Long-term changes in functional properties and biodiversity of plankton in Lake Greifensee (Switzerland) in response to phosphorus reduction

Abstract

Lake Greifensee, a dimictic lake situated near Zürich, Switzerland, has been monitored chemically and biologically since 1971, a period during which total phosphorus concentration changed dramatically. Based on analysis of cores of the lake sediment, this lake was oligotrophic 120 years ago. Due to a rapid increase in population in the catchment area to about 100,000 inhabitants, the phosphorus input from sewer systems eventually exceeded the tolerable P-load of 2.3 t y-1. In the early 1970s, this lake was hypertrophic (ca. 600 mg m-3 total phosphorus). Due to P-elimination in sewage treatment plants and a ban of P-containing detergents, phosphorus concentrations declined to less than 50 mg m-3 in spring 2000 (overturn value). Currently we observe growth-limiting phosphorus concentrations in the trophogenic layer from May to November. Therefore, primary productivity dropped from > 700 g C m-2 y-1 at the beginning of our investigations (1971) to approximately 400 g C m-2 y-1 in 2000, which is still in the eutrophic range. Ammonia concentrations in the hypolimnion decreased significantly over this period. However, anaerobic conditions still regularly occur in the hypolimnion during May/June. Although total biomass values of phytoplankton or zooplankton did not show any correlation with the long-term decline of phosphorus concentrations, the composition of different systematic groups showed clear trends. During the period with Ptotal concentrations of >250 mg m-3 phytoplankton biomass was dominated by chlorophytes and cryptomonads, with lesser amounts of diatoms and dinoflagellates. Cyanobacteria and chrysomonads were of minor importance. With declining P load cyanobacteria biomass increased while chlorophytes and dinoflagellates decreased. Chrysomonads and pennate diatoms attained substantial portions, Zooplankton community did not change markedly. Main fraction was contributed by the herbivorous crustaceans, about 20% of total zooplankton biomass was carnivorous. Functional properties of phytoplankton showed closer relationship with P decline than biomass of systematic groups of algae. Under hypertrophic conditions r-strategists were dominant, especially during spring and early summer. K-strategists were more abundant under moderate eutrophic conditions in the second half of the year. Motility and buoyancy are very important in lakes with thin trophogenic layer (5 m in Lake Greifensee). Nonmotile plankton could survive during overturn (enhanced turbulence) and during clear water periods, whereas motile plankton groups dominated during thermal stratification. The best criteria for measuring resilience of Lake Greifensee are species richness, biomass-based diversity and evenness indices of phytoplankton and the strength of coupling between different trophic levels. With declining phosphorus content, species richness and diversity indices of phytoplankton increased.