Experimental eutrophication of a shallow acidic mining lake and effects on the phytoplankton

Acidic mining lakes offer an opportunity to investigate ecological development under extreme geochemical conditions. Low pH combined with high ionic and metal concentrations allows only a small number of species, in particular nanoflagellates, to occur. In these lakes, important nutrients such as phosphorus and inorganic carbon are scarce and limit primary production. In order to investigate the neutralization potential of sulphate reducing processes and controlled eutrophication, in Lake Koyne 113 (Brandenburg, Germany), a new technique for the remediation of acidic waters was tested by adding organic material to the lake in jute bags (``Biobags''), which should form anoxic microbial reaction compartments and increase primary production by adding nutrients to the water. This treatment was expected to have profound effects on the lake ecosystem. The hydrochemical and biological effects were observed within an accompanying monitoring programme. The experiment led to an increase in the organic carbon and total phosphorus concentrations. The influence on other physical and chemical variables was not obvious. Phytoplankton species composition remained unchanged and was still determined by the high acidity. Algal biomass rapidly increased after nutrient addition and chlorophytes replaced chrysophytes as the dominant phytoplankton group.