Iron (Fe) plays a key role in lake ecosystems by regulating the availability of phosphorus (P) as a limiting factor for primary production. This study investigates how microbial sulphate reduction in Lake Stechlin, a dimictic and initially oligotrophic lake in northeast Germany (zₘₐₓ = 69.5 m), has altered the mobility of Fe and P over several decades. Using long-term monitoring data combined with sedimentological investigations, the study examines long-term geochemical focusing, defined as the net accumulation of redox-mobilised Fe in the deepest basin driven by reductive dissolution, lateral transport, and re-precipitation. Under oligotrophic conditions prior to the 1960s, focusing of Fe toward the deepest site likely involved: (1) reductive dissolution of sedimentary Fe oxides in shallow areas, (2) diffusion of dissolved Fe into overlying water, (3) re-oxidation/complexation and stepwise lateral transport within the water column, and (4) final deposition and burial at the deepest zone. Chemical and mineralogical evidence indicates that Fe burial was facilitated by authigenic formation of stable Fe(II) minerals, most likely ankerite (CaFe[CO3]2) and vivianite (Fe3[PO4]2·8H2O). Fe enrichment at the deepest site has strongly weakened, coinciding with intensified pyrite (FeS2) formation in anoxic littoral and profundal sediments. This shift was likely driven by sufficient supply of organic matter and elevated sulphate inputs, which stimulated microbial sulphate reduction. Elevated S/Fe ratios indicate that sulphide efficiently competed with P for binding to Fe, potentially increasing susceptibility to external and internal P loading.

Symptoms of eutrophication are increasingly evident in remote clearwater lakes. To identify the sources and dynamics of phosphorus release, we measured total phosphorus (TP) in sediments and soluble reactive phosphorus (SRP) fluxes across the sediment–water interface at 54 locations in a deep temperate lake. Once renowned for its clear waters, Lake Stechlin has experienced a fourfold increase in water column TP over the past decade. SRP fluxes from sediments generally increased with water depth across all three lake basins, although there were significant variations in SRP concentrations, up to threefold, among sampling locations at the same depth. Notably, the lake´s total mean SRP flux in June (1.12 mg m−2 day−1) was higher than that determined in October (0.74 mg m−2 day−1). This result can be attributed to the substantial contribution (about 31

We present 50 years of monitoring data on water quality of Lake Stechlin, a deep, dimictic hardwater lake in northeastern Germany known for its exceptionally clear water. Although located in a forested catchment, the lake has undergone major changes in recent decades, including a period of massive heating of surface water when receiving cooling water from a nearby nuclear power plant (1966–1990), accompanied by a greatly shortened water residence time from more than 40 years to less than 300 days. These changes are superimposed by a long-term trend of surface water warming and a concomitant decrease in winter ice cover. Total phosphorus concentrations have quadrupled since 2010 and zones of deep-water oxygen depletion have greatly expanded. The presented dataset covers basic water-chemical and physical records taken at monthly to fortnightly intervals from 1970 to 2020, documenting limnological changes during that period. Furthermore, it serves as a valuable basis to assess and project potential consequences of climate change and other types of environmental change on deep clearwater lakes in temperate climates.

