INFLUENCE OF SIZE-DEPENDENT C-14 UPTAKE RATES BY PHYTOPLANKTON CELLS IN ZOOPLANKTON GRAZING MEASUREMENTS

C-14 is currently used to label natural phytoplankton food in zooplank ton grazing experiments. Carbon content per unit volume and rate of ca rbon uptake both decrease with cell size, following different regressi ons. When working with multi species phytoplankton assemblages, this p hysiological property results in a disproportional distribution of tra cer to phytoplankton carbon biomass. Because most zooplankton organism s select certain sizes of cells, this disproportionate distribution of tracer results in bias of clearance rates determined on the total phy toplankton population with the C-14 method. This paper combines experi mentally determined clearance rates of the copepods A. tonsa and T. lo ngicornis measured on natural particulate matter distributions in comb ination with theoretical calculations of the C-14 clearance rates. It is shown that in these examples, clearance rates measured by the C-14 method would underestimate the grazing pressure on the phytoplankton c arbon stock by 7 to 24%.