Seasonal variability of the microzooplankton biomass and community composition on the Northeast US Shelf

Abstract Grazing on phytoplankton by microzooplankton is a critical process in making energy available to higher trophic levels, shaping marine food webs, and biogeochemical cycling. Despite its key role, the drivers of grazing remain poorly constrained due to the concurrent influences of biotic and abiotic factors. To elucidate the impact of microzooplankton biomass and community composition on grazing rates, we measured ciliate and dinoflagellate biomass in boreal summer and winter from 2018-2023, at three stations spanning the Northeast US Shelf from coastal Rhode Island, USA to the shelf break. This record of microzooplankton biomass encompasses the seasonal and spatial variability from shore to shelf. Dinoflagellate biomass showed a clear seasonal shift from larger (>20 μm) cells in winter to smaller ones in summer, while ciliate biomass lacked seasonal trends. Anomalies in community composition coincided with disruptions in the typical seasonal phytoplankton size structure, suggesting that microzooplankton size structure was linked to the size of available prey. In winter, microzooplankton biomass was positively correlated with microzooplankton grazing, indicating that the mechanism moderating the planktonic food web structure changes seasonally. These results inform our understanding of planktonic food webs by identifying the linkage between grazing and seasonal changes in phytoplankton community structure that impact energy transfer to higher trophic levels.