Charophytes (stoneworts) often dominate the submerged vegetation in nutrient-poor hardwater lakes, where they support high benthic phosphorus (P) retention. As a consequence, epilimnion P concentrations remain low and water clarity high, resulting in a positive feedback. When perennial charophytes, retaining P all year round, are replaced by vascular macrophytes, P is retained only during summer but rapidly released during decomposition in the autumn and winter. Epilimnion P thus becomes available to phytoplankton in the following growing season, reducing lake water clarity and further shifting plant dominance from bottom-dwelling charophytes to taller vascular species. We tested the hypothesis that changes in lake P concentrations may be partly related to changes in the structure of submerged macrophytes in a deep hardwater lake that experienced a dramatic increase in total P (TP) concentrations over a decade with no evidence of changes in external P loading. We repeatedly measured water transparency, macrophyte maximum colonisation depth (MCD) and abundance between 2008 and 2022 and determined macrophyte tissue P content in 2020 to test whether changes in submerged vegetation could explain gaps in the lake's P budget and shifts in P sink/source functions of littoral areas. In 2008, charophyte communities were dominated by wintergreen Nitellopsis obtusa. The subsequent decline of the species was negatively correlated with the spring TP concentration in the upper water layer (0-20 m) of the lake and positively correlated with Secchi depth and macrophyte MCD, which decreased from 13.5 to 9.5 m. In contrast, the cover of annual vascular macrophytes (mainly rootless hornwort, Ceratophyllum demersum) increased and was positively correlated with P losses from the upper water layer in summer and negatively correlated with Secchi depth and MCD. Budget calculations showed that the littoral zone was a P source during the period when N. obtusa declined. Despite the relatively low contribution of P directly released from lost charophyte biomass, declines of N. obtusa partly explained the temporary P source function of the littoral zone and the increasing P concentrations in the upper water layer at the beginning of May. This was most probably due to the positive effects of charophytes on particle retention and the negative effects on sediment resuspension. Conversely, the growth of C. demersum can explain P losses from the upper water layer, turning the littoral zone into a temporal P sink during summer. However, most of this P is likely to return to the upper water layer during the decomposition of plant biomass after the growing season. In conclusion, our data indicate that replacement of charophytes by submerged vascular plants can facilitate rapid lake eutrophication due to changes in the feedback loop between submerged vegetation, benthic P retention, epilimnion P availability, water transparency, and MCD.

Many clearwater lakes increasingly show symptoms of eutrophication, but the underlying causes are largely unknown. We combined long-term water chemistry data, multi-year sediment trap measurements, sediment analyses and simple mass balance models to elucidate potential causes of eutrophication of a deep temperate clearwater lake, where total phosphorus (TP) concentrations quadrupled within a decade, accompanied by expanding hypolimnetic anoxia. Discrepancies between modeled and empirically determined P inputs suggest that the observed sharp rise in TP was driven by internal processes. The magnitude of seasonal variation in TP greatly increased at the same time, both in surface and deep water, partly decoupled from deep water oxygen conditions. A positive correlation between annual P loss from the upper water column and hypolimnetic P accumulation could hint at a short-circuited P cycle involving lateral TP transport from shallow-water zones and deposition and release from sediments in deep water. This hypothesis is also supported by P budgets for the upper 20 m during stable summer stratification, suggesting that sediments in shallow lake areas acted as a P net source until 2018. These changes are potentially related to shifts in submerged macrophytes from wintergreen charophyte meadows ( Nitellopsis obtusa ) to annual free-floating hornwort ( Ceratophyllum demersum ) and to increased sulfide formation, promoting iron fixation in the sediments. Iron bound to sulfur is unavailable for binding P, resulting in a positive feedback between P release in shallow lake areas, primary productivity, macrophyte community structure and redox-dependent sediment biogeochemistry. Overall, our results suggest that relationships more complex than the commonly invoked increase in internal P release under increasingly anoxic conditions can drive rapid lake eutrophication. Since the proportion of littoral areas is typically large even in deep stratified lakes, littoral processes may contribute more frequently to the rapid lake eutrophication trends observed around the world than is currently recognized.

