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

Abstract

Observations from the Hawaii Ocean Time series indicate that a large fraction of the nitrogen supply to the euphotic zone is presently from nitrogen fixation. There is growing evidence, some presented here, that the main phosphorus flux is from the upper thermocline via eddy-induced mixing processes. These mechanisms of supply create a non-steady state nutrient flux in which the N:P ratio in particulate organic matter exiting the upper ocean (N:P = 23) is greater than the ratio of dissolved nitrogen to phosphorus that is remineralized in the upper thermocline (N:P = 13-14). Carbon to nitrogen ratios in the particulate export flux have remained between 7 and 8 in spite of these N:P changes, suggesting that the biological carbon export may have increased along with nitrogen fixation even though Redfield Ratio arguments would suggest phosphorus should be limiting. A simple model of the upper ocean circulation and carbon export demonstrates the sensitivity of apparent oxygen utilization (AOU) in the upper thermocline, atmospheric pCO2 and atmospheric pO2 to changes in the carbon pump. Comparison of AOU distributions 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 invoking changes in the biological pump or upper-ocean ventilation by as much as 30-50%. Estimated atmospheric pCO2 and pO2 responses to this change are <10% and ~ 15%, respectively, of the observed trends in the 1990s. 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 biological pump over that expected from nutrient delivery via ocean circulation. This suggests that at least on decadal timescales, the biological pump is not limited by phosphate concentrations in the Redfield ratio (N:P = 15-16).