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The impact of light and nutrients (such as silicate and iron) availability onnitrogen uptake and primary production vary seasonally and regionally in theSouthern Ocean. The seasonal cycle of nitrogen uptake by phytoplankton in theSouthern Ocean is not fully resolved over an annual scale due to the lack ofwinter in situ measurements. In this study, nitrate and ammonium uptake rateswere measured using 15N tracers during a winter cruise in July 2012 anda summer cruise in February–March 2013. The winter cruise consisted of twolegs: leg 1 extended from Cape Town to the ice margin along the GoodHope lineand leg 2 stretched from the ice margin to Marion Island. The summer cruisewas mostly focused on the subantarctic zone of the Atlantic sector. Inwinter, nitrogen uptake rates were measured at 55 and 1% of the surfacephotosynthetically active radiation (sPAR). The summer uptake rates weremeasured at four light depths corresponding to 55, 30, 10 and 3% sPAR. Theintegrated nitrate uptake rates during the winter cruise ranged from0.17 to 5.20 mmol N m−2 d−1 (average 1.14 mmol N m−2 d−1) while theammonium uptake rates ranged from 0.60 to 32.86 mmol N m−2 d−1 (average6.73 mmol N m−2 d−1). During the summer cruise, the mean-integrated nitrate uptake rate was 0.20 mmol N m−2 d−1 witha range between 0.10 and 0.38 mmol N m−2 d−1. The integratedammonium uptake rate averaged 4.39 mmol N m−2 d−1 and rangedfrom 1.12 to 9.05 mmol N m−2 d−1. The factors controlling nitrogen uptake in winter and summer wereinvestigated. During the winter cruise, it was found that the different nitrogenuptake regimes were not separated by the fronts of the Antarctic CircumpolarCurrent (ACC). Light (in terms of day length) and ammonium concentration had themost influence on the nitrogen uptake. In the summer, increases in the mixed layer depth (MLD) resulted in increased nitrogen uptake rates. This suggeststhat the increases in the MLD could be alleviating nutrient limitationsexperienced by the phytoplankton at the end of summer

A geographical and seasonal comparison of nitrogen uptake by phytoplankton in the Southern Ocean