The isotopic ratio O-18/O-16 of dissolved O-2 in aquatic systems is affected by the preferential biological uptake of O-16 (epsilon). Studies over the past six decades reveal that during incubation experiments, the isotopic effect of microorganism respiration (epsilon(organism)) varies in the range of -18 parts per thousand to -22 parts per thousand. In contrast, natural variations in the deep-ocean O-2 concentration and delta O-18 levels show a considerably weaker effect (similar to -10 parts per thousand). The differences between these observations have been explained to result from either O-2 uptake by sediments or organic particles that, due to diffusion-limited respiration, are expected to weakly fractionate oxygen isotopes, by mixing processes or by weak fractionation at low temperatures. To gain better insight, we studied oxygen demand and delta O-18 in the deep, cold hypolimnion of Lake Stechlin between 2018 and 2021 as well as in various laboratory incubations. Our incubation results demonstrate an epsilon(organism) of about -24 parts per thousand. Simple model calculations demonstrate a sediment O-2 demand isotope effect (epsilon(SOD)) of about -8.4 parts per thousand, and a variated water-column O-2 demand isotope effect (epsilon(WOD)) which is lower than epsilon(organism), ranging from -13.9 parts per thousand in 2019 to -23 parts per thousand in 2021. Accompanying experiments indicate that the lower magnitude of epsilon(WOD) may be related to respiration at organic particles lending to a weaker fractionation effect. Thus, variations in epsilon(WOD) may reflect a changing partitioning of hypolimnion oxygen uptake between suspended particles and their containing microorganisms. Based on own incubation experiments with Daphnia carcasses, we discuss how possible changes of particle rigidity might influence epsilon(WOD).

Not much is known about early responses of pelagic fish populations to rapidly increasing nutrient concentrations in originally oligotrophic lakes. Here we report on population parameters of a pair of pelagic ciscoes (Coregonus spp.) exposed to accelerating eutrophication of the deep (69.5 m) and stratified Lake Stechlin (northeast Germany), in which total phosphorus (TP) concentration increased from similar to 15 to >60 mg m(-3) within 10 years. With increasing TP concentrations, lower Secchi depths, declining oxygen concentrations in deeper water layers, higher phytoplankton, and higher zooplankton biomass were observed. Correlation analyses supported strong bottom-up directed effects of higher TP concentrations on primary and secondary producers and negative effects of TP on water quality parameters. Annual hydroacoustic records and midwater trawling showed an increase in average individual fish size. No temporal trend was found in fish biomass, but we observed cyclic annual biomass fluctuations. Diel vertical migration amplitudes of ciscoes declined, primarily by the occurrence of more fish in shallower waters than in previous years during daytime. We propose that release from density-dependent food limitation, due to increased plankton food abundances, resulted in increased individual sizes for both cisco species, which in turn increasingly blurs species discrimination because length distributions of both species are no longer substantially different. Our data show the value of long-term lake monitoring, including fish population surveys, to understand the initially subtle changes in biotic communities that may precede more drastic, potentially irreversible changes from anthropogenic pressures.

The concurrent short-term fluctuations of methane (CH4) and carbon dioxide (CO2) concentrations and emissions in lakes are not well studied. Quantifying the differences in daytime versus nighttime dynamics may provide information about the controls over greenhouse gas production and may have relevance to lake carbon budgets. In this study, the diel variation of CH4 and CO2 emissions and concentration patterns were determined along the water column of 2 temperate lakes with different trophic states during July 2015. The results showed that the shallow eutrophic lake had significantly higher CH4 and CO2 releases than the deep oligo-mesotrophic lake. No significant diel fluctuations were observed in atmospheric CH4 emissions from either lake or in CO2 emission from the oligo-mesotrophic lake, but significantly higher CO2 emission from the eutrophic lake occurred during the daytime compared to nighttime. Based on a diffusion-reaction model, it was possible to estimate the diel variation of the sources and sinks of dissolved CH4 throughout the water column, suggesting lateral transport is a potential source of CH4 in oxygenated water layers in the oligo-mesotrophic lake. Sources and sinks throughout the water column should be considered in future studies for a better understanding of CH4 dynamics within the lakes.

Methane production (MP) in aquatic systems is positively related to temperature and trophic state. Global warming and eutrophication are predicted to potentiate MP in freshwater reservoirs. The simultaneous impact of rising temperature and carbon and nutrient additions on MP were examined in a tropical semiarid hydropower reservoir. Sediments cores from 3 locations (profundal, littoral, and intermediate) with differing water depth were sectioned and slurries incubated at 20, 30, and 40 degrees C with or without additions of carbon, nitrogen, or phosphorus, or all combined. Maximal MP (4.2 mu mol g DW-1 d(-1)), occurred under carbon addition, and mean MP was about twice as high than in the control, independent of temperature. The effect of carbon additions manifested differently at the 3 locations, with enhancement of MP greater in upper sediment layers of the profundal location and in deeper layers (4-8 cm) of littoral and intermediate locations. Without carbon addition, MP was slower and positive effects of warming were more frequent, especially in littoral. These results suggest that the combined effect of warming and land use changes, principally on carbon loads, will increase the MP and methane emissions potential in this semiarid reservoir. Differences in effects are linked to location in the reservoir.

