Nutrient utilisation and chlorophyll distribution in the Atlantic sector of the Southern Ocean during Austral summer 1995-96

Abstract

The chemical fields in the Eastern Bellingshausen Sea, Gerlache and Bransfield Straits were studied for nutrients, apparent oxygen utilisation (AOU) and total inorganic carbon (TIC) in the upper mixed layer of the water column during the FRUELA 95 (December 1995–January 1996) and FRUELA 96 (January–February 1996) cruises. In Bellingshausen Sea area, a strong silicic acid gradient clearly traced the Southern Boundary of the ACC front during the two cruises. In the Bransfield Strait, the lowest concentrations of nutrients, AOU and TIC (NO3o28 mmol kg 1, PO4o2 mmol kg 1, Si(OH)4o80 mmol kg 1, AOUo10 mmol kg 1, TICo2170 mmol kg 1) corresponded to transition zonal water with Bellingshausen Sea influence. The southeastern part, which was dominated by transition zonal water with Weddell Sea influence, had the highest levels of these properties. In the Gerlache Strait, the chemical field was marked by low-salinity water in the southwestern extreme, which had low nutrient levels. Though the chemical field strongly reflected the thermohaline field, significant nutrient consumption was observed where there was high chlorophyll a and particulate organic matter. During the FRUELA 95 cruise, the highest nutrient consumption in the upper mixed layer was localised in the Gerlache Strait (DNO3=772 mmol kg 1, DPO4=0.570.1 mmol kg 1, DSi(OH)4=673 mmol kg 1, DAOU=65715 mmol kg 1, DTIC=4078 mmol kg 1) and in the Southern Boundary of the ACC front (DNO3=1073 mmol kg 1, DPO4=0.670.2mmol kg 1, DSi(OH)4=1.971.6 mmol kg 1, DAOU= 78710 mmol kg 1, DTIC=69717 mmol kg 1), which was associated with high biomass. During FRUELA 96 cruise, the highest nutrient consumption also was observed in the Gerlache Strait (DNO3=1473 mmol kg 1, DPO4= 1.170.2 mmol kg 1, DSi(OH)4=1275 mmol kg 1, DAOU=94722 mmol kg 1, DTIC=70716 mmol kg 1), with average chlorophyll a of 772mgm 3. Using a simple approach, we can ascertain whether the estimated nutrient consumption can account for the observed biomass accumulation during the two cruises.