LAUSR

Abstract 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.