Aspects of phytoplankton succession and spatial distribution in an acidic mining lake (Plessa 117, Germany)

In Germany, several hundred mining lakes have been formed in the last century. The majority of these water bodies is strongly influenced by high concentrations of dissolved ions and high acidity, and shows pH values around 3. Hereby, a first evaluation is given on the relevance of different parameters, regulating the phytoplankton composition and seasonal succession in the extremely acidic Lake Grünewalde, Lusatia. The phytoplankton biocoenosis, which was analysed for the period April 1996–July 2001, was of low biodiversity with a dominance of the phytoflagellates Ochromonas (Chrysophyceae), Chlamydomonas (Chlorophyta) and Gymnodinium (Dinophyceae). The development of the phytoplankton biomass was low with an average biovolume of 0.64 mm 3 l –1. The phytoplankton succession revealed a clear dependence from the inorganic carbon, which is, in combination with phosphorus, the limiting major nutrient in acidic lakes. Higher TIC concentrations occurred during ice covering and stagnation periods in the hypolimnion, and were responsible for rapid increases of the planktonic populations with a maximum biovolume around 2–3 mm 3 l –1. Vertical gradients of hydrochemical parameters and plankton distribution were assessed during the summer stratification period. A daily migration pattern was shown for the dominant mixotrophic nanoflagellate Ochromonas, living in deeper layers of the epi- and metalimnion and migrating to the lake surface during lowering light conditions in the evening. This behaviour is explained as a strategy to receive optimal light supply and to overcome nutrient stress in periods of strong physico-chemical gradients.