Feldberger Haussee (NE Germany) was polluted for almost a century. During the late 1970s, the nutrient input reached a maximum of approximately 1.9/11.5 g TP/TN M-2 yr(-1). As a result, the lake became a hypertrophic ecosystem and had largely lost its recreational value. In 1980, the sewage discharge was stopped, decreasing the external loading by approximately 90%. Because of vast amounts of phosphorus stored in the sediment, the lake remained highly eutrophic until 1985 with a TP concentration of ca. 1 mg L-1. To accelerate recovery, biomanipulation was applied from 1985 to 2002 but was successful to only a minor extent. Eventually, due to sediment sequestration and discharge to downstream lakes the TP spring maximum (2006-2010) dropped to 0.112-0.078 mg L-1. However, given the trend, it was obvious that it would take another 10-15 years for the concentration to approach the desired mesotrophic level. Thus, it was suggested to inactivate the surplus phosphorus by treating the lake with poly-aluminium chloride (PAC) as precipitant. To ensure good water quality, the objective was to decrease concentrations below 0.035 mg TP L-1, while optimising the amount of PAC applied (as much as needed, but as little as possible). As a prerequisite, the status of the lake was carefully studied; external phosphorus loading and the amount of mobile phosphorus stored in the sediment being of specific interest. Laboratory experiments, modelling studies and field observations eventually resulted in an estimated dosage of 27 g Al M-2 (molar Al/P 12). Following the treatment in April 2011, prime water quality parameters showed two opposing trends: (1) TP concentration immediately dropped below the restoration target (<= 0.025/0.035 mg L-1), primary production and phytoplankton biomass declined substantially. (2) However, water clarity did not improve for another four years. The likely reason for the delay was the structure of the phytoplankton community. It was dominated by cyanobacteria with the potential to trigger intensive calcite precipitation and thus impair transparency. In spring 2015, the cyanobacteria suddenly disappeared and transparency increased significantly. We speculate that these changes of the planktonic community shifted the ratio of assimilation and respiration in favour of the latter. This allowed an increase of free CO2* (dissolved CO2 & dissociated carbonic acid) altering the carbonate buffering system and thus halting the formation of calcite crystals. Also, the phytoplankton, now represented by small readily ingestible taxa, promoted a flourishing Daphnia population inflicting heavy grazing losses, resulting in clear water stages (2015 June, 4.20 m; 2016, August 3.70 m; 2017 August, 4.5 m). We conclude that the drastic decrease of phosphorus availability in concert with structural and functional changes of the plankton community eventually improved the water quality of Haussee significantly. The lake is now in a mesotrophic status; well in accordance with the aim of the restoration project.

Climate forecasts project a global increase in extreme weather events, but information on the consequences for ecosystems is scarce. Of particular significance for lakes are severe storms that can influence biogeochemical processes and biological communities by disrupting the vertical thermal structure during periods of stratification. An exceptional storm passing over northern Germany in July 2011 provided an opportunity to assess the consequences and underlying mechanisms of such extreme events on the interplay between the physics and ecological characteristics of a deep, nutrient-poor lake. Wind speeds were among the most extreme on record. A suite of variables measured throughout the event consistently indicates that a cascade of processes pushed the clear-water lake into an exceptionally turbid state. Specifically, thermocline deepening by the storm-entrained cyanobacteria of a deep chlorophyll maximum located at about 8 m depth into the surface mixed layer. Released from light limitation, intense photosynthesis of the cyanobacteria boosted primary production, increased algal biomass, raised the pH and thus induced massive calcite precipitation to a level never observed within three decades of lake monitoring. As a consequence, water transparency dropped from 6.5 to 2.1 m, the minimum on record for 40 years, and the euphotic zone shrank by about 8 m for several weeks. These results show that cyanobacterial blooms not only are promoted by climate warming, but can also be triggered by extreme storms. Clear-water lakes developing a deep chlorophyll maximum appear to be particularly at risk in the future, if such events become more intense or frequent.

