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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.

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.

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 sim...

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.

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 1 cm 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 1 cm-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.

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 i...

Von 2001 bis 2005 wurde der Tiefwarensee mithilfe der hypolimischen Einbringung von Al- und Ca-Salzen restauriert. Die Behandlung unterdruckte beinahe vollstandig 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 losliches, reaktives P (SRP) welches in diese Schicht diffundiert. Die jahrliche Ablagerung von frischem Sediment fuhrt zu einer allmahlichen ?berdeckung der P absorbierenden Sedimentschichten und gelegentlich zu hohen SRP-Konzentrationen an der Sedimentoberflache und daher zu einem Anstieg der P-Freisetzung aus dem Sediment. Bisher blieb die P-Akkumulation im Hypolimnion wahrend der Sommerstagnation auf niedrigem Niveau, steigt jedoch allmahlich 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. Keywords: Seenrestaurierung; Sedimente; Phosphor; P-Bindung; Aluminiumbehandlung; Kalziumbehandlung lake restoration; sediments; P-binding forms; Al-treatments; Ca-treatments

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 50 g m −2 of NO 3 − –N and 66 g m −2 of Fe 3+ ensured availability of nitrate at the sediment surface during a 2-months period. As a result the phosphate release from the anoxic sediments was completely suppressed even 1 year after the application. The hypolimnetic deoxygenation was unaffected by the Depox ® Fe addition. However, sulfur reduction and methanogenesis were inhibited and the phosphatase activity increased.

The Feldberger Haussee, a highly eutrophic stratified hard-water lake located in the eastern part of Germany's Baltic lake district, was selected for a restoration programme combining external nutrient loading reduction and long-term biomanipulation. In 1980 the external phosphorus loading (1.8 g TP m−2 a−1) decreased by 90%, but water quality did not improve significantly within the following 6 years. In 1985 biomanipulation was initiated, with manual removal of cyprinid fish coupled with piscivore introductions as the principal measures. The expected changes in the pelagic community and improvements of water quality occurred after a delay of several years. Despite intensive manual removal of cyprinids and stocking of piscivores, standing stocks of cyprinid fish remained relatively high (130–260 kg ww ha−1) after some years of decline. Compared to the pre-biomanipulation period, mean seasonal (May-September) Daphnia spp. biomass roughly doubled (0.037 g C m−3 vs. 0.084 g C m−3). However, the predominance of small (

(2002). Primary production of phytoplankton and nutrient metabolism during and after thermal pollution in a deep, oligotrophic lowland lake (Lake Stechlin, Germany) SIL Proceedings, 1922-2010: Vol. 28, No. 2, pp. 569-575.

Die Gewassereutrophierung stellt in vielen Teilen der Welt ein noch immer gravierendes Problem dar. Wenn in einem Gewasser mehr Algen wachsen als wunschenswert ist, kann ihre Menge zum Beispiel durch eine Erhohung der Verluste des Phytoplanktons verringert werden. Der Beitrag erlautert zunachst die trophischen Zusammenhange, die zu einer Verringerung des Phytoplanktons fuhren konnen und beschreibt die Moglichkeiten der Biomanipulation uber den Fischbesatz. Anschliesend werden Anwendungsempfehlungen gegeben und am Beispiel des Feldberger Haussees erlautert. Es wird gezeigt, dass die Biomanipulation in Kombination mit Belastungsverminderungen den okologisch besten, gutewirtschaftlich nachhaltigsten und finanziell gunstigsten Weg einer Seerestaurierung darstellen kann. Keywords: Besatzfisch; Biomanipulation; Cyanobakterien; Daphnia; Eutrophierung; Feldberger Haussee; Klarwasserstadium; Makrophyten; Nahrungsnetz; Nahrstoffbelastung; Phytoplankton; Planktologie; Raubfischpopulation

The vertical distribution of various phosphorus (P)-binding forms, associated potential P-binding partners and the composition of dry material were investigated in the bottom sediments of the dimictic oligotrophic Lake Stechlin and the dimictic eutrophic Lake Feldberger Haussee. Reductant soluble P (Fe- and Mn-bound) at the sediment surface (0−1 cm) was considerably higher in the oligotrophic Lake Stechlin (1.29 g kg−1) than in the eutrophic Lake Haussee (0.32 g kg−1). The amounts of dissolved, loosely adsorbed, metal oxide- and calcium carbonate bound P were higher in the eutrophic lake. The depth profiles of the investigated P species indicated that the mobilization of Fe- and Mn-bound P is the most important mechanism of P-release in oligotrophic lakes, whereas the mobilization of recently sedimented labile organic bound P seems to be the driving force of P-release in eutrophic lakes. In both lakes autochthonous calcite precipitations occurs during the summer months. The coprecipitation of P with calcite is an important self-cleaning mechanism in eutrophic hardwater lakes and contributes to the permanent burial of P in the sediments. Although, the precipitation of calcite is inhibited by the presence of high concentrations of soluble reactive P, the coprecipitation of P with calcite seems to be enhanced.

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 c...

The vertical distribution of various P-binding forms, potential P-binding partners and the concentration of soluble reactive P in the interstitial water were investigated in the bottom sediments of the oligotrophic Lake Stechlin (north-eastern Germany). The most important fraction of total P in the top centimeter of the sediment is the redox-sensitive Fe- and Mn-bound P with 1.29 g/kg (54% total P), which decreased to about 0.23 g/kg at a depth of 10-30 cm. Its decrease is responsible for a corresponding decrease in total P with depth. The release of P due to diagenesis of labile P-com-pounds was calculated to be 69 mg P m -2 a -1 . This is concordant with the mean accumulation rate of soluble reactive P in the hypolimnion during summer stratification (70 mg P m -2 a -1 ). The mobilization of reductant-soluble P due to the shift of the top oxic sediment layers into deeper anoxic zones (burial) accounts for 81% of the calculated P-release of labile P-compounds (86 mg P m -2 a -1 , 1970-1996). Although, reductant-soluble P does not add to the permanent burial of P, the thickness of the top oxic sediment layer and its redox-dependent P-binding capacity influence the present concentration of P in the whole water body, and thus contribute to maintaining the oligotrophic status of Lake Stechlin.

The aim of this investigation was to study the temporal variation in phosphorus release from the sediments and its influence on water quality of stratified lakes. The concentrations of soluble reactive phosphorus (SRP), calcium and sulfate in the interstitial water and the pH in the wet sediments of dimictic lakes were investigated during the spring circulation and at the end of summer stratification. Multiple regression analysis using the calculated diffusive fluxes of SRP out of the sediments and the morphometric characteristics of the lakes (reduced water depth), explained 73 % of the variance of the SRP-accumulation in the hypolimnia during summer stagnation. At the end of surnmer stratification diffusive fluxes of SRP out of the sediments increased and pH-values and sulfate-concentrations decreased at the sediment surface (0–2 em) and in the hypolimnia. The maximum diffusive flux of SRP was calculated to be 5.8 mg/m2/d at the end of summer stagnation. Prob able reasons for these higher diffusive fluxes of SRP at the end of summer stagnation are higher supply of labile organic matter and thereby higher mineralization rates. lower redox potential and thus higher dissolution of redox sensitive P-binding forms and/or dissolution of phosphorus being bound to Ca-phases at lower pH.