The biological pump in the subtropical North Pacific Ocean: Nutrient sources, Redfield ratios, and recent changes

Observations from the Hawaii Ocean Time series indicate that a large fracti on of the nitrogen supply to the euphotic zone is presently from nitrogen f ixation. There is growing evidence, some presented here, that the main phos phorus flux is from the upper thermocline via eddy-induced mixing processes . These mechanisms of supply create a non-steady state nutrient flux in whi ch the N:P ratio in particulate organic matter exiting the upper ocean (N:P = 23) is greater than th,. ratio of dissolved nitrogen to phosphor-us that is remineralized in the upper thermocline (N:P = 13-14). Carbon to nitroge n ratios in the particulate export flux have remained between 7 and 8 in sp ite of these N:P changes, suggesting that the biological carbon export may have increased along with nitrogen fixation even though Redfield Ratio argu ments would suggest phosphorus should be limiting. A simple model of the up per ocean circulation and carbon export demonstrates the sensitivity of app arent oxygen utilization (AOU) in the upper thermocline, atmospheric pCO(2) and atmospheric pO(2) to changes in the carbon pump. Comparison of AOU dis tributions from four meridional transects through the northeast subtropical Pacific between 1980 and 1997 suggests there has been an increase of 20-25 % over the past two decades. These results cannot be explained without invo king changes in the biological pump or upper-ocean ventilation by as much a s 30-50%. Estimated atmospheric pCO(2) and pO(2) responses to this change a re < 10% and similar to 15%, respectively, of the observed trends in the 19 90s. Taken together, changes in the organic matter C:N:P ratios and in the AOU within the upper thermocline of the eastern North Pacific indicate that the onset of nitrogen fixation has resulted in an increase in the biologic al pump over that expected from nutrient delivery via ocean circulation. Th is suggests that at least on decadal timescales, the biological pump is not limited by phosphate concentrations in the Redfield ratio (N:P = 15-16).