The investigation of reservoirs within the Guanting basin aimed to evaluate the possible changes of reservoir water quality in a scenario period 2008-2037 regarding the simulation of climate changes and the assumed socio-economic development (Section 9.2.3).In a first step we investigated the recent water quality state, the geographical conditions in the catchments as well as the utilization and management of the reservoirs to understand the limnological system and the causes for the water quality. This was done by the analysis of limnological and geographical data from different sources as well as by own geographical investigations during field trips and interviews with Chinese reservoir managers and experts from water authorities. As a result an elementary geographical and limnological classification of the reservoirs has been composed.An important key for the understanding of the nutrient metabolism of the reservoirs were our own investigations on the chemical composition and behavior of the reservoir sediments. Sediments as well as dispersed suspended solids are able to bind high amounts of dissolved phosphorus thus trapping a huge amount of nutrients (phosphorus). Thereby the sediments and suspended solids play a very important role in the nutrient metabolism of the reservoirs as well as for the river network.The second step of our work was based on the results of the climatic scenarios (STARS & CCLM, Chapter 3), eco-hydrological (SWIM, Chapter 4), water quantity management (WBalMo, Chapter 7), water quality monitoring (Chapter 8) and water quality management (MONERIS, Chapter 9) models as well as the socio-economic development scenarios (Chapter 6). From the results of the models we could deduce the probable impacts on the water balance and water quality of the reservoirs in the scenario period and we evaluated the likely effects on the water quality of the reservoirs. Since water balance and quality of reservoirs is mainly influenced by the amount and quality of the discharge, we assessed the verbalized management measures from WBalMo and MONERIS concerning their impact on water quality of the reservoirs.Besides traditional utilizations, the future management of the water balance and quality have to consider also the newly discovered function as a nutrient trap. However, to describe the immanent processes of the phosphorus binding by sediments and suspended solids further scientific work is necessary.Finally, we discuss the aims for the water quality of the reservoirs in the future.

Von 2001 bis 2005 wurde der Tiefwarensee mithilfe der hypolimischen Einbringung von Al- und Ca-Salzen restauriert. Die Behandlung unterdrückte beinahe vollständig die P-Freisetzung aus den Sedimenten. Die Menge von an Al und Ca gebundenem Phosphor in den Sedimenten stieg erheblich an. Die Sedimentschichten zwischen 5 und 10 cm binden immer noch lösliches, reaktives P (SRP) welches in diese Schicht diffundiert. Die jährliche Ablagerung von frischem Sediment führt zu einer allmählichen ?berdeckung der P absorbierenden Sedimentschichten und gelegentlich zu hohen SRP-Konzentrationen an der Sedimentoberfläche und daher zu einem Anstieg der P-Freisetzung aus dem Sediment. Bisher blieb die P-Akkumulation im Hypolimnion während der Sommerstagnation auf niedrigem Niveau, steigt jedoch allmählich seit dem Ende der Restaurierung an. In 2001–2005 Lake Tiefwarensee was recovered by a combined hypolimnetic addition of alum and calcium salts. The treatment almost completely suppressed the phosphorus (P) release from the sediments and the amount of P bound to Al and Ca in the sediments considerable increased. The sediment layers between 5-10 cm are still binding soluble reactive P (SRP) which is diffusing into this layer. Yearly deposition of fresh sediment leads to the gradually burial of the P adsorbing sediment layers and occasionally to high SRP-concentrations at the sediment surface and thus, to an increase of P-release from the sediments. So far, the P-accumulation in the hypolimnion during summer stagnation remained at a low level but is gradually increasing since the end of the restoration.

