Description of the biogeochemical features of the subtropical southeastern Atlantic and the Southern Ocean south off South Africa during the austral summer of the International Polar Year

Abstract

Abstract. Meridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (chl a) phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively) four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC) and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of chl a and phaeopigments never exceeded, 0.5 μg l−1 and 0.25 μg l−1 respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, of the nutrients isolines. Mesoscale eddies can bring episodic pulse of nutrients into the photic zone. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area chl a level of 0.4–0.5 μg l−1 distributed homogenously within the mixed layer, and POC and PON accumulated to values up to 10 μM and 1.5 μM, respectively; still indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a postbloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 to values up to 1.25 μM at 100 m depth centred on the PF and the accumulation of BSi up to 0.5 μM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. Similar southward displacement of the silicic acid depletion beyond the PF has been previously observed throughout the productive period, associated with the development and extension of the seasonal bloom of diatoms. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9 ± 6.5 mmol m−2 d−1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre occasionally displayed relative surface depletion of silicic acid suggesting a late stage of a relatively minor diatom bloom, possibly driven by iceberg drifting releases of iron. An accumulation of BSi up to 0.5 μM was recorded in the top 350 m of the southern branch of the ACC and in the Weddell Gyre which may be seen as the presence of heavily silicified diatoms due to lack of iron in this HNLC area. In this domain the estimated range of silica daily net production rate (e.g. 21.1 &plusmn 8.8 mmol m−2 d−1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region.