A REFINED DILUTION TECHNIQUE FOR MEASURING THE COMMUNITY GRAZING IMPACT OF MICROZOOPLANKTON, WITH EXPERIMENTAL TESTS IN THE CENTRAL EQUATORIAL PACIFIC

The standard dilution technique can provide unbiased estimates of phytoplankton growth and microzooplankton grazing rates only when certain restrictive assumptions are met. The most important of these assumptions - that grazing impact Varies in direct proportion to the dilution of grazer population density - can be easily violated when clearance rate of individual grazers and/or growth response of the grazer population vary significantly with food concentration over the course of the incubation. We have developed a modified protocol which now allows the dilution technique to be applied unambiguously, even when its original assumptions may be violated. The new protocol uses flow-cytometry measured disappearance of fluorescently labeled tracer cells (FLB or FLA) as an internal measure of 'relative grazing activity' in each dilution treatment. Coefficients of phytoplankton growth and mortality rates are determined from Model II regression analyses of 'net growth' versus 'relative grazing', rather than the usual Model I regressions of 'net growth' versus 'dilution factor'. Tests of this hybrid experimental design in the central equatorial Pacific during an EQPAC cruise in August 1992 gave results essentially identical to the standard dilution interpretation.