Knowledge about the contribution of food web structure and nutrient concentration in lakes to phosphorus (P) sedimentation and remobilisation at the sediment surface is still poor. Using four large enclosures placed in a eutrophic, thermally stratifying lake, we studied the effects of the structure of the planktonic food web (with and without planktivorous fish, ±F treatments) and nutrient concentration (with and without fertilisation, ±N treatments) on P sedimentation. We investigated the total P content and P binding forms in settling material (TPSM) and of the uppermost 1cm sediment layer (TPSed) during three consecutive stratification periods (2005–2007). Additionally, epilimnetic P (SRPEpi, TPEpi), chlorophyll a and biomass of total crustacean and Daphnia were measured. On a seasonal scale, Daphnia biomass tended to negatively influence chlorophyll a, sedimentation rate of total particulate matter and of P, but the latter two criteria did not differ significantly between treatments due to large fluctuations within each enclosure. The contents of TPSM and loosely adsorbed P in settling material decreased in the following order: −F/+N>+F/+N>−F/−N>+F/−N, indicating greater effects of nutrient addition than of food web structure. In sediments, organically bound P was 9–23% higher in −F variants compared to the corresponding +F treatments, thus indicating an effect of food web structure. Furthermore, positive correlations between SRPEpi, TPEpi, TPSM, TPSed, sediment reductant-soluble P and calcite bound P revealed an effect of the epilimnetic P concentration on P sedimentation and specific P binding forms. Compared to the composition of different P binding forms in the settling material, a considerable decrease of loosely adsorbed P (12–26%) and reductant-soluble P (14–21%), as well as an increase of organic P (14–26%) were observed in the uppermost 1cm-layer of the sediments in all treatments. We conclude that both nutrient enrichment (+N) and food web structure (−F) enhance the P sedimentation and P content at the sediment surface. However, in addition to food web effects on organic P content in settling matter and sediments, factors like iron concentration and calcite precipitation might be of importance for P sedimentation and storage in sediments in complex systems such as lakes.

Wauer, G., T. Gonsiorczyk, M. Hupfer and R. Koschel. 2009. Phosphorus balance of Lake Tiefwarensee during and after restoration by hypolimnetic treatment with aluminum and calcium salts. Lake Reserv. Manage. 25:377-388.Between 2001 and 2005, the recovery of Lake Tiefwarensee from eutrophic to mesotrophic state was successfully accelerated by the stepwise hypolimnetic addition of 137 g aluminum and 154 g calcium per square meter of profundal sediment. In response to the treatment, an 8-cm sediment cover was formed, which almost completely suppressed the phosphorus (P) release from the sediments, and is still present. The spatial variability of the sediments was analyzed at eight sampling points at different lake depths. With increasing lake depth, soluble reactive phosphorus decreased in the pore water, whereas the total phosphorus (TP) increased in treated sediment. Total P in the upper sediment layer (0-10 cm) increased by about 3 tons during the treatment period, consistent with the simultaneous decrease in the water from 0.223 mg/L in 1998 to 0.013 mg/L in 2005 (annual mean values for the whole water body). After initial settling, the drastic TP decrease in the water column can be attributed to an increase in the sediment P-binding capacity, which is related to a decrease of the mobile P pool (NH(4)Cl-TP) and a strong increase in the Al:P ratio in sediment. In the 3 years after completion of the treatment, the lake TP concentration was well described by the Vollenweider model, indicating that a sustainable state of nutrient equilibrium was achieved.

