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 Investigati ons and fourteen subsequent cruises, have allowed the description of s patial and temporal variability of temperature, salinity, phosphate, s ilicate, nitrate and chlorophyll a in the surface waters around the su bantarctic island of South Georgia. Measurements made in Antarctic Cir cumpolar Current water were compared with others made in Weddell Sea w ater, and profiles from shelf, shelf-slope and oceanic sites were cons idered separately. In summer, Weddell Sea surface water was significan tly colder than that of the Antarctic Circumpolar Current (1.66 and 2. 59 degrees C, respectively), but no changes of temperature corresponde d with bathymetry. There were no systematic differences between the sa linity measurements made in Weddell Sea surface water and those in Ant arctic 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 t emperature, and Weddell Sea surface water concentrations were substant ially higher than those of the Antarctic Circumpolar Current (51 and 2 5 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 syst ematic differences in any of the nutrient levels were attributable to bathymetry. Although summer chlorophyll a levels appeared to be highes t in Antarctic Circumpolar Current water over the shelf and shelf-slop e (2.7-3.0 mg m(-3)), no significant differences were attributable to water-mass or bathymetry. A clear seasonal pattern was evident, with t he warmest seawater conditions, minimum nutrient concentrations and hi ghest chlorophyll a levels found between December and March. Phosphate and nitrate were never exhausted: the lowest recorded phosphate conce ntrations were around 0.6 mmol m(-3) and for nitrate 11 mmol m(-3). Ho wever, low concentrations of silicate (similar to 1.0 mmol m(-3)) were evident during some summer surveys from the 1920s through to the pres ent day. Average nutrient deficits calculated either between winter an d summer mixed-layer concentrations, or between summer mixed-layer and T-min 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 sat isfactory 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 J anuary 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 condition s around South Georgia in summer and the preceding winter's fast-ice d uration at the South Orkney Islands, which implied that some of this v ariability was attributable to large-scale change over the Scotia Sea as a whole, as opposed to local influences. Copyright (C) 1996 Elsevie r Science Ltd.