365 Days of Climate Awareness 48 - Dissolved Oxygen

Oxygen (O₂) is not overly soluble in water. Well-oxygenated surface water has roughly 8 grams of O₂ per liter (g/l). Photosynthetic marine organisms manufacture oxygen globally, and the ocean is also bounded by an atmosphere composed of 21% O₂. Transfer of oxygen between ocean and atmosphere occurs slowly, and depends on whether the ocean water is saturated or not.

Solubility of oxygen is not uniform in seawater. It is more soluble in cold water, which partly explains why cold regions of the ocean are so biologically productive: there is more oxygen. It is also more soluble in fresher water. In very warm parts of the ocean, animals use up a large portion of dissolved O₂, and there are fewer photosynthesizing organisms, so oxygen levels drop. Oxygen levels tend also to be higher in the upper portion of the water column, especially in the upper 200 m known as the "euphotic zone", meaning the well-illuminated part of the sea where photosynthesis occurs. Oxygen levels tend to drop with depth, as oxygen is consumed but not produced there.

Oxygen is produced in the ocean mostly by phytoplankton, a gigantic grouping of organisms ranging from bacteria to kelp. Phytoplankton are primary producers, meaning they assemble organic from inorganic components, using light (photosynthesis) for energy. These primary producers are the foundation of oceanic food webs. They are also the source of most of the oxygen in the ocean. It is estimated that 50-80% of all oxygen production on earth occurs in the ocean.

A term used to describe oxygen in the ocean is "flux", that is the movement of matter or energy across a boundary. In this case, flux of the molecule O₂ between the ocean and atmosphere, which appears to be seasonal. Global data on oxygen content is somewhat spotty, so no comprehensive budget has been developed, but fluxes appear to be seasonal. The ocean contributes oxygen to the atmosphere in the spring and summer, when the rates of photosynthesis are greatest and the water becomes saturated with O₂ molecules. The ocean draws down oxygen from the atmosphere in fall and winter, when photosynthesis rates slow but marine animals still require O₂ to live.

Tomorrow: eutrophication.

Be well!