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The superiority of large zooplankton in suppressing phytoplankton growth has often been inferred from the Size Efficiency Hypothesis (S.E.H.). The S.E.H. has originally been formulated to account for the competitive superiority of large to small zooplankton under food limiting conditions. Extrapolation of its predictions to the suppression ofphytoplankton by zooplankton under high food availability, should bedone with care. In an attempt to assess the relevance of the S.E.H. to biomanipulation theory in hypertrophic systems, a fish exclosure experiment was carried out in which the efficiency of two differently structured zooplankton communities in reducing phytoplankton biomass was examined. By inoculating part of the enclosures with laboratory grown Daphnia magna, a community dominated by this large cladoceran species could be compared with a community mainly consisting of Bosminaand smaller Daphnia species. After the exclusion of fish, there was an exponential increase of total zooplankton biomass. Phytoplankton growth was efficiently suppressed to equal levels in both treatments, though there was a difference in timing: chlorophyll-a levels in the enclosures inoculated with D. magna dropped one week earlier than in non-inoculated enclosures. The time-lag was even more pronounced whenlarge phytoplankton was considered. In accordance with the S.E.H., the time lags could be explained by differences in population growth potential as well as by differences in zooplankton grazing rates (indirectly measured as the minimal zooplankton biomass needed to suppressphytoplankton growth) and food particle size range. [ABSTRACT FROM AUTHOR]

Cladoceran populations can respond to changing predation regimes by a phenotypical response as well as by shifts in genotype frequencies.In this study, we investigated the phenotypic plasticity exhibited by life history traits of D. galeata in response to the presence of predator kairomones, as well as the extent to which natural selection may act on these traits and their phenotypic plasticity, In a life-table experiment, seven clones of a natural D. galeata population were subjected to kairomones from fish (Perca), from an invertebrate predator (Chaoborus) or a mixture of both. Life history traits were affected by the kairomones of both predators, but effects of Chaoborus were neutralised by Perca in the kairomone mix. No apparent trade-off was found between growth- and reproduction related traits: although daphnids from the Chaoborus treatment grew faster than daphnids from the other treatments, no reduction in the reproductive output was observed. Broad-sense heritabilities were found to be relatively high for some life history traits (size at maturity, neonate size, number of neonates) as well as for the phenotypic plasticity response of these traits. This reflects the evolutionary potential of life history traits and their phenotypic response to predator kairomones in the D. galeatapopulation. [ABSTRACT FROM AUTHOR]