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

Abstract

As part of the Bonus-Good Hope (BGH) campaign, <sup>15</sup>N-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. Nitrogen uptake (ρN) in the oligotrophic Subtropical Zone (STZ) was dominated by ρ urea, which contributed up to 70 % of ρN. High regenerated ρN in the STZ resulted in low <i>f</i>-ratios (<i>f</i> = 0.2). 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 ρN (10.3 mmol m<sup>−2</sup> d<sup>−1</sup>), mainly due to enhanced light and 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> = 0.2), indicate a higher contribution of ρNO<sub>3</sub> relative to total ρN and potentially higher export production. Greater contribution of regenerated uptake to ∫ρN in the northern sector of the cruise resulted from increased ambient regenerated nutrient concentrations, shallow mixed layers in the north (~40 m) relative to the regions further south (~100 m). Higher ρ∫N rates also correspond with higher surface iron concentrations. No clear correlation was observed between carbon export estimates derived from new production and <sup>234</sup>Th flux. In addition, export derived from <sup>15</sup>N estimates were 2–20 times greater than those based on <sup>234</sup>Th 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

Primary productivity, new productivity, and their relation to carbon flux during two Southern Ocean Gas Exchange tracer experiments

“…There, nitrate and phosphate concentrations have been observed to be similar to those of SO GasEx, while silicate concentrations averaged around 10 mmol m −3 and could be much higher than those of SO GasEx (compare our measurements that were consistently <4 mmol m −3 and often ∼1 mmol m −3 ) [ Whitehouse et al , 1996, 2012; Korb et al , 2012]. SO GasEx ammonium concentrations, on the other hand, were generally much higher than the <1 mmol m −3 often reported for open ocean sites [e.g., Olson , 1980; Atkinson and Whitehouse , 2001; USJGOFS AESOPS , 2003; Cochlan , 2008; Joubert et al , 2011] but comparable with the northern reaches offshore of South Georgia Island in autumn [ Whitehouse et al , 2012].…”

Seasonal Depletion of the Dissolved Iron Reservoirs in the Sub‐Antarctic Zone of the Southern Atlantic Ocean

“…Nevertheless, the water mass characterization (Figure b) between each occupancy is relatively consistent. However, the December occupation exhibited a temperature‐salinity signature of warmer (>9.0 °C), saltier (>34.5) subtropical water (Boye et al, ; Chever, Bucciarelli, et al, ; Joubert et al, ), suggesting an intrusion of Agulhas water. Due to the distinct water mass sampled in December, calculated nutrient inventories do not form part of the budget calculations; profiles are nonetheless included to explore seasonal changes in nutrient inventories.…”

Nitrogen dynamics and phytoplankton community structure: the role of organic nutrients

“…Evidence suggests that although primary production by the smaller size fractions is important and often dominant, especially in oligotrophic regions, the contribution of the larger size fraction increases and often dominates in coastal waters, where NO 3 -levels are higher (Lomas and Glibert 1999a;Maguer et al 2009;Malone 1971;Wilkerson et al 2000). Also, the smaller size fractions appear to contribute more to regenerated N uptake, while the larger size fractions to new N uptake (Glibert et al 2016;Joint et al 1986;Joubert et al 2011). Our data show that in the\10 lm fraction, NO 3 -contributed little to N uptake (up to 28% during spring and summer but generally much lower, Fig.…”