Phytoplankton nutrient uptake, size structure and biomass distribution in the spawning region of Atlantic Bluefin Tuna in the oligotrophic Gulf of Mexico

Western Atlantic Bluefin Tuna (ABT) are an important commercial fishery in the Northern Atlantic. Although ABT migrate throughout the Atlantic Ocean to forage, the western stock spawns almost exclusively in extremely oligotrophic regions of the Gulf of Mexico (GoM). The partitioning of nitrogen sources supporting plankton communities in these regions were previously unknown, with possibilities including nitrogen fixation, nitrate upwelled at the edges of mesoscale eddies, or horizontal advection of nutrients from the Mississippi River Plume. Here we describe results from 2 month-long cruises in the GoM (May 2017 and 2018) conducted in open ocean waters where we used in situ and deckboard incubations along with flow cytometric cell counts of phytoplankton to investigate phytoplankton community composition, primary productivity, nitrate uptake, and ammonium uptake. The phytoplankton community was dominated by Prochlorococcus in all regions during both cruises. Prochlorococcus and Synechococcus abundance showed mixed layer maxima ranging from 15 to 25 meters while pico-eukaryotic phytoplankton were maximal deeper. Preliminary results show that ammonium uptake rates were higher than nitrate uptake rates, suggesting that the community relies primarily on recycled nitrogen. Next generation stock assessment models should incorporate predicted changes in phytoplankton community composition into future recruitment scenarios because a future ocean may become more oligotrophic leading to reduced productivity and food availability at multiple trophic levels throughout the ecosystem.