Thomas Gonsiorczyk

Reduction of nutrient loading and biomanipulation as tools in water quality management: Long-term observations on Bautzen Reservoir and Feldberger Haussee (Germany)

Abstract 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).

Reduction of nutrient loading, planktivore removal and piscivore stocking as tools in water quality management: The feldberger haussee biomanipulation project

The Feldberger Haussee, a highly eutrophic stratified hard-water lake located in the eastern part of Germanys 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 (< 1 ram) D. cucullata throughout the whole investigation period indicated that planktivory was still substantial. Paired observations between edible phytoplankton biomass and Daphnia spp. indicated that a significant decline in algal stocks would only occur if herbivorous biomass was above a certain threshold (0.2 g C m 3). Reduced external and internal loading in concert with pelagic calcite precipitation were most likely responsible for the decline in lake phosphorus concentrations, thereby substantially improving the water quality of Feldberger Haussee. Although this is not supported by quantitative evidence, we hypothesize that resource-related water quality improvements were caused by changes in the structure of the pelagic community leading to increased calcite precipitation. In agreement with the results of other investigations, we conclude that because stabilising mechanisms such as macrophyte growth were lacking in Feldberger Haussee, biomanipulation in stratified lakes may not be as successful as has been observed in shallow lakes. However, in hard-water lakes, calcite precipitation may act as another stabilising resource-related mechanism. Phosphorus associated with sedimenting calcite particles is insensitive to redox-conditions and may therefore not be re-mobilised from lake sediments even if hypolimnetic oxygen is depleted.

Phosphorus balance of Lake Tiefwarensee during and after restoration by hypolimnetic treatment with aluminum and calcium salts

Abstract 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 (NH4Cl-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.

Primary production of phytoplankton and nutrient metabolism during and after thermal pollution in a deep, oligotrophic lowland lake (Lake Stechlin, Germany)

From 1966 to 1989 a nuclear power plant (NPP) was operated in the Lake Stechlin area. lt had an external circulation system for its cooling water, which was taken from the mesotrophic Lake Nehmitz. The heated cooling water was pumped into the oligotrophic Lake Stechlin. Canal connections between both lakes completed the system and from Lake Stechlin the water was led back to Lake Nehmitz (see Table 2). From an ecological point of view, this manipulation ofLake Stechlin can be considered as a long-term experiment. Comparisons of biological, chemical and physical conditions before, during and after the cooling water circulation gave insights into material and energy balances as well as the dynamic biological imeractions in the lake. Thus, there would be a solid basis for research prograrns on basic limnological problems. Lake Stechlin proved to be a sensitive laboratory in which to test the biotic and abiotic behavior of an aquatic ecosystem under various external influences, for exarnple, changes of water circulation patterns, nutrient input and waste heat. The Lake Stechlin limnological studies, which now span more than 40 years, have covered a large range of subjects in understanding of the structure and function of an oligotrophic lowland lake in the temperate climate zone, while maintaining a general focus o n influences of thermal and nutrient loadings on biocoenoses and bacterial, primary and secondary production as well as on heat and water balances. The operation of the NPP and its cooling water release into Lake Stechlin was terminated in 1989 and the relationships between those new conditions and subsequent changes of limnological effects and reactions in the lake have been studied as a quasiexperiment. The present paper is based on studies that were intended to define the conditions ofLake Stechlin as it adjusted to a major alteration in hydrological stabiliry, nutrient metabolism, primary production and succession o f phytoplankton over a period o f l 0-30 years. The objective of the study was to summarize severallong-term reactions and changes in the chemical composition and the phytoplankton productiviry of a dimictic, oligotrophic lowland lake during and after the operation of cooling water circuits, and the subsequent consequences in trophic conditions.

Methane production increases with warming and carbon additions to incubated sediments from a semiarid reservoir

Abstract 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 °C with or without additions of carbon, nitrogen, or phosphorus, or all combined. Maximal MP (4.2 μmol 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.