FOOD-WEB MANIPULATION IN SHALLOW, EUTROPHIC LAKES - BRIDGING THE GAP BETWEEN THE WHOLE-LAKE APPROACH AND BEHAVIORAL AND DEMOGRAPHIC-STUDIES

Even in the most thorough biomanipulation studies, which cover the res ponse of all trophic levels to manipulation at the top of the food cha in, the mechanisms underlying the success or failure of an experiment often remain obscure. The case of a small eutrophic Lake Wirbel in Pol and is an example of how useful it could be to apply demographic and b ehavioural analysis of dominant planktonic herbivores to explain the s equence of events following biomanipulation measures. At a certain sta ge of a seven-year (1988-1994) biomanipulation experiment, rotenone wa s applied to remove all fish. Following fish removal, summer algal bio mass in the 'edible' fraction of phytoplankton (< 30 mu m) was on aver age 2.5 times lower. Besides the significant increase in the density o f herbivorous zooplankton (above all the largest cladoceran Daphnia cu cullata), we identified three other phenomena underlying the observed reduction in algal biomass which may contribute to the overall zooplan kton grazing efficiency: (1) the significant increase in the mean body size of the dominant cladoceran Daphnia cucullata, calculated for the whole water column, (2) the clear shift in the vertical distribution of Daphnia towards the surface and (3) the increase in mean Daphnia si ze in the surface waters. We postulate that this type of analysis shou ld be performed along with the measures conventionally used in biomani pulation studies, first, as a tool to fully understand the sequence of events generated by manipulations at the top of the food chain and, s econd, as a perfect opportunity to study population and individual res ponses (e.g. morphological, life-history and behavioural) to varying p redatory pressure.