On the Role of the Amazon River for N-2 Fixation in the Western Tropical Atlantic

The high rates of N-2 fixation observed in the Western Tropical Atlantic Ocean are powered, at least in parts, by large influx of nutrients from the Amazon River. To disentangle the impact of the Amazon on different factors controlling N-2 fixation in the region, we use a high-resolution regional model (Regional Oceanic Modeling System-Biogeochemical Elemental Cycling model) that includes two diazotrophic phytoplankton classes (Trichodesmium and Diatom-Diazotroph-Assemblages, DDAs). In our simulations, the Amazon enhances marine N-2 fixation by 74% (3.8 Tg N yr(-1)), with most of the enhancement driven by DDAs that represent 90% of the diazotrophic community in plume waters. To determine how the Amazon creates the conditions in which DDAs thrive, we analyze bottom-up and top-down controls on phytoplankton along the plume pathway and how the phytoplankton competition is disrupted by the delivery of nutrients by the river. Along the entire plume pathway, DDAs out-compete Trichodesmium as their higher maximum growth makes them more efficient in building biomass. Nevertheless, DDAs never account for more than 25% of the total phytoplankton biomass as they face the competition of other non-diazotrophic phytoplankton types in the use of the riverine nutrients. Overall, DDAs thrive in the offshore plume region where their nutrient uptake advantages (N-fixation, DOP uptake) is concomitant with a relaxed grazing pressure.