Dissolved Oxygen

The circulation of the ocean is driven by inputs of dense water, fluxes of heat and fresh water at the surface, and by the winds. Embedded in this general circulation are dissolved gases such as oxygen, carbon, nitrogen, etc. Dissolved oxygen (DO) is essential for all aerobic marine biogeochemical processes in the water column, the air-sea boundary, and the sediment. DO concentration results from physical and biogeochemical processes that balance sources and sinks. Photosynthetic plants produce oxygen during photosynthesis. When marine organisms die and decompose, their organic tissue breaks down releasing nutrients and carbon into the water while consuming dissolved oxygen in the process. As you would expect, dissolved oxygen is generally highest at or near the surface because of biological production (photosynthesis) in the photic zone, the layer where sunlight penetrates allowing photosynthesis to take place. All organisms in the ocean contain more or less constant proportions of nutrients, oxygen, and carbon.

Dissolved oxygen (DO) is essential for all aerobic marine biogeochemical processes in the water column, the air-sea boundary, and the sediment. DO concentration results from physical and biogeochemical processes that balance sources and sinks. Photosynthetic plants produce oxygen during photosynthesis. When marine organisms die and decompose, their organic tissue breaks down releasing nutrients and carbon into the water while consuming dissolved oxygen in the process. As you would expect, dissolved oxygen is generally highest at or near the surface because of biological production (photosynthesis) in the photic zone, the layer where sunlight penetrates allowing photosynthesis to take place. For comparison, notice the difference between DO abundance at the surface and that at 1000 m depth in the Atlantic Ocean. Surface waters are generally more depleted of nutrients than those at deeper levels. When deep water circulates to the surface, a process called upwelling, a spike in oceanic productivity usually occurs, associated with plankton blooms. For example, the west coast of the U.S. is a high upwelling region particularly during summer, and thus contains high marine production levels. Excessive runoff of nutrients from the land, mostly due to agricultural fertilizers, and carried out to sea by rivers or wind deposition can create an overabundance of the nutrient, a process called eutrophication. As plankton bloom and subsequently die off, dead zones may occur as the bacteria decomposing organic matter consume available dissolved oxygen.  Upwelling events of nutrient-rich but extremely oxygen-poor deep waters onto the continental shelf may result water conditions detrimental for aerobic processes.

The dissolved oxygen maps represent the large-scale horizontal distribution of this gas at selected standard depth levels of the world ocean on a one-degree latitude-longitude grid in the World Ocean Atlas (Garcia et al., 2009). These maps were computed by objective analysis of all scientifically quality-controlled historical dissolved oxygen data in the World Ocean Database (Boyer et al., 2009). The online version of World Ocean Atlas 2009 Figures contains a collection of "JPEG" images of objectively analyzed fields and statistics generated from the World Ocean Atlas 2009. The ocean variables included in the atlas are: temperature, salinity, oxygen (dissolved oxygen, apparent oxygen utilization, and percent oxygen saturation), and dissolved inorganic nutrients (phosphate, nitrate, and silicate).

Boyer, T. P., J. I. Antonov , O. K. Baranova,H. E. Garcia, D. R. Johnson, R. A. Locarnini, A. V. Mishonov, T. D. O’Brien, D. Seidov, I. V. Smolyar, M. M. Zweng, 2009. World Ocean Database 2009. S. Levitus, Ed., NOAA Atlas NESDIS 66, U.S. Gov. Printing Office, Wash., D.C., 219 pp., DVDs. Available at NODC World Ocean Database 2009.