The production and fate of phytoplankton size fractions in the plume of the Hudson River, New York Bight1

Abstract

Seasonal effects of water column stability, phytoplankton growth, and copepod grazing on the biomass of phytoplankton size fractions are described in the context of variations in particulate organic carbon (POC). Net plankton biomass was more variable in time (between weeks and months), while nanoplankton biomass was more variable in space (between surface and bottom layers and between stations). Biomass was highest and most uniformly distributed during February–March when net plankton diatoms dominated and doubling times approximated the flushing time of the plume. Biomass declined rapidly with depth and distance from the mouth of the estuary during June–July when nanoplankton dominated and doubling times were short relative to the flushing time of the plume. Variations in POC were well correlated with phytoplankton biomass when net plankton dominated and biomass was limited by sinking and mixing. When nanoplankton dominated, biomass was limited mainly by grazing and correlations between POC and biomass were poor. It appears that most of the biomass produced by net plankton diatoms sinks from the surface layer before the summer thermocline develops, while most of the biomass produced by nanoplankton during the summer enters pelagic food chains before sinking from the surface layer.