抄録
Grazing impacts of cladoceran Simocephalus vetulus population on phytoplankton standing crop were evaluated from the rates of phytoplankton biomass decrease and of organic matter sedimentation during a period between succesive sampling days in four experimental enclosures. Specific grazing rates of animal population estimated from the incorporation rates of 14C labelled algae were correlated with a regression coefficient smaller than 1 with the specific decrease rates of algal biomass and particulate organic carbon in the enclosures. Examination of the budget of organic carbon in the enclosures demonstrated pronounced temporal changes in the proportion of algal biomass increase, sedimentation loss and grazing loss in the net production of phytoplankton. In the initial phase of the experiment, algal increase accounted for 25-80% of the net production, while sedimentation loss for 19-68% and grazing loss for only 0.4-2%. Increasing animal grazing pressure and sedimentation loss after the mid-period of the experiment, which accounted for the consumption of organic matter in 176-418% and 76-170% of the algal net production, were identified as largely responsible for the pronounced decreases of phytoplankton biomass in the enclosures.
