Nitrogen uptake by phytoplankton in the Atlantic sector of the Southern Ocean during late austral summer

Abstract

As part of the Bonus-GoodHope (BGH) campaign, ¹⁵N-labelled nitrate, ammonium and urea uptake measurements were made along the BGH transect from Cape Town to ~60° S in late austral summer, 2008. Our results are categorised according to distinct hydrographic regions defined by oceanic fronts and open ocean zones. High regenerated nitrate uptake rate in the oligotrophic Subtropical Zone (STZ) resulted in low <i>f</i>-ratios (<i>f</i> = 0.2) with nitrogen uptake being dominated by &rho;urea, which contributed up to 70 % of total nitrogen uptake. Size fractionated chlorophyll data showed that the greatest contribution (>50 %) of picophytoplankton (<2 μm) were found in the STZ, consistent with a community based on regenerated production. The Subantarctic Zone (SAZ) showed the greatest total integrated nitrogen uptake (10.3 mmol m⁻² d^&minus;1), mainly due to enhanced nutrient supply within an anticyclonic eddy observed in this region. A decrease in the contribution of smaller size classes to the phytoplankton community was observed with increasing latitude, concurrent with a decrease in the contribution of regenerated production. Higher <i>f</i>-ratios observed in the SAZ (<i>f</i> = 0.49), Polar Frontal Zone (<i>f</i>= 0.41) and Antarctic Zone (<i>f</i> = 0.45) relative to the STZ (<i>f</i> = 0.24), indicate a higher contribution of NO₃^&minus;-uptake relative to total nitrogen and potentially higher export production. High ambient regenerated nutrient concentrations are indicative of active regeneration processes throughout the transect and ascribed to late summer season sampling. Higher depth integrated uptake rates also correspond with higher surface iron concentrations. No clear correlation was observed between carbon export estimates derived from new production and ²³⁴Th flux. In addition, export derived from ¹⁵N estimates were 2–20 times greater than those based on ²³⁴Th flux. Variability in the magnitude of export is likely due to intrinsically different methods, compounded by differences in integration time scales for the two proxies of carbon export.