Primary productivity and nutrient utilization ratios in the Pacific sector of the Southern Ocean based on seasonal changes in seawater chemistry

In the austral winter (September–October) of 1994, a hydrographic cruise in the Pacific sector of the Southern Ocean (NBP94-5) was completed, complementing two austral summer cruises (February–March) in 1992 (WOCE S4-P) and 1994 (NBP94-2) in the same area. During all three cruises, the partial pressure of carbon dioxide and concentration of total carbon dioxide dissolved in seawater were determined throughout the water column, accompanied by oxygen and nutrient measurements. Seasonal changes in CO2 and nutrients observed in the mixed layer above the remnant winter water were used to estimate utilization ratios, primary productivity, and new production. The mean rate of primary production over 120 days ranged from 6 to 24 mmol C m-2d-1, consistent with the notion of the Southern Ocean as a low productivity area. Primary production was highest in the northern limb of the Ross Sea Gyre, where the wintertime vertical stability of the upper water column was lower. The circulation patterns and nutrient distributions in the region studied appear to influence the primary production rates observed. The ratio of biogenic silica to carbon was found to be 0.66±0.02. The observed N/P utilization ratio of 13.0±1.2 and C/P utilization ratio of 91.4±7.9 are both significantly lower than the traditional Redfield ratios of 16 and 106, respectively. The former is, however, consistent with the ΔN/ΔP regeneration ratio of 14.0±0.3 discerned for deep waters south of the Polar Front. Hence, the classical C/P and N/P Redfield ratios do not appear to hold in Antarctic waters. On the other hand, the C/N utilization ratio of 6.9±0.6 does agree with the Redfield ratio of 6.6 (106/16). This C/N ratio also agrees with the ratio obtained for surface particulate organic matter (POM) taken during the WOCE S4-P cruise along 67°S. Accordingly, the biological cycles of carbon and nitrogen in the Southern Ocean appear to be “Redfieldian”.