Long-term response of phytoplankton to reduced nutrient load in the flushed Lake Müggelsee (Spree system , Germany)
Köhler, J.; Behrendt, H.; Hoeg, S.
published: Apr 14, 2000
ArtNo. ESP141014802003, Price: 29.00 €
Biomass and structure of phytoplankton of a flushed, shallow lake were compared between a decade with hypertrophic conditions (1980-1990) and a period with 40-50 % reduced external nutrient load (1991-1997). Nutrient concentrations in the inflow declined less than loads because of reduced discharge. At the same time, the phytoplankton biovolume decreased by 33 % in spring (March-May) and 40 % in summer (June-September, average 1980-1990 versus 1991-1997). The total phytoplankton biomass declined despite heavy internal phosphorus load in summer, an absence of macrophytes and reduced biomass of daphnids. Nutrient-based carrying capacities of Müggelsee diminished from the 1980s to recent years, but were still rarely fully saturated. Filamentous cyanobacteria dominated the phytoplankton under hypertrophic conditions but were much less abundant in recent years. Diatoms were less affected by nutrient reduction and became the dominant algal group in the 1990s. Phytoplankton development depended more strongly on the start conditions than on the seasonal averages of nutrient concentrations. Earlier exhaustion of phosphorus by diatoms was the only proper explanation of the shorter or lacking spring development of cyanobacteria from 1991-1997. During summer, cyanobacteria grew at the same rates as in the previous decade, but attained lower maxima because of their lower starting biomass. Obviously, temporary phosphorus limitation in spring might suppress the development of species with low growth rates in such flushed systems during the whole season.