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Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Søbygaard

The impact of fish-mediated changes on the structure and grazing of zooplankton on phytoplankton and bacterioplankton was studied in Lake S0bygaard during the period 1984-92 by means of in vitro grazing experiments (C-labelled phytoplankton, H-labelled bacterioplankton) and model predictions. Measured zooplankton clearance rates ranged from 0-25 ml I' h" on phytoplankton to 0-33 ml H h" on bacterioplankton. The highest rates were found during the summer when Daphnia jpp. were dominant. As the phytoplankton biomass was substantially greater than that of bacterioplankton throughout the study period, ingestion of phytoplankton was 26-fold greater than that of bacterioplankton. Multiple regression analysis of the experimental data revealed that Daphnia spp., Bosmina longirostru and Cyclops vidnus, which were the dominant zooplankton, all contributed significantly to the variation in ingestion of phytoplankton, while only Daphnia spp. contributed significantly to that of bacterioplankton. Using estimated mean values for clearance and ingestion rates for different zooplankters, we calculated zooplankton grazing on phytoplankton and bacterioplankton on the basis of monitoring data of lake plankton obtained during a 9 year study period. Summer mean grazing ranged from 2 to 4% of phytoplankton production and 2% of bacterioplankton production to maxima of 53 and 88%, respectively. The grazing percentage decreased with increasing density of planktrvorous fish caught in August each year using gill nets and shore-line electrofishing.The changes along a gradient of planktivorous fish abundance seemed highest for bacterioplankton. Accordingly, the percentage contribution of bacterioplankton to the total ingestion of the two carbon sources decreased from a summer mean value of 8% in ZJap/tnia-dominated communities at lower fish density to 0.7-1.1% at high fish density, when cyclopoid copepods or Bosmina and rotifers dominated. Likewise, the percentage of phytoplankton production channelled through the bacteria varied, it being highest (5-8%) at high fish densities. It is argued that the negative impact of zooplankton grazing on bacterioplankton in shallow lakes is highest at intermediate phosphorus levels, under which conditions Daphnia dominate the zooplankton community.