Eutrophication problems and their potential solutions in the artificial shallow lake Altmuhlsee (Germany)

Lake Altmuhlsee in Middle Franconia, Germany, was constructed as part of a water transportation system and flooded in 1986 by River Altmuhl. Supply of nutrient rich water resulted in a hypertrophic state characterized by low Secchi depth (<0.5 m in summer). Massive blooms of phytoplankton in summer were mainly caused by cyanobacteria (Aphanizomenon flos-aquae, Anabaena flos-aquae, Microcystis aeruginosa) and by chlorophytes (Oocystis ssp., Monoraphidium ssp., Planktosphaeria gelatinosa), respectively. Fish assemblage was characterized by the dominance of planktivorous bream. Hence, zooplankton community mostly consisted of small-bodied cladocerans due to predation pressure by fish. Investigations of the phosphorus (P) cycle of Lake Altmuhlsee revealed high P-turnover rates in the water body and at the immediate sediment-water interface, an outstanding importance of external P-load, and a minor importance of the internal P-load. The lake supports recreational and social functions which, however, are restricted due to the low water quality. Improvement of water quality is necessary to increase attractiveness. Our study presents some proposals for an effective and economically justifiable water quality management, especially, biomanipulation in accordance to the holder of the fishing rights and local establishment of macrophytes.