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

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 (NO3<28 μmol kg−1, PO4<2 μmol kg−1, Si(OH)4<80 μmol kg−1, AOU<10 μmol kg−1, TIC<2170 μmol 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 (ΔNO3=7±2 μmol kg−1, ΔPO4=0.5±0.1 μmol kg−1, ΔSi(OH)4=6±3 μmol kg−1, ΔAOU=65±15 μmol kg−1, ΔTIC=40±8 μmol kg−1) and in the Southern Boundary of the ACC front (ΔNO3=10±3 μmol kg−1, ΔPO4=0.6±0.2μmol kg−1, ΔSi(OH)4=1.9±1.6 μmol kg−1, ΔAOU=78±10 μmol kg−1, ΔTIC=69±17 μmol kg−1), which was associated with high biomass. During FRUELA 96 cruise, the highest nutrient consumption also was observed in the Gerlache Strait (ΔNO3=14±3 μmol kg−1, ΔPO4=1.1±0.2 μmol kg−1, ΔSi(OH)4=12±5 μmol kg−1, ΔAOU=94±22 μmol kg−1, ΔTIC=70±16 μmol kg−1), with average chlorophyll a of 7±2 mg m−3. Using a simple approach, we can ascertain whether the estimated nutrient consumption can account for the observed biomass accumulation during the two cruises.