TAXON-SPECIFIC ANALYSIS OF MICROZOOPLANKTON GRAZING RATES AND PHYTOPLANKTON GROWTH-RATES

We performed laboratory experiments to test the accuracy of using HPLC to estimate taxon-specific grazing and growth rates obtained by the d ilution technique. In seven of nine experiments, this technique undere stimated microzooplankton grazing rates. The average estimated grazing rates were 52% of the true grazing rates for chlorophyll a and 33% fo r taxon-specific pigments. With the food sources Phaeodactylum tricorn utum, Isochlysis galbana, Gymnodinium sp., and Emiliania huxleyi, the clearance rates for Oxyrrhis marina were between 2 x 10(-3) and 7 x 10 (-3) ml grazer(-1) d(-1), at initial food concentrations of similar to 5 x 10(3) cells ml(-1). At higher food concentrations (1 x 10(5) cell s ml(-1)) clearance rates decreased to similar to 1 x 10(-4) ml grazer (-1) d(-1). In field experiments performed in September 1991 and March 1992 in Monterey Bay, California, 21-55% of the phytoplankton standin g stock was consumed daily by microzooplankton grazing. The underestim ation of grazing rates in the laboratory experiments implies that rate s obtained in field incubations may be in error.