FISH-INDUCED CHANGES IN ZOOPLANKTON GRAZING ON PHYTOPLANKTON AND BACTERIOPLANKTON - A LONG-TERM STUDY IN SHALLOW HYPERTROPHIC LAKE SOBYGAARD

The impact of fish-mediated changes on the structure and grazing of zo oplankton on phytoplankton and bacterioplankton was studied in Lake So bygaard during the period 1984-92 by means of in vitro grazing experim ents (C-14-labelled phytoplankton, H-3-labelled bacterioplankton) and model predictions. Measured zooplankton clearance rates ranged from 0- 25 ml(-1) h(-1) on phytoplankton to 0-33 ml l(-1) h(-1) on bacteriopla nkton. The highest rates were found during the summer when Daphnia spp . were dominant. As the phytoplankton biomass was substantially greate r than that of bacterioplankton throughout the study period, ingestion of phytoplankton was 26-fold greater than that of bacterioplankton. M ultiple regression analysis of the experimental data revealed that Dap hnia spp., Bosmina longirostris and Cyclops vicinus, which were the do minant zooplankton, all contributed significantly to the variation in ingestion of phytoplankton, while only Daphnia spp. contributed signif icantly to that of bacterioplankton. Using estimated mean values for c learance and ingestion rates for different zooplankters, we calculated zooplankton grazing on phytoplankton and bacterioplankton on the basi s of monitoring data of lake plankton obtained during a 9 year study p eriod. Summer mean grazing ranged from 2 to 4% of phytoplankton produc tion and 2% of bacterioplankton production to maxima of 53 and 88%, re spectively. The grazing percentage decreased with increasing density o f planktivorous fish caught in August each year using gill nets and sh ore-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 Dap hnia-dominated communities at lower fish density to 0.7-1.1% at high f ish density, when cyclopoid copepods or Bosmina and rotifers dominated . Likewise, the percentage of phytoplankton production channelled thro ugh the bacteria varied, it being highest (5-8%) at high fish densitie s. It is argued that the negative impact of zooplankton gazing on bact erioplankton in shallow lakes is highest at intermediate phosphorus le vels, under which conditions Daphnia dominate the zooplankton communit y.