The Agulhas Current enhances the productivity of the subtropical Indian Ocean: evidence from coupled flow cytometry-high sensitivity nitrogen isotope analysis

Kolisa Yola Sinyanya, University of Cape Town, Department of Oceanography, Cape Town, South Africa, Ruan Parrott, University of Cape Town, Oceanography, Cape Town, South Africa, Dr. Raquel Flynn, University of North Carolina at Chapel Hill, Earth, Marine, and Environmental Sciences, Chapel Hill, North Carolina, United States, David Y Walker Dr, Cape Peninsula University of Technology, Conservation and Marine Sciences, Cape Town, South Africa, Yeongjun Ryu, Princeton University, Princeton, United States, Daniel Mikhail Sigman, Princeton University, Geosciences, Princeton, NJ, United States and Sarah Fawcett, University of Cape Town, Oceanography Department, Cape Town, South Africa
The Agulhas Current system plays a crucial role in global overturning circulation and climate. However, little is known of its biogeochemistry, particularly the role of different phytoplankton in carbon production and export. The nitrogen (N) isotope ratio (δ15N) of bulk particles suspended in surface waters in the Agulhas region suggests that recycled ammonium fuels >60% of surface productivity. From a mass balance perspective, such ammonium-fuelled production yields no net carbon export, in contrast to phytoplankton growth supported by nitrate mixed up from below. Interpreting bulk PN δ15N is complicated, however, because this pool includes diverse living and dead autotrophic and heterotrophic material. For samples collected across the Agulhas Current and into the southwest Indian subtropical gyre, we use fluorescence-activated cell sorting to separate prokaryotic and eukaryotic (<20 μm) phytoplankton prior to measuring their δ15N. We find that the prokaryote, Synechococcus, is generally low in δ15N (-0.07 ± 0.5‰), consistent with a high reliance on ammonium. By contrast, eukaryotic phytoplankton, which constitute >50% of the biomass at all stations, have a higher and more variable δ15N (3.0 ± 2.6‰), indicative of high reliance on subsurface nitrate; on average across the region, eukaryotes appear to rely on nitrate for 55% of their N. Eukaryote δ15N is highest within and at the edges of the Agulhas Current where we hypothesize that boundary shear injects subsurface nitrate into the euphotic zone. Here, we estimate that nitrate supports ~80% of eukaryote growth despite its low concentration. In the open subtropical gyre, nitrate fuels a lower, albeit still significant, fraction of eukaryotic productivity (~30%). Our data imply that 1) the Agulhas Current promotes nitrate consumption in the subtropical Indian Ocean, enhancing its productivity, and 2) small eukaryotic phytoplankton exploiting low levels of nitrate is a widespread feature of the subtropical ocean.