The 3-spined stickleback Gasterosteus aculeatus often dominates fish communities in eutrophic, brackish, north-temperate lakes where it is thought to contribute significantly to the low zooplankton biomass and low grazing pressure on phytoplankton. To elucidate its effect on zooplankton community structure and phytoplankton biomass (chl a), we conducted a 5.5 mo enclosure experiment with sticklebacks at different densities in high nutrient concentrations and a salinity of 2 psu. Increasing fish density (0 to 10.2 sticklebacks m -2 ) led to substantial changes in the zooplankton community structure, an increase in phytoplankton chl a, and a reduction in water clarity. At densities below 4 to 6 sticklebacks m -2 , large-bodied cladocerans dominated the zooplankton biomass, the zooplankton:phytoplankton biomass ratio was high (>1), and chl a was accordingly low (< 20 µg l -1 ). At higher stickleback densities, large cladocerans disappeared and the abundance of small cladocerans was low. Instead, cyclopoid copepods and rotifers dominated, the zooplankton:phytoplankton biomass ratio was well below 0.5, and chl a exceeded 200 µg l -1 in late summer. The fish density level for a shift from a clear to a turbid state is low compared to the densities observed in several eutrophic, brackish lagoons. We conclude that fish have a strong impact on water clarity in shallow, eutrophic, brackish lakes with low salinity, which may explain the usually low zooplankton:phytoplankton biomass ratio and the turbid state of such aquatic environments. However, our results also indicate that once the density level declines to below 4 to 6 sticklebacks m

Stratified eutrophic lakes often suffer from hypolimnetic oxygen depletion during summer. This may lead to low redox conditions and accumulation of phosphate and ammonia in the hypolimnion. Hypolimnetic oxygenation has been used as a lake management strategy to improve the water quality in five eutrophic dimictic Danish lakes where oxygenation was conducted for 4-20 years. In one lake, the hypolimnetic oxygen concentration clearly improved by oxygenation, whereas the other four lakes still exhibited low mean summer levels (\2.2 mg O 2 l -1 ). Oxygenation generally increased the hypolimnetic water temperature by 0.5-2°C, but in one lake it increased by 4-6°C. In all lakes, oxygenation significantly reduced the hypolimnetic concentrations of phosphorus and ammonia during stratification. The accumulation of phosphorus and ammonia typically decreased by 40-88%. In two lakes oxygenation was stopped for 1-2 years and here hypolimnion concentrations of both phosphorus and ammonia increased again. Surface water quality only improved in one lake, but was likely also influenced by simultaneously occurring changes in external nutrient loading. Overall, it is concluded that hypolimnetic oxygenation reduces the hypolimnetic accumulation of phosphorus and ammonia and may prevent anoxia in the deeper parts of the lake. However, long-term oxygenation is required and it is uncertain whether the overall lake water quality can be improved by oxygenation. Reduction of the external nutrient loading is still essential to improve lake water quality.

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