Phytoplankton selection in a river-lake system during two decades of changing nutrient supply

Phytoplankton dynamics in Lake Muggelsee, a eutrophic and polymictic lake in Berlin, and in the inflowing lowland River Spree have been comprehensively investigated during the last two decades. Zooplankton dynamics, nutrient supply, light climate, duration of ice cover and of summer stratification have also been regularly measured to help to explain phytoplankton development. The first period (1978-1990) was characterised by high nutrient loads and dominance of cyanobacteria from spring to autumn. Since then, loads of phosphorus and nitrogen have been lowered by 40-50%. Oscillatoria-like cyanobacteria (Limnothrix redekei, Planktothrix agardhii) were favoured under hypertrophic conditions in both the polymictic lake and the river, but they have disappeared nearly completely after nutrient reduction. Development of these species depended on meteorological conditions and nutrient supply in spring rather than on seasonal averages of nutrient concentrations. Diatoms have became dominant and chlorophytes have increased their share of the biomass since the nutrient load was reduced. Species composition changed even within the algal groups. Retention time of water and duration of thermal stratification of the water column modified phytoplankton structure. Mobile algae like Microcystis or Ceratium occurred in the lake during stratification periods. Otherwise, species composition in the shallow, polymictic lake was very similar to that in the inflowing lowland river. Species with high starting biomass, fed by high riverine import, resting stages or perennation were selected in this flushed system.