Chemical tracers of biological processes in shallow waters of North Pacific: Preformed nitrate distributions

Distributions of nitrate and Apparent Oxygen Utilization in the upper subtropical North Pacific Ocean reveal a layer with negative values of preformed nitrate. This layer occurs at depths just below the 1% light level and above the density of sigma theta 25.6. We show that large-scale spatial patterns in the distribution of this feature are determined by an interaction between light penetration and the depth of isopycnal surfaces which are ventilated in nutrient rich surface waters. Although the data alone are insufficient to distinguish between several possible causes, we believe the geographic and depth distributions of the negative preformed nitrate feature are most readily explained by respiration of nitrogen-poor dissolved organic matter (DOM) from the surface ocean with the possible accompaniment of nitrate uptake. Dissolved organic carbon gradients and transport calculations suggest that a significant fraction of the carbon flux out of the euphotic zone may be via DOM, indicating that the processes responsible for creating the negative preformed nitrate feature could alter the metabolite stoichiometry in upper subtropical Pacific Ocean