Relevance of sporadic upwelling events on primary productivity the key role of nitrogen in a gulf of SW Atlantic Ocean

Abstract In order to assess the relevance of sporadic upwelling events on primary productivity, the availability of nitrogen (N) sources and their uptake by phytoplankton were studied. Incubations of 15N-enriched samples were performed at 3 depths in 12 stations in San Jorge Gulf (SJG, Patagonia Argentina) aboard of the R/V “Puerto Deseado” during November 2016. Additional coastal stations were sampled to analyze the upwelling event. The total uptake of N sources (ρNO3- + ρNH4+) was higher on the surface than in the deep layer. The ρNO3- by phytoplankton doubled that of ρNH4+ suggesting SJG acted as a CO2 sink during the period studied. However, NH4+ played an important role in maintaining primary productivity where NO3− was low. In most stations, phytoplankton showed a preference for the uptake of the less available N source, likely resulting from adaptation to low nutrient concentrations. However, in the southwestern sector of the gulf, where a wind-induced upwelling occurred, phytoplankton preferred to uptake NO3−, this being the most abundant N source. As a consequence, the integrated primary productivity in the water column of that sector was high. Results suggest that such high productivity was governed by the optimal relationship between the preference and availability of N, established by the presence of elevated NO3− and low NH4+ ambient concentration. This would be a physiological process regardless of community composition. The new primary productivity generated by upwelling events could be key to feeding the biological resources of the gulf.