INFLUENCES OF THE LOWLAND RIVER SPREE ON PHYTOPLANKTON DYNAMICS IN THE FLOW-THROUGH LAKE MUGGELSEE (GERMANY)

The influences of imports of nutrients and planktonic algae from the River Spree on the dynamics of phytoplankton were examined in the shallow, eutrophic Muggelsee, which has a retention time of only 42 days. Phytoplankton biomass and nutrient concentrations were measured in both the lake and its inflow from 1980-1990. On a long-term average, mean biomass as well as vitality of most dominant phytoplankton populations in the lake were not significantly different from those in the river. Nevertheless, during distinct periods the external rates of biomass change of single lake populations (due to dilution or enrichment) were as high as the lake internal ones. The import of inocula populations from the river probably induced the formation of the typical community structure in the lake. Growth and decay of phytoplankton populations in the river strongly influenced the load of dissolved nutrients and thus indirectly the dynamics of planktonic algae in the downstream lake. For example, intensive assimilation of phosphorus by riverine algae in spring intensified the P-shortage and supported possible P-limitation of algal growth in the lake at that time. In years with high vernal biomass of centric diatoms in the river, and thus diminished import of dissolved silicon, the growth of diatoms was suppressed and that of cyanobacteria was favoured in the lake during summer.