Microzooplankton grazing on phytoplankton in Rhode River, Maryland: noninear feeding kinetics

Abstract

The dilution technique estimates microzooplankton grazing on phytoplankton by reducing encounter rates between the microzooplankton predators and their phytoplankton prey. The technique was extended to the case in which grazing is a nonlinear function of phytoplankton concentration. By allowing microzooplankton clearance rates to vary between, but not within, dilut~on fractions, and assuming microzooplankton density remains constant, it was shown that the grazing impact on apparent phytoplankton growth rate bears a simple relationship to the funchonal response of the microzooplankton. Experiments in the Rhode River. Maryland (USA) demonstrated saturated feeding kinetics. Half-saturation chlorophyll concentrations occurred within a relatively small fraction (0.1 to 0.2) of initial concentrations, despite a 4-fold range in the latter, suggesting tight coupling between microzooplankton communities and their phytoplankton prey. Simulations with time-dependent equations coupling phytoplankton and microzooplankton growth laws indicated that inferences about the shape of the microzooplankton functional response from measurements of phytoplankton apparent growth rate are insensitive to change in microzooplankton density and to changes in clearance rates along the functional response. However, estimates of specific grazing coefficient, g, are very sensitive to these changes Equations were solved for the case when the specific ingestion rate of the microzooplankton is food-saturated. The solution allows for proper estimation of gin eutrophic systems whenever microzooplankton growth rate is observed.