Seasonal Variability in Planktonic Food Web Structure and Function of the Northeast U.S. Shelf

Herbivorous consumption of primary production is a key transformation in global biogeochemical cycles, directing matter and energy either to higher trophic levels, export production, or remineralization. Grazing by microzooplankton is often poorly constrained, particularly in dynamic coastal systems. Temperate coastal areas are seasonally and spatially variable, which presents both challenges and opportunities to identify patterns and drivers of grazing pressure. Here we report on two winter and one summer week-long cruises (2018-2019), as part of the new Northeast U.S. Shelf Long-Term Ecological Research program. During both seasons, coastal waters were colder and fresher, and had higher phytoplankton biomass than waters at the shelf break. The phytoplankton community was dominated by large cells in winter and by small cells in summer. Phytoplankton growth rates ranged from < 0.5 d(-1) in winter and up to 1.4 d(-1) in summer and were strongly correlated to temperature, to light availability, and to phytoplankton community size-structure. Grazing rates were not correlated with total chlorophyll a, which points to other biological drivers, including species composition in predator-prey interactions at the first trophic level. The percentage of primary production consumed (%PP) indicated higher trophic transfer in winter (%PP > 50%) than during summer (%PP < 20%), highlighting seasonal shifts in planktonic food web structure and function. These results imply that predictable shifts in environmental conditions can be linked to ecosystem shifts in net primary production. Hierarchies of variability, from localized to interannual and long-term climate driven, can be understood within the context of sustained measurements of ecosystem properties and function.