Regulation by low temperature of phytoplankton growth and nutrient uptake in the Southern Ocean

During oceanographic cruises in 1996 and 1998, phytoplankton from 15 stations in the southwest Atlantic sector of the Southern Ocean were incubated at ambient temperature and 2 elevated temperatures (ambient plus 3 degreesC, and ambient plus 6 degreesC). Rates of growth, nutrient depletion, N-15-nitrate uptake and nutrient interaction were all studied. Microalgal growth rate showed a strong positive relationship to temperature elevation, indicating ambient temperatures were sub-optimal for the phytoplanktonic community as a whole. Ratios of silicate uptake to chlorophyll a increase were high at ambient temperatures and showed a strong negative relationship with temperature elevation. Nitrate uptake rates, measured by 15N-nitrate incorporation, showed a consistent trend of increased uptake rate at elevated temperature. Specific nitrate-depletion rates, N-15-nitrate uptake rates, and the f ratio all showed an inverse relationship to increasing ammonium concentration. The results of this study imply that ambient temperature, in addition to direct iron limitation, is important in the maintenance of the high nutrient, low chlorophyll conditions common to the Southern Ocean.