P(H2O-TP), redox-sensitive P (BD-TP), Fe- and Al-bound P (NaOH-SRP), organic bound P (NaOH-NRP), and Ca-bound P (HCl-TP) – ofduplicate undisturbed sediment cores of L. Müggelsee from August 6th 1993 as used for the microcosm experiments (left) and after ‘dredging’(see Figure 1) and one year incubation from August 6th 1994 (right), respectively.

only discussed with data from the first set of experi-ments, where the net P flux across the interface wasobserved over a period of only 38 days. Second, thewater temperature was close to the in-situ temperat-ure. Third, the amount of labile organic matter addedto the cores in the form of algae detritus (2.37 g Cm−2) was equivalent to that which would have beendeposited over 2–5 days in the lake (Kozerski & Klee-berg, 1998). Moreover, the water movement allowedan additional, but small restricted supply of sestonicparticles due to biweekly water replacements. Never-theless, this limitation should be borne in mind duringthe following interpretation of the results.

The distinct lowering in P release (Figure 3) ofthe ‘dredged’ cores is in the same order of magnitudeas determined by laboratory investigations for L. Tre-hörningen (Ryding, 1982), where the mean P releaserate under anaerobic conditions (15 mg P m−2 d−1)was 46.7% higher than those of the ‘dredged’ coresand was 50% higher under aerobic conditions in the

undisturbed cores compared to the ‘dredged’ cores (6mg P m−2 d−1). Peters & van Liere (1985) simu-lated dredging by suctioning the uppermost 10 – 23cm sediment reaching P release rates averaged over180 days of 2.22 mg P m−2 d−1 for Spaghnumpeatof Ankeveense Plassen and 60 mg P m−2 d−1 for thegyttja of eutrophic Uitgeestermeer before ‘dredging’,and 0.22 mg P m−2 d−1 and 5.56 mg P m−2 d−1 after‘dredging’, corresponding to a decrease of 90%.

The reduction in P release for the ‘dredged’ L.Müggelsee cores by 82% (−10 cm) up to 99.7% (−50cm) in comparison to the intact cores under anaerobicconditions and by 67% (−10 cm) up to 99.5% (−50cm) in comparison to the intact cores under aerobicconditions is mainly due to a progressed diagenesis,and much less the result of a decreasing content ofTP with depth (Figure 4). Beside an increasing com-paction and dehydratisation of sediment with depth(Figure 1), there was obviously no considerable break-down of the organic matter exposed once again. It

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Figure 5. Mean vertical profile of interstitial water phosphate concentration of duplicate sediment cores of L. Müggelsee (6.0 m depth). Thein-situ cores represent the actual in-lake situation from August 26th 1994. The control and the ‘dredged’ cores are from August 6th 1993 afterone year incubation.

needs obviously a longer time of a continuous mat-ter supply to the sediment until a new equilibrium atthe sediment water interface is reached as shown bythe changes in the P pools (Figure 4), and the ver-tical pore-water PO3−4 distribution (Figure 5). Becausenot all sediment P is exchangeable or biological avail-able and some forms are tightly bound or actuallyincorporated as part of the structural matrix (Psen-ner et al., 1984), the sediment TP (bulk analysis)is of limited use in determining the potential for Prelease (Peterson, 1981). More operationally, but use-ful are the P fraction in relation to relevant sedimentparameters as TFe and LOI (Sondergaard et al., 1996).

The increase of the H2O-P and of the BD-P portion(in deeper horizons) of TP in the ‘dredged’ cores afterone year incubation (Figure 4) emphasizes the post-depositional mobility of P as shown by Carignan &Flett (1981). The H2O-P fraction is very dynamic andshould be used as an indicator in provisions of long-term dredging control despite the fact that it usuallyamounts only a few percent on TP (Psenner et al.,1984; Jensen et al., 1992).

