Plankton population dynamics and food web structure on the Northeast US Shelf (NES-LTER): early indication of seasonal shifts in production regimes

Within the framework of the Northeast US Shelf (NES) Long-Term Ecological Research (LTER) project, we investigated plankton population dynamics measuring phytoplankton growth and microzooplankton grazing rates. During two winter (Feb. 2018 and Feb. 2019) and a summer (Jul. 2018) week-long cruises, we performed series of dilution experiments (at 5-6 stations located along a transect from Martha’s Vineyard to the shelf break) to quantify in-situ phytoplankton growth and grazing mortality. During both winter and summer, coastal waters were colder and fresher than waters at the shelf break. The phytoplankton community structure was dominated by large cells (> 20 µm) in winter and by small cells (< 5 µm) in summer, with higher biomass near the coast than offshore. Growth rates (µ) appeared to be temperature dependent with values below 0.5 d^-1 in winter and up to 1.4 d^-1 in summer, whereas grazing rates (g) were not. Phytoplankton community size structure was another critical driver to predict phytoplankton growth. The percentage of primary production consumed (%PP) suggested a more efficient transfer of energy at the first trophic levels in winter (%PP > 50%) than during summer (%PP < 20%), suggesting high trophic transfer in winter and a microbial loop dominated food web during summer resulting in low export of carbon across trophic levels. We observed spatial variability along the shelf between coastal and offshore waters, as well as interannual variability between winter cruises. Such variability supports the need for an optimized sampling strategy over the shelf and the requirement of long-term research efforts to identify the mechanisms that control primary production and its relative contribution to trophic transfer and carbon export.