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

Chlorophyll a and nutrient concentrations along with temperature and salini ty values were measured at 22 CTD stations along a 735-km transect running to the northwest of the island of South Georgia, Southern Ocean. Measuremen ts were repeated during five summer surveys (January and February 1994, Jan uary 1996, December 1996, January 1998) and one spring survey (October 1997 ). The transect sampled Sub-Antarctic Zone water in the north, Polar Fronta l Zone water and Antarctic Zone water in the south. Chlorophyll a concentra tions were lowest to the north of the transect and frequently high (up to 1 7 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-surfa ce waters (0-50 m), the warmest water occurring in the north (similar to 12 degrees C), and the coolest in the Antarctic Zone (similar to 2 degrees C) . There was also a well-defined latitudinal gradient in summer near-surface silicate concentrations (similar to 2, 4, and 10 mmol m(-3) in the Sub-Ant arctic Zone, the Polar Frontal Zone and the Antarctic Zone, respectively), increasing to > 20 mmol m(-3) near South Georgia. Distinct differences in s ilicate concentrations were also evident in all three zones to a depth of 5 00 m. Nearsurface nitrate and phosphate concentrations were relatively low to the north of the transect (similar to 14 and 1 mmol m(-3), respectively) and higher in the Polar Frontal Zone and Antarctic Zone (similar to 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 p eaks, 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 (similar to 6 m) spring nutrient measurements p rovided an indication of pre-bloom conditions; ammonium and nitrite concent rations were low (similar to 0.27 and 0.28 mmol m(-3), respectively), while silicate, nitrate and phosphate concentrations were high and similar to pr eviously measured winter values (e.g. similar to 26, 23, 2 mmol m(-3), resp ectively in the Antarctic Zone). Although the values measured were very var iable, and there was some evidence of a seasonal growth progression, the ch lorophyll a and nutrient distribution patterns were dominated by intercruis e (interannual) factors. Approximate nutrient depletions (spring minus summ er) appeared similar in the Polar Frontal Zone and Antarctic Zone for nitra te and phosphate, while silicate showed a marked latitudinal increase from north to south throughout the transect. Highest chlorophyll a concentration s coincided with the highest apparent silicate depletions over the deep oce an of the Antarctic Zone. In this area, relatively warm, easterly flowing A ntarctic Circumpolar Current water meets cooler, westerly flowing water tha t is influenced by the Weddell-Scotia Confluence and is rich in nutrients, especially silicate.