Changes in Phytoplankton Community Size Structure Across the Shelfbreak Front of the Middle Atlantic Bight

Several upwelling mechanisms are associated with the continental shelfbreak front of the Middle Atlantic Bight that are hypothesized to supply nutrients to the surface ocean and increase primary productivity in the region. However, while increased chlorophyll concentration along the shelfbreak front has been observed in a few synoptic measurements, time-averaged climatologies of the region do not indicate local enhancement of phytoplankton. Grazing by zooplankton may prevent the accumulation of phytoplankton biomass on seasonal timescales by transferring the upwelling-driven productivity to higher trophic levels. We hypothesize that this high-productivity, high-grazing regime is characterized by a higher proportion of large phytoplankton in the community, even in the absence of an increased chlorophyll signal. To test this hypothesis, we conducted three cruises over two years to collect a broad suite of environmental data along a transect crossing the shelfbreak front. We collected phytoplankton community size structure data using both imaging and traditional flow cytometry. We sampled the phytoplankton community at the surface continuously via the ship’s underway system to provide high-resolution measurements across the shelfbreak front. These measurements were supplemented with CTD discrete samples that provided lower resolution measurements of the entire water column. Preliminary analysis of these data indicates that an increase in larger phytoplankton size classes does occur in the vicinity of the shelfbreak front. The degree of this increase appears highly variable in time and may be linked to physical processes, such as interactions with warm core rings, that affect the structure of the front.