Food-web manipulation in shallow, eutrophic lakes: Bridging the gap between the whole-lake approach and behavioural and demographic studies

Abstract

Even in the most thorough biomanipulation studies, which cover the response of all trophic levels to manipulation al the top of the food chain, the mechanisms underlying the success of failure of an experiment often remain obscure. The case of a small eutrophic Lake Wirbel in Poland is an example of how useful it could be to apply demographic and behavioural analysis of dominant planktonic herbivores to explain the sequence of events following biomanipulation measures. At a certain stage of a seven-year (1988-1994) biomanipulation experiment, rotenone was applied to remove all fish. Following fish removal, summer algal biomass in the 'edible' fraction of phytoplankton (<30 μm) was on average 2.5 times lower. Besides the significant increase in the density of herbivorous zooplankton (above all the largest cladoceran Daphnia cucullata), we identified three other phenomena underlying the observed reduction in algal biomass which may contribute to the overall zooplankton 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 size in the surface waters. We postulate that this type of analysis should be performed along with the measures conventionally used in biomanipulation studies, first, as a tool to fully understand the sequence of events generated by manipulations al the top of the food chain and, second, as a perfect opportunity to study population and individual responses (e.g. morphological, life-history and behavioural) to varying predatory pressure.