THE RELEVANCE OF SIZE EFFICIENCY TO BIOMANIPULATION THEORY - A FIELD-TEST UNDER HYPERTROPHIC CONDITIONS

The superiority of large zooplankton in suppressing phytoplankton grow th has often been inferred from the Size Efficiency Hypothesis (S.E.H. ). The S.E.H. has originally been formulated to account for the compet itive superiority of large to small zooplankton under food limiting co nditions. Extrapolation of its predictions to the suppression of phyto plankton by zooplankton under high food availability, should be done w ith care. In an attempt to assess the relevance of the S.E.H. to bioma nipulation 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 examine d. By inoculating part of the enclosures with laboratory grown Daphnia magna, a community dominated by this large cladoceran species could b e compared with a community mainly consisting of Bosmina and smaller D aphnia species. After the exclusion of fish, there was an exponential increase of total. zooplankton biomass. Phytoplankton growth was effic iently suppressed to equal levels in both treatments, though there was a difference in timing: chlorophyll-a levels in the enclosures inocul ated with D. magna dropped one week earlier than in non-inoculated enc losures. The time-lag was even more pronounced when large phytoplankto n was considered. In accordance with the S.E.H., the time lags could b e explained by differences in population growth potential as well as b y differences in zooplankton grazing rates (indirectly measured as the minimal zooplankton biomass needed to suppress phytoplankton growth) and food particle size range.