LONG-TERM DYNAMICS OF SMALL-BODIED AND LARGE-BODIED CLADOCERANS DURING THE EUTROPHICATION OF A SHALLOW RESERVOIR, WITH SPECIAL ATTENTION FOR CHYDORUS-SPHAERICUS

Eutrophication in Tjeukemeer involved a gradual increase in chlorophyl l concentrations from ca. 30 mg m(-3) in 1968-69 to 125 mg m(-3) in 19 76. From 1976 onwards, chlorophyll concentrations remained at a high l evel fluctuating between 100-225 mg m(-3). Hillbricht-Illkowska (1977) hypothesized that small-bodied species will become increasingly abund ant and dominant over large-bodied species with increasing eutrophicat ion. We tested this hypothesis using observations from life history ex periments on Chydorus sphaericus, combined with data from 25 years of field observations on the population dynamics of cladocerans in Tjeuke meer. In life history experiments with C. sphaericus, the fitness meas ure r in treatments with natural lake seston and laboratory cultured g reen algae was significantly higher on lake seston from Tjeukemeer, co ntaining a high proportion of detritus. This suggests that detrital pa rticles are good quality food for C. sphaericus. Field observations du ring the period 1968-1976 showed that all three categories of cladocer ans: C. sphaericus, 'other' small-bodied cladocerans (predominantly Bo smina spp.) and large-bodied cladocerans (predominantly Daphnia galeat a), increased in biomass with increasing chlorophyll concentration. Ho wever, 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 decrea sed in biomass with increasing chlorophyll concentration, whereas the biomass of the large-bodied cladocerans significantly increased with i ncreasing 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 i n detritus and filaments of Cyanobacteria than small-bodied cladoceran s, but is more vulnerable to fish predation. Densities of 0+ zooplankt ivorous fish show strong annual fluctuations in Tjeukemeer, and becaus e of hydrological conditions, 0+ fish abundance in this lake is probab ly negatively related to chlorophyll content.