No abstract

1. The responses of nutrient concentrations, plankton, macrophytes and macrozoobenthos to a reduction in external nutrient loading and to contemporary climatic change were studied in the shallow, moderately flushed Lake Mü ggelsee (Berlin, Germany). Weekly to biweekly data from 1979 to 2003 were compared with less frequently collected historical data. 2. A reduction of more than 50% in both total phosphorus (TP) and total nitrogen (TN) loading from the hypertrophic to the eutrophic period (1997)(1998)(1999)(2000)(2001)(2002)(2003) was followed by an immediate decline in TN concentrations in the lake. TP concentrations only declined during winter and spring. During summer, phosphorus (P) release from the sediments was favoured by a drastic reduction in nitrate import. Therefore, Mü ggelsee acted as a net P source for 6 years after the external load reduction despite a mean water retention time of only 0.1-0.16 years. 3. Because of the likely limitation by P in spring and nitrogen (N) in summer, phytoplankton biovolume declined immediately after nutrient loading was reduced. The formerly dominant cyanobacteria (Oscillatoriales) Limnothrix redekei and Planktothrix agardhii disappeared, but the mean biovolume of the N 2 -fixing species Aphanizomenon flosaquae remained constant. 4. The abundance of Daphnia spp. in summer decreased by half, while that of cyclopoid copepod species increased. Abundances of benthic macroinvertebrates (mainly chironomids) decreased by about 80%. A resource control of both phytoplankton and zooplankton is indicated by significant positive correlations between nutrient concentrations and phytoplankton biovolume and between phytoplankton and zooplankton biomass. 5. Water transparency in spring increased after nutrient reduction and resulted in recolonisation of the lake by Potamogeton pectinatus. However, this process was severely hampered by periphyton shading and grazing by waterfowl and fish. 6. Water temperatures in Mü ggelsee have increased in winter, early spring and summer since 1979. The earlier development of the phytoplankton spring bloom was associated with shorter periods with ice cover, while direct temperature effects were responsible for the earlier development of the daphnid maximum in spring.

Regime shifts between clear and turbid water states are commonly found in shallow lakes. These shifts are attributed to a positive feedback between water clarity and submerged macrophytes (underwater plants). Altering the retention time of the water may influence these interactions and thus potentially reduce the probability of alternative stable states. Here we assessed the effect of water retention time on the occurrence of alternative states in water quality of flushed lakes, chains of lakes and rivers using a spatially explicit simple model. Our results indicate that increased flushing of lakes rapidly decreases the range of parameters with alternative stable states up to their total disappearance at a flushing rate of about 50% the algal growth rate. Similarly, in a chain of lakes or in rivers with low flowing velocity, our model predicts that alternative stable states can only occur for systems with a high retention time. Despite the lack of hysteresis at lower water retention times, we predict that abrupt changes between clear and turbid states are still possible both in time and in space. Over a wide range of parameters, the equilibrium state of the chain of lakes shows a steep gradient of vegetation cover. Further, the transient dynamics of the model often include rapid shifts in time. For example, a local regime shift that occurs upstream may propagate through the whole lake chain or river due to a domino effect. All results of the simple model could qualitatively be reproduced with a more mechanistic model. The abrupt rather than gradual response of submerged macrophytes to reduced turbidity levels still makes river systems rather resilient to management measure. The importance of the initial turbidity and the observed domino effect suggest that restoration measures should start upstream and that these measures should eventually trigger regime shifts downstream.

The sediments and exchange processes of the shallow (mean depth 4.9 m), polymictic, eutrophic Miiggelsee have been studied over more than 30 years. Here, previous results are summarised and completed by new data. Sediment analyses were used to identify changes in loadings and migration processes within the sediments of the 12000 year old lake. The surficial oligohumic sediments are rich in Fe (8.8 f 1.9% DW) and in P (2.7 ? 0.5 mg P g-' DW). Mass balances of P and Fe as well as sediment trap measurements and sinking velocity analyses showed that the retention of matter strongly varies within seasons and years. The lake amplifies the seasonal fluctuations of P in the River Spree system by a strong redox-controlled, benthic P-release in summer. Since 1988, P-release has exceeded retention so that the lake changed from a P-sink to a source. The retention rate dropped in summer to -7.3 mg P m-* d-' and in winter to 1.4 mg P m-2 d-I. Water Sampling and AnalysisThe water of the lake and of River Spree is regularly studied (weekly from March to October and biweekly in winter from 1979 to 1994) at the Spree inflow station (Muggelhort), at the deepest site in the lake (M7) and at the Spree outflow station (Spreetunnel) presented in Fig. I . Since 1987, samples from 21 single points (5 sites (M 3, 5 , 7, 8, and 10) at different depths) have been pooled to decrease the variance caused by spatial inhomogeneities (DRIESCHER et ul. 1993).After filtration, phosphorus (P) was measured as soluble reactive P (SRP) according to MURPHY and RILEY (1962). For the determination of total P (TP) and total dissolved P (TDP) unfiltered and filtered samples were digested (H,SO,, 2 h at 170 "C) as described in GELBRECHT et al. (1991) prior to measuring SRP (see list of abbreviations). Particulate P (PP) is calculated as the difference between T P and TDP (KOZERSKI et al., 1993). Total iron (TFe) was analyzed according to LEGLER et af. (1986). Sediment Sampling and AnalysisTo study the horizontal and vertical distribution of different sediment parameters, 23 sampling stations were investigated. Intact sediment cores up to a length of 0.5 m were taken with a UWITEC-corer (flutter valve, acrylic glass tubes i.d.

No abstract