Hypoxia in the world ocean as recorded in the historical data set

Abstract

As of December 1986, the National Oceanographic Data Center (NODC), Washington, DC, listed over 2 million observations of dissolved oxygen concentratiions in the world ocean (including adjacent seas) collected between 1905 and 1982. Hypoxia, defined as concentrations below 0.2 ml l −1, accounts for 27,413 of the recorded oxygen observations. The spatial distribution of this NODC-based hypoxia is more extensive than previous compilations suggest. Although the more isolated data points may represent spurious data, the great majority of the observations fall within the ocean regions bounded by contours of oxygen concentrations <1.5 ml l −1 or of <20% oxygen saturation in published contour maps of the oxygen minimum surface. In the eastern Pacific, the contour boundary reduces to 0.5 ml l −1 or 7.5% saturation. Some seasonality is detected, especially at high latitudes. The depth distribution of hypoxia exhibits a typical marine ( ≥28 ppt ) range from near-surface to about 1500 m depth but with occasional occurrences at greater depths; the estuarine ( <28 ppt ) range extends from near-surface to the bottom of the basin in question. The apparent oxygen utilization leading to hypoxia varies widely (4.0–9.5 ml l −1) in the world ocean. The highest values occur at higher latitudes in coastal environments. Nitrate deficits ( >10 μ m ) calculated for restricted marine areas (28–38 ppt) generally occupy subzones nested within the major historically identified hypoxic regions in the world ocean, but the data are inadequate for a definitive comparison in all areas. These marine nitrate deficits occur in water with a somewhat more restricted range of apparent oxygen utilization than marine hypoxia.