The BD-P extracted – representing mainly iron-bound P – is important in relation to O2 with regardto P retention in general (e.g. Jensen et al., 1992;Sondergaard et al., 1996), and particularly for theiron-rich sediment (Figure 2) of L. Müggelsee with re-

spect to NO−3 which also controls the actual P release(Kleeberg & Dudel, 1997). The BD-P fraction did notchange significantly within the uppermost sedimentlayer, i.e. in the undredged control cores (Figure 4).This redox-sensitive P is usually released at suboxic,reduced conditions in deeper sediment layers. Sincethe O2 and NO−3 concentration during the course ofexperiments were always comparably low, the abilityof the BD pool in the exposed ‘dredged’ sediments,i.e. from−10 cm to−40 cm to incorporate P after-wards is difficult to interprete. Such an increase in a Ppool is also in contrast to the P model by Hosomi &Sudo (1992) which considers P transformations, min-eralization, sediment accumulation and compaction. Itshows, even if the sediment–water interface would bechanged significantly as a result of dredging, bacterialactivity and adsorption–desorption capacity in the toplayer in the sediment are not different from those of thecontrol simulation. However, our results are in con-cordance with the findings of Sondergaardet al. (1996)who found that part of the P in deeper sediment layersis still present as a particulate iron P complex, eitherprotected from reduction in the absence of O2 andNO−3 by coating of FePO4, or in reduced form as vivi-anite. Moreover, the relative portion of iron-bound Pwas more or less independent of depth, averaging 20%of TP at depth below 50 cm, where in L. Müggelsee

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sediment cores the BD-P did also not change after‘dredging’. Sondergaard et al. (1996) argued, assum-ing that their NaOH-P represents iron bound P, thatits occurrence in deeper parts of the sediments (> 50cm, where interactions with lake water should not bepossible) is related to a permanently immobilized Pportion in iron-bound form and consequently contrib-utes to P retention. Due to the phytoplankton detritusadditions mineralization processes were slightly stim-ulated in relation to the transformation of organicbound P (Figure 4) and related to an increasing or-ganic matter content with depth (Figure 1). This issupported by an increasing C:N ratio from 7.8 to 9.6with depth in the uppermost 2 cm layer of the corebefore ‘dredging’ in comparison to 9.7 to 10.5 in thecorresponding layers after ‘dredging’ and incubation.The NaOH-NRP portion on TP, representing the or-ganic bound P, decreased significantly and part of thisP are obviously bound by reoxidized iron compoundsas BD-P (Figure 4).

Despite the restricted transferability of the labor-atory experiments ontoin-situ conditions, the exper-imental approach has shown that dredging the up-permost sediment layer of 30–50 cm would be suf-ficient to effectively reduce the internal loading ofL. Müggelsee. However, since the net P release isa function of the P supply via sedimentation, afterat least one or two years of non-restricted biologicalproduction and sedimentation a new equilibrium withpossibly a little higher BD-P portion (Figure 4) at ahigher Fe:P ratio in the depth (Figure 2) is reached.This is supported by the dephosphorization process ofthe uppermost sediment layers following loading re-duction (Keizer & Sinke, 1992; Kleeberg & Kozerski,1997). Sondergaard et al. (1996) found that surfacesediment TP was highly and positively correlated tothe external P load, but not or only weakly related toother sediment parameters. In addition, in the modelby Hosomi & Sudo (1992) the 50% reduction of Pdeposition was the most effective measure for thecontrol of benthic P release among the four cases artifi-cial aeration, aluminium sulfate application, sedimentdredging and bottom sealing.

In conclusion, the P dynamic of the upper fewsediment centimeters is only slightly determined byP transformations of exchangeable P forms after re-vealing deeper layers. Hence, an intensive P cycle andrelease is mostly regulated by the particulate P trans-portation to the sediment. Consequently, the reductionof the P deposition rate by means of reducing the ex-ternal P loading in the catchment area of River Spree

at a short water residence time of L. Müggelsee wouldbe more effective than a dredging measure.

Acknowledgements

The logistic support of the staff of the Institute of Bio-logy (Ecological Group), Humboldt-University Berlinis gratefully acknowledged. The collegues R. Henze,H. Rolletschek and M. Dewender, R. Kräft, K. Awe,H. Stenzel are sincerely acknowledged for field andlaboratory assistance, respectively. These investiga-tions were funded in part by the Senate for MunicipalDevelopment and Environmental Protection of Berlin(SenStadtUm).

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