Mesoscale and Sub-mesoscale Variability in Phytoplankton Community Composition in the Sargasso Sea

The Sargasso Sea is a highly dynamic physical environment in which strong seasonal variability combines with forcing by mesoscale (~ 100 km) eddies. Over four cruises (2011 and 2012), we investigated links between water column structure, nutrient regimes, and phytoplankton community composition at a range of time and space scales in the Sargasso Sea. On all cruises, cyanobacteria (Prochlorococcus and Synechococcus) dominated the phytoplankton numerically and haptophytes were the dominant eukaryotes (up to 60% total chl-a). We demonstrated significant effects of mesoscale and inferred sub-mesoscale forcing on phytoplankton community composition. Downwelling (in anticyclones) was coincident with Prochlorococcus abundances 22-66% higher compared to outside stations. We observed a shift in the taxonomic composition of haptophytes between a coccolithophores dominated community in a cyclone in spring 2012 (C2) and a non-coccolithophore community at BATS. At the center of cyclone C1 (summer 2011), we observed significantly higher haptophyte chl-a (non-coccolithophores) and lower cyanobacteria chl-a biomass at the center and edge as compared to BATS. In summer 2012, downwelling associated with anticyclone AC2 occurred at the edge of the eddy and we found significantly lower Synechococcus abundances and higher eukaryote chl-a compared to the center of AC2 and BATS. These along-transect nuances demonstrate the significance of small-scale perturbations that significantly alter phytoplankton community structure. Therefore, while seasonality in the North Atlantic is the primary driver of broad-scale trends in phytoplankton community composition, the effect of transient events must be considered when studying planktonic food webs in the Sargasso Sea.