Abstract:
The Southern Ocean is a High Nutrient Low Chlorophyll (HNLC) region and comprises about 15% of the world's ocean area. It is characterised by high nitrate concentrations in surface waters but with phytoplankton biomass remaining fairly low throughout the growing season. While nitrogen (N) is the limiting nutrient for primary production in most of the global ocean, the Southern Ocean is thought to be limited by a combination of iron and light availability. Nonetheless, evaluation of the Southern Ocean's role in global ocean fertility and carbon (C) sequestration relies on measurements of N cycle processes. Phytoplankton primary production supported by nitrate that is supplied to the euphotic zone (the sunlit upper layer) from the deep ocean is termed "new production" and primary production fuelled by N that is recycled within the euphotic zone (e.g., ammonium) is termed "regenerated production". Annually, new production is balanced by the export of sinking organic matter from surface waters, which acts to sequester C in the ocean interior. Quantifying Southern Ocean new production thus speaks to CO2 removal by an oceanic region that is disproportionately important for global C cycling and climate. In addition, given that our view of Southern Ocean productivity is currently biased towards the summer, there is a need to investigate the N cycle of the winter season in order to fully understand the role of the Southern Ocean in global ocean fertility. On two cruises aboard the R/V SA Agulhas II in winter 2015 and summer 2015/2016, new and regenerated N uptake, net primary production and C export were quantified using 15N and 13C tracer techniques. Here, we present a summer-winter comparison from the Atlantic Southern Ocean of total, new, and regenerated production rates throughout the euphotic zone, and discuss the implications for seasonal and annual Southern Ocean productivity and C removal. In addition, regional differences in uptake rates are discussed. Finally, the potential for euphotic zone N regeneration (i.e., remineralization of ammonium and nitrification) is considered with respect to our estimates of new production and C export.
