Phytoplankton community composition and biomass in the oligotrophic Gulf of Mexico Journal of Plankton Research

Biomass and composition of the phytoplankton community were investigated in the deep-water Gulf of Mexico (GoM) at the edges of Loop Current anticyclonic eddies during May 2017 and May 2018. Using flow cytometry, high-performance liquid chromatography pigments and microscopy, we found euphotic zone integrated chlorophyll a of ~10 mg m−2 and autotrophic carbon ranging from 463 to 1268 mg m−2, dominated by picoplankton (<2 μm cells). Phytoplankton assemblages were similar to the mean composition at the Bermuda Atlantic Time-series Study site, but differed from the Hawaii Ocean Times-series site. GoM phytoplankton biomass was ~2-fold higher at the deep chlorophyll maximum (DCM) relative to the mixed layer (ML). Prochlorococcus and prymnesiophytes were the dominant taxa throughout the euphotic zone; however, other eukaryotic taxa had significant biomass in the DCM. Shallower DCMs were correlated with more prymnesiophytes and prasinophytes (Type 3) and reduced Prochlorococcus. These trends in ML and DCM taxonomic composition likely reflect relative nutrient supply—with ML populations relying on remineralized ammonium as a nitrogen source, and the taxonomically diverse DCM populations using more nitrate. These spatially separated phytoplankton communities represent different pathways for primary production, with a dominance of picoplankton in the ML and more nano- and microplankton at the DCM. Grant no. NA15OAR4320071 Grant no. NA16NMF4320058