Abstract:
Phytoplankton growth and diversity are highly dependent on nutrient availability and can be hindered by an inadequate supply of biologically-available forms of nitrogen (N. such as nitrate and ammonium. The Agulhas System Climate Array (ASCA. transect in the Indian Ocean was developed to provide long-term observations of the hydrography of the greater Agulhas Current system, including volume, heat and salt transport. However, little is known of the biogeochemistry of this region, particularly the role of different phytoplankton in carbon production and export. In these subtropical waters, upper ocean density stratification obstructs the upward flux of nitrate into the euphotic zone so that phytoplankton presumably rely mostly on ammonium recycled in surface waters. Over an appropriate timescale, this ammonium-fuelled production yields no net carbon export to deep waters, in contrast to growth supported by upwelled nitrate, which is quantitatively linked to carbon removal. Here, we use fluorescence-activated cell sorting to separate prokaryotic and eukaryotic phytoplankton from bulk particle samples collected along the ASCA transect during winter 2016. Subsequent measurements of the organic N isotopes of these populations can be used to deduce the dominant N source supporting their growth, with the expectation that eukaryotic phytoplankton will be the primary consumers of nitrate. Preliminary results show that on average, eukaryotic phytoplankton abundance decreases with distance from the coast along with a decline in the nitrate concentration, while the prokaryotic genus, Synechococcus, becomes more dominant as conditions become more oligotrophic. Both populations are most abundant at the southern edge of the Agulhas Current, likely due a higher upward supply of nutrients driven by boundary shear. This is supported by depth profiles of chlorophyll and the concentration and N isotopes of nitrate that indicate higher biomass accumulation and a higher degree of nitrate consumption in current-edge waters. - Abstract as displayed in the - Abstract booklet. The presentation on the day may differ from the - Abstract.
