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 crui se in the Pacific sector of the Southern Ocean (NBP94-5) was completed , complementing two austral summer cruises (February-March) in 1992 (W OCE S4-P) and 1994 (NBP94-2) in the same area. During all three cruise s, the partial pressure of carbon dioxide and concentration of total c arbon dioxide dissolved in seawater were determined throughout the wat er column, accompanied by oxygen and nutrient measurements. Seasonal c hanges in CO2 and nutrients observed in the mixed layer above the remn ant winter water were used to estimate utilization ratios, primary pro ductivity, and new production. The mean rate of primary production ove r 120 days ranged from 6 to 24 mmol Cm-2 d(-1), consistent with the no tion of the Southern Ocean as a low productivity area. Primary product ion was highest in the northern limb of the Ross Sea Gyre, where the w intertime 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 o f biogenic silica to carbon was found to be 0.66 +/- 0.02. The observe d N/P utilization ratio of 13.0 +/- 1.2 and C/P utilization ratio of 9 1.4 +/- 7.9 are both significantly lower than the traditional Redfield ratios of 16 and 106, respectively. The former is, however, consisten t with the Delta N/Delta P regeneration ratio of 14.0 +/- 0.3 discerne d for deep waters south of the Polar Front. Hence, the classical C/P a nd N/P Redfield ratios do not appear to hold in Antarctic waters. On t he other hand, the C/N utilization ratio of 6.9 +/- 0.6 does agree wit h 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 degrees S. Accordingly, the biolo gical cycles of carbon and nitrogen in the Southern Ocean appear to be ''Redfieldian''. (C) 1998 Elsevier Science Ltd. All rights reserved.