Seasonal and annual change in seawater temperature, salinity, nutrient and chlorophyll a distributions around South Georgia, South Atlantic
Data collected between 1926 and 1990, during the Discovery Investigations and fourteen subsequent cruises, have allowed the description of spatial and temporal variability of temperature, salinity, phosphate, silicate, nitrate and chlorophyll a in the surface waters around the subantarctic island of South Georgia. Measurements made in Antarctic Circumpolar Current water were compared with others made in Weddell Sea water, and profiles from shelf, shelf-slope and oceanic sites were considered separately. In summer, Weddell Sea surface water was significantly colder than that of the Antarctic Circumpolar Current (1.66 and 2.59°C, respectively), but no changes of temperature corresponded with bathymetry. There were no systematic differences between the salinity measurements made in Weddell Sea surface water and those in Antarctic Circumpolar Current water; however, oceanic waters were always more saline than those over the shelf-slope and shelf (33.91, 33.89 and 33.86, respectively). Silicate levels correlated well with seawater temperature, and Weddell Sea surface water concentrations were substantially higher than those of the Antarctic Circumpolar Current (51 and 25 mmol m−3, respectively, in winter; 29 and 13 mmol m−3 in summer). No such differences were found for phosphate or nitrate, and no systematic differences in any of the nutrient levels were attributable to bathymetry. Although summer chlorophyll a levels appeared to be highest in Antarctic Circumpolar Current water over the shelf and shelf-slope (2.7–3.0 mg m−3), no significant differences were attributable to water-mass or bathymetry. A clear seasonal pattern was evident, with the warmest seawater conditions, minimum nutrient concentrations and highest chlorophyll a levels found between December and March. Phosphate and nitrate were never exhausted: the lowest recorded phosphate concentrations were around 0.6 mmol m−3 and for nitrate 11 mmol m−3. However, low concentrations of silicate (∼ 1.0 mmol m−3) were evident during some summer surveys from the 1920s through to the present day. Average nutrient deficits calculated either between winter and summer mixed-layer concentrations, or between summer mixed-layer and Tmin values, produced similar estimates of carbon fixation for both phosphate and silicate, while nitrate appeared to underestimate carbon production. Phosphate and silicate deficits were considered to be satisfactory predictors of carbon production, which was about 30–40 g C m−2 year−1 in a mixed-layer depth of 50 m. Considerable interannual variability was found, with winter-like conditions prevailing until January on some occasions, and apparent year-to-year variability in the timing and magnitude of nutrient utilisation (especially silicate) by phytoplankton. A relationship was found between sea surface conditions around South Georgia in summer and the preceding winter’s fast-ice duration at the South Orkney Islands, which implied that some of this variability was attributable to large-scale change over the Scotia Sea as a whole, as opposed to local influences.