Nitrogen fixation can support a substantial fraction of pelagic primary production in Brazilian waters

Nitrogen fixers are commonly found in near-shore waters off the coast of Brazil in the South Atlantic Ocean (e.g., Trichodesmium spp, unicellular cyanobacteria, diazotroph-diatoms associations). Their relatively high abundances suggest that nitrogen fixation could be substantial in these waters. In February 2018, we did two zonal transects perpendicular to the coasts of Brazil at 5° and 11°S aboard RV Meteor. There, we measured concurrent rates of nitrogen fixation and primary production at 10m depth using stable tracers (¹³C and ¹⁵N) and nitrogen stable isotopes in seston (δ¹⁵N, air ‰) throughout the euphotic layer along with the hydrographic structure of the water column. In both transects, the rates of nitrogen fixation were variable, depicted no clear zonal pattern, and ranged between 0.5 and 10 nmol N L⁻¹ d⁻¹ (0.01−0.14 mg N m⁻³ d⁻¹). Primary production showed the same behavior and ranged between 1 and 8 mg C m^-3 d^-1. Nitrogen fixation supported less than 15% of primary production in all stations but one at 11°S, where it seemed to exceed the requirements of primary producers (>100%). However, the δ¹⁵N of seston was consistently below 4‰ in the euphotic layer, pointing to a persistent supply of nitrogen by nitrogen fixation through time.