Chlorophyll/nutrient characteristics in the water masses to the north of South Georgia, Southern Ocean

Abstract

Chlorophyll a and nutrient concentrations along with temperature and salinity values were measured at 22 CTD stations along a 735-km transect running to the northwest of the island of South Georgia, Southern Ocean. Measurements were repeated during five summer surveys (January and February 1994, January 1996, December 1996, January 1998) and one spring survey (October 1997). The transect sampled Sub-Antarctic Zone water in the north, Polar Frontal Zone water and Antarctic Zone water in the south. Chlorophyll a concentrations were lowest to the north of the transect and frequently high (up to 17 mg m−3) in the deep open ocean of the Antarctic Zone. Sub-surface peaks were measured in all zones and chlorophyll a was detectable to a depth of 150 m. There was a clear latitudinal temperature gradient in the near-surface waters (0–50 m), the warmest water occurring in the north (∼12 °C), and the coolest in the Antarctic Zone (∼2 °C). There was also a well-defined latitudinal gradient in summer near-surface silicate concentrations (∼2, 4, and 10 mmol m−3 in the Sub-Antarctic Zone, the Polar Frontal Zone and the Antarctic Zone, respectively), increasing to >20 mmol m−3 near South Georgia. Distinct differences in silicate concentrations were also evident in all three zones to a depth of 500 m. Near-surface nitrate and phosphate concentrations were relatively low to the north of the transect (∼14 and 1 mmol m−3, respectively) and higher in the Polar Frontal Zone and Antarctic Zone (∼18 and 1.4 mmol m−3, respectively). Ammonium and nitrite were restricted to the upper 200 m of the water column, and exhibited sub-surface concentration peaks, the lowest being in the Sub-Antarctic Zone (0.68 and 0.25 mmol m−3, respectively) and the highest in the Antarctic Zone (1.72 and 0.29 mmol m−3, respectively). Surface (∼6 m) spring nutrient measurements provided an indication of pre-bloom conditions; ammonium and nitrite concentrations were low (∼0.27 and 0.28 mmol m−3, respectively), while silicate, nitrate and phosphate concentrations were high and similar to previously measured winter values (e.g. ∼26, 23, 2 mmol m−3, respectively in the Antarctic Zone). Although the values measured were very variable, and there was some evidence of a seasonal growth progression, the chlorophyll a and nutrient distribution patterns were dominated by intercruise (interannual) factors. Approximate nutrient depletions (spring minus summer) appeared similar in the Polar Frontal Zone and Antarctic Zone for nitrate and phosphate, while silicate showed a marked latitudinal increase from north to south throughout the transect. Highest chlorophyll a concentrations coincided with the highest apparent silicate depletions over the deep ocean of the Antarctic Zone. In this area, relatively warm, easterly flowing Antarctic Circumpolar Current water meets cooler, westerly flowing water that is influenced by the Weddell-Scotia Confluence and is rich in nutrients, especially silicate.