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

Even in the most thorough biomanipulation studies, which covertheresponse of all trophic levels to manipulation at the top ofthefood chain, the mechanisms underlying the success or failureof anexperiment often remain obscure. The case of a small eutrophicLakeWirbel in Poland is an example of how useful it could be toapplydemographic and behavioural analysis of dominant planktonicherbivores to explain the sequence of events followingbiomanipulation measures.At a certain stage of a seven-year (1988–1994)biomanipulationexperiment, rotenone was applied to remove all fish. Followingfishremoval, summer algal biomass in the ‘edible’ fraction ofphytoplankton (<30 µm) was on average 2.5 times lower.Besides the significant increase in the density of herbivorouszooplankton (above all the largest cladoceran Daphniacucullata), we identified three other phenomena underlyingtheobserved reduction in algal biomass which may contribute totheoverall zooplankton grazing efficiency: (1) the significantincrease in the mean body size of the dominant cladoceran Daphnia cucullata, calculated for the whole water column,(2) theclear shift in the vertical distribution of Daphniatowardsthe surface and (3) the increase in mean Daphnia size in thesurface waters.We postulate that this type of analysis should be performed alongwith themeasures conventionally used in biomanipulation studies,first, asa tool to fully understand the sequence of events generated bymanipulations at the top of the food chain and, second, as aperfect opportunity to study population and individualresponses(e.g. morphological, life-history and behavioural) to varyingpredatory pressure.