Crustacean zooplankton in Lake Constance from 1920 to 1995 : response to eutrophication and re-oligotrophication

During the first three quarters of this century, the trophic state ofLake Constance changed from oligotrophic to meso-/eutrophic conditions. The response of crustaceans to the eutrophication process is studied by comparing biomasses of crustacean zooplankton from recent years, i.e. from 1979-1995, with data from the early 1920s (AUERBACH et a1. 1924, 1926) and the 1950s (MUCKLE & MUCKLEROTTENGATTER 1976). This comparison revealed a several-fold increase in crustacean biomass. The relative biomass increase was more pronounced from the early 1920s to the 1950s than from the 1950s to the 1980s. Most important changes of the species inventory included the invasion of Cyclops vicinus and Daphnia galeata and the extinction of Heterocope borealis and Diaphanosoma brachyurum during the 1950s and early 1960s. All species which did not become extinct increased their biomass during eutrophication. This increase in biomass differed between species and throughout the season which resulted in changes in relative biomass between species. Daphnids were able to enlarge their seasonal window of relative dominance from 3 months during the 1920s (June to August) to 7 months during the 1980s (May to November). On an annual average, this resulted in a shift from a copepod dominated lake (biomass ratio cladocerans/copepods = 0.4 during 1920/24) to a cladoceran dominated lake (biomass ratio cladocerans/copepods = 1.5 during 1979/95). The biomass of cyclopoid copepods increased strongly during the first half of the year owing to the invasion of Cyclops vicinus, which caused a strong relative decline ofEudiaptomus. In contrast to the pronounced response to eutrophication, crustaceans have not yet shown an unambiguous response to the beginning re-01igotrophication of Lake Constance.