Restoration of anthropogenically eutrophied lake ecosystems is difficult due to feedback mechanisms that stabilize the trophically degraded state. Here, we show rapid recovery of a eutrophic stratified lake in response to multiple restoration that targeted the feedback mechanisms of high external and internal nutrient loads, lack of a trophic cascade, and lack of structured littoral habitats. Lake Tiefwarensee (Germany) was exposed to aluminium and calcium treatment and fisheries management over 5 years. Within this period, in-lake phosphorus concentrations declined by more than 80%, and transparency, zooplankton biomass and fish assemblage structure and biomass responded immediately and almost linearly to the reduction in phosphorus concentrations. Phytoplankton biomass and chlorophyll a (chl a) concentrations likewise decreased in response to restoration, but the declining trend was interrupted by one recovery year with unusually high phytoplankton biomasses. The zooplankton:phytoplankton biomass ratio and the chl a:phosphorus ratio approached values observed in other stratified lakes during natural recovery from eutrophication. The slow response of Tiefwarensee to the reduction of external load, and the quick response to the chemical treatment suggest that the disruption of internal P recycling and loading was the decisive restoration measure in Tiefwarensee. The external load reduction was a necessary but not sufficient measure, at least in the short-term, whereas the low-effort fisheries management was of minor importance. A comparison with other case studies confirms that measures aiming to inactivate phosphorus are the most efficient approaches to restore stratified lakes in the short-term, but a shift to a permanent near-pristine state is possible only by additional P input control.

Long-term (1976-1999) biomanipulation in Bautzen Reservoir (BR) revealed that a combination of piscivore stocking and catch restrictions for piscivores led to the desired effects of low planktivorous fish biomass and enhanced biomass of large filter feeders (Daphnia galeata). Despite the hypertrophic status of BR, fisheries management shifted the planktivore-dominated fish community into a piscivore-dominated community. High winter (Jan-Mar) Daphnia biomass was a sensitive indicator of reduced planktivory. Although edible phytoplankton was suppressed by elevated Daphnia biomass, mean seasonal (May-Oct) total phytoplankton biomass remained unchanged due to growth of large inedible algae and cyanobacteria. Inedible and total phytoplankton biomass was primarily controlled by phosphorus availability. However, during clear water periods a reduction of total phytoplankton was achieved with drastically increased Secchi readings. In Feldberger Haussee (FH), despite intensive long-term manual removal of cyprinids (1985-2002) and stocking of piscivorous fish (1988-2002), biomanipulation only had restricted, delayed, or transient effects on the ecosystem. Mean proportion of piscivores within total yield increased but was below 20% in most years. Planktivore cyprinid yields dropped until 1990 and remained constant thereafter. Daphnia biomass slightly increased after biomanipulation became effective (1987-1989) but declined to pre-biomanipulation levels later in the experiment (1996-2005). Beginning in 1997, both edible and inedible phytoplankton biomass started to decrease. Finally, when biomanipulation had been implemented after a delay of 8 years water clarity increased significantly. Although external loading reductions and biomanipulation in both lakes resulted in moderate phytoplankton biomass reduction and Secchi depth enhancement, the reasons for the observed changes were different. Even though strong cascading effects were detected at the top of the food web in BR, the trophic cascade was largely decoupled between phyto- and zooplankton. External plus internal phosphorus loading still exceeded a critical threshold below which a top-down-induced indirect effect of phosphorus sedimentation and finally limitation could have reduced phytoplankton biomass. In constrast to BR, the critical phosphorus loading threshold in FH has probably been approached. Nonetheless, cascading effects were weak due to insufficient reduction of planktivorous cyprinids. Improved water quality was primarily a result of resource-related effects. Thus, the 2 long-term experiments reveal that (1) biomanipulation cannot be applied successfully without reducing nutrient loading below a critical threshold (BR), and (2) for successful biomanipulation, 30-40% piscivores within total fish standing stocks are required (BR and FH). Without optimum piscivory, manual removal of planktivores will hardly produce sustained cascading effects (FH).

The influence of a further developed inlake restoration method on the P-immobilisation and microbial activities, especially under anoxic conditions was investigated. The impact of nitrate and iron dosing with a newly developed nitrate storage compound (Depox®Fe) was tested in enclosures in the eutrophic dimictic Lake Dagowsee, Germany. Additions of 50gm−2 of NO3−–N and 66gm−2 of Fe3+ ensured availability of nitrate at the sediment surface during a 2-months period.