REPLACEMENT OF HERBIVORE ZOOPLANKTON SPECIES ALONG GRADIENTS OF ECOSYSTEM PRODUCTIVITY AND FISH PREDATION PRESSURE

In general the pooling of major taxa did not provide a basis for classifying zooplankton communities in 342 large Norwegian lakes, as neither cladocerans nor calanoids varied systematically with lake productivity or fish predation pressure. At the species level, most herbivorous cladocerans and calanoids, which constituted three quarters of the metazoan zooplankton biomass, differed in their preference for lake productivity and fish community and could be distinctly grouped according to these variables by canonical correspondance analysis. The analysis pointed out one oligotrophic and one eutrophic specialist among the herbivorous cladocerans, while two of the calanoids were oligotrophic specialists. The biomasses of cladocerans, calanoids, or daphnids were poorly correlated with both lake productivity and fish predation, whereas shifts in average size and species distribution could be attributed to these variables. At low lake productivity, chemical variables such as pH and Ca, as well as the species' physiological adaptations, appear as the main determinants for the competitive advantage and relative success of herbivorous species. Fish community composition changes with increasing lake productivity, bur only at very high fish predation intensity (cyprinid communities) did the effects of predation become the main determinant of the zooplankton community, superimposed on the lake productivity.