Amazon River influence on nitrogen fixation and export production in the western tropical North Atlantic

As part of the ANACONDAS program, we investigated the role of the Amazon plume in stimulating offshore nitrogen fixation and export production during the river's high-discharge period (May-June 2010). Using the shipboard underway system, we performed high-resolution sampling of over 450,000 km(2) of surface waters, characterizing the distribution of nutrients, phytoplankton, particulate organic matter (POM), and stable carbon and nitrogen isotopes of POM in the offshore plume. We found distinct regional variations in diazotroph communities, with the Diatom-Diazotroph Associations (DDA) Hemiaulus hauckii - Richelia intracellularis dominating the low N : P mesohaline waters to the northwest of the plume axis and Trichodesmium spp. primarily occupying oceanic waters to the east. Nutrient availability broadly shaped diazotroph distributions along the salinity gradient, but habitat longevity may also play a role in the finer-scale distributions of communities, particularly of DDAs. H. hauckii and Trichodesmium spp. affected the nitrogen and carbon budgets in fundamentally different ways within the plume-influenced regions, with H. hauckii making much greater contributions to the particulate nitrogen pool and to CO2 drawdown than Trichodesmium spp., leading to much higher export fluxes. Our findings provide an important constraint on the role of the Amazon plume in creating distinct niches and roles for diazotrophs in the nutrient and carbon budgets of the western tropical North Atlantic.