Biological Effects of Incomplete Destratification of Hypertrophic Freshwater Reservoir

This paper describes results of partial destratification of a large reservoir with complex morphology. In order to reduce phytoplankton growth by light limitation, the upper 20 m of the water column were destratified in the Bleiloch Reservoir (Thuringia, Germany). Temperatures, phytoplankton, and zooplankton were investigated before and during aeration to determine the effects of the partial destratification on plankton. The measure induced an increase of phytoplankton dynamics. Since 1991 nuisance cyanobacteria blooms increased in the hypertrophic Bleiloch reservoir (Thuringia, Federal Republic of Germany), as light conditions improved due to a reduction of coloured wastewaters supplied by a cellulose mill. Warm inflowing water formed an upper layer, which reached the bubble plume near the dam, and was only weakly affected by the deeper mixed water body. The water column stability decreased, which caused an overturn 2–3 weeks earlier, because the hypolimnion warmed up faster. These physical effects of the destratification delayed the seasonal succession of phytoplankton. Cyanobacteria were suppressed but diatoms and chlorophytes became more abundant. Copepods maintained higher population densities throughout the summer. During artificial mixing, the algal diversity increased, not only because k-strategists selected a stratified water column, but also because episodic thermal instability enabled r- and k-strategists to co-exist in a competitive environment.