EFFECTS OF ZOOPLANKTON GRAZING ON PHYTOPLANKTON SIZE-STRUCTURE AND BIOMASS IN THE LOWER HUDSON RIVER ESTUARY

The impact of mesozooplankton (>210 mu m, mostly adult copepods and la te-stage copepodites) and micrometazoa (64-210 mu m, mostly copepod na uplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various C-14-labeled algal speci es as tracers of gazing on natural phytoplankton. During spring and su mmer, zooplankton grazing pressure, defined as % = mg C ingested m(-2) h(-1)/mg C produced m(-2) h(-1) (depth-integrated rates) x 100, on to tal phytoplankton ranged between 0.04% and 1.9% for mesozooplankton an d 0.1% and 6.6% for micrometazoa. The greatest grazing impact was meas ured in fall when 20.2% and 44.6%, respectively, of the total depth-in tegrated primary production from surface water phytoplankton was graze d. Mesozooplankton exhibited some size-selective gazing on phytoplankt on, preferentially grazing the diatom Thalassiosira pseudonana over th e larger diatom Ditylum brightwelli, but this was not found for microm etazoa. Neither zooplankton group grazed on the dinoflagellate Amphidi nium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estu ary. Differences in the growth coefficients of various phytoplankton s ize-fractions-not grazing selectivity-may be the predominant factor ex plaining community size-structure.