From nutrient deplete to replete: phytoplankton community composition from the oligotrophic open ocean to eutrophic coastal waters

Nutrient availability plays an important role in structuring phytoplankton communities. Although ecosystem models generally predict that phytoplankton species diversity increases from oligotrophic to coastal waters, data detailing shifts in in situ community composition from different nutrient regimes are rare, particularly in the North Atlantic. In order to examine changes in phytoplankton community composition with shifts in nutrient concentrations, a transect was sampled across the Sargasso Sea (SS), Gulf Stream (GS), and coastal waters (CW) of the Northeast shelf. Community composition was determined using a combination of flow imaging microscopy and high-throughput sequencing. Preliminary flow imaging microscopy data revealed increases in eukaryotic species richness and Shannon diversity from the SS to CW mirroring increases in chlorophyll a concentration and an increasing nutrient gradient along the transect. Additional taxonomic resolution will be obtained using high-throughput sequencing. The GS and SS regions were most similar to each other both in terms of nutrient concentrations and community composition and they differed most from CW. We identified several phytoplankton taxa that were present across the entire transect as well as those present in only oligotrophic or coastal waters suggesting that the metabolic capacity of organisms in these communities is likely diverse and also malleable to intermittent and prolonged shifts in nutrient conditions.