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Abstract

Eutrophication in Tjeukemeer involved a gradual increase in chlorophyll concentrations from ca. 30 mg m⁻³ in 1968-69 to 125 mg m⁻³ in 1976. From 1976 onwards, chlorophyll concentrations remained at a high level fluctuating between 100-225 mg m⁻³. Hillbricht-Illkowska (1977) hypothesized that small-bodied species will become increasingly abundant and dominant over large-bodied species with increasing eutrophication. We tested this hypothesis using observations from life history experiments on Chydorus sphaericus, combined with data from 25 years of field observations on the population dynamics of cladocerans in Tjeukemeer.

In life history experiments with C. sphaericus, the fitness measure r in treatments with natural lake seston and laboratory cultured green algae was significantly higher on lake seston from Tjeukemeer, containing a high proportion of detritus. This suggests that detrital particles are good quality food for C. sphaericus. Field observations during the period 1968-1976 showed that all three categories of cladocerans: C. sphaericus, 'other' small-bodied cladocerans (predominantly Bosmina spp.) and large-bodied cladocerans (predominantly Daphnia galeata), increased in biomass with increasing chlorophyll concentration. However, of these three cladoceran categories only C. sphaericus showed a distinct and significant increase whereas the other two only showed a marginally significant increase. During the period 1977-1992, both 'other' small-bodied cladocerans and C. sphaericus significantly decreased in biomass with increasing chlorophyll concentration, whereas the biomass of the large-bodied cladocerans significantly increased with increasing chlorophyll content. These observations are not in agreement with the hypothesis that small-bodied zooplankton become increasingly abundant with increasing eutrophication. We suggest that the observed trends are partially caused by a food effect, and partially caused by predation pressure. Daphnia shows a better response to the increase in detritus and filaments of Cyanobacteria than small-bodied cladocerans, but is more vulnerable to fish predation. Densities of 0+ zooplanktivorous fish show strong annual fluctuations in Tjeukemeer, and because of hydrological conditions, 0+ fish abundance in this lake is probably negatively related to chlorophyll content.