Award: OCE-1737128

The plume of the Amazon River, the largest river in the world, covers an area greater than a millions square kilometers for a few months of the year and thus impacts the biogeochemistry and productivity of the Western Tropical North Atlantic (WTNA). The plume supplies essential nutrients but is impoverished in nitrogen compared to phosphorus and thus setting up the WTNA as a major hot spot for nitrogen fixing cyanobacteria (diazotrophs). The goal of this project was to understand the processes that promote distinct communities of diazotrophs and other phytoplankton and to understand the processes that lead to niche separation. We conducted 11 nutrient amendment experiments in the plume waters to measure phytoplankton response to additions of N, P, Si and combinations of these nutrients at salinities ranging from 19.1 to 35.9 PSU. Throughout the Amazon plume, the phytoplankton communities are serially limited, first by nitrogen and then by phosphorous. Though the amount of growth varied, the communities showed significant growth in all of the treatments with addition of nitrogen with even greater growth when nitrogen and phosphorus were added in conjunction. The composition of the community changed, with cyanobacteria comprising a smaller percentage at the end of the 48 hour experiment. However, this change is not directly linked to the addition of any specific nutrient(s). Generally, we found the highest abundances of Synechococcus spp., picoeukaryotes, and hetrotrophic bacteria with little or no Prochlorococcus spp in the freshest surface plume waters (15-28 ppt). In the transition of surface salinities from 28 ppt to 32 ppt, a population of Prochlorococcus spp. began to form below the surface plume while Synechococcus spp. abundances at the surface remained unchanged and picoeukaryotes, and heterotrophic bacteria abundances decreased. As the surface salinity climbed over 32 ppt, the Prochlorococcus spp. abundance was uniformly high throughout the euphotic zone. On the other hand, Synechococcus spp. abundances at the surface gradually decreased as salinity increased from 32 ppt, while picoeukaryote and hetrotrophic bacterial abundances remained constant. We generated a framework for dynamic delineation of phytoplankton habitats in river plumes and outlying oceanic waters and evaluated the phytoplankton communities that occupy them. The method is based on five biologically relevant and yet commonly measured environmental variables ? sea surface temperature, sea surface salinity, mixed layer depth, depth of the chlorophyll maximum, and nitrate availability to the euphotic zone. These variables span both conservative and emergent properties of a system, meaning that this approach is both simple to implement and widely applicable. When applied to the Amazon River plume, we identified seven niches ? river input (RI), young plume core (YPC), old plume core (OPC), western plume margin (WPM), eastern plume margin (EPM), and oceanic water (OSW). Using data from three previous cruises in addition to the two conducted as a part of this project showed that these niches are robust across years and seasons. Our habitat classifications were consistent with major patterns of the phytoplankton community composition in the Amazon River Plume region, generating consistent niches that support distinct phytoplankton communities. These community differences were most clear at the high taxonomic level captured by HPLC diagnostic pigments and in-vivo fluorescence measurements made with CLASS. Group-specific direct counts differed less among habitats, which may reflect a mismatch between often rapid changes in physical and chemical properties and the growth responses of the phytoplankton communities. Last Modified: 05/15/2022 Submitted by: Ajit Subramaniam