Low nitrate: phosphate ratios in the global ocean

The distribution and availability of the nutrients nitrate and phosphate exert a strong control over primary production in the world ocean. Here we use a recently compiled global data set of oceanographic observations—a database that is at least 40 times larger than currently used data sets (for example, ref. 2)—to analyse the variation of the nitrate:phosphate concentration ratio with depth and geographical location. Although the nutrient distributions confirm the dominant influence of aerobic decomposition, in agreement with the observations of Redfield, we also identify a hitherto unreported secondary trend at low nitrate: phosphate ratio (2–3). These conditions of low nitrate:phosphate ratio are associated with low oxygen concentrations and are probably caused by denitrification. Examination of the geographical distribution of these low nitrate:phosphate data suggests that denitrification in the western and northern North Pacific Ocean may have been previously overlooked, and that a reassessment of the global oceanic denitrification budget may therefore be required