The above figure shows the oxygen concentration as measure of the salinity of the ocean. The salinity is influenced by evaporation and by freshwater dilution. The ocean also contains dissolved Nitrogen and Argon, 445 and 11.5 µM concentrations at 25C..

The concentration of Oxygen in the ocean is not constant at all depths, but rather changes due to the combination of mixing, and oxygen enrichment and consumption pathways. The figure above shows the characteristics of this behaviour although it is region specific. Oxygen enrichment of water is provided either by direct exchange with the atmosphere at the ocean surface, or from photosynthesis in the photic zone. The photic zone can produce >100% oxygen saturated concentrations if conditions are optimal (nutrients, sunlight). The surface oxygenated layers of water water are then mixed down. However, a strong oxygen consumption is also present with the biological degradation (oxidation) of organic material and detritus. The oxygen minimum zone occurs not in the deepest part of the ocean but at intermediate depths. The reason for this is postulated to be due to a number of possibilities: a region of minimal mixing (resting boundary); a region where density and specific gravity of the organic detritus are such that detritus accumulation takes place at this depth and maximises oxygen consumption; the density of water laden with organic material is such that it descends to intermediate depth where maximal biological consumption facilitates a depleted zone.

Oxygen Measurements in the Ocean

Photosynthesis is a mechanism growth by Plants, Cyanobacteria and algae that results from the capture of energy (light) and conversion to chemical energy. This process globally represents the capture of ~10¹⁴ W of energy per year, that is used to generate O₂ alone (and more if the other chemistry is considered). On earth about 46% of this photosynthesis takes place in the oceans, and the oceans therefore are an extremely vital source of life.

The diversity of photosynthetic organisms chemically is low. That is to say they all perform the chemistry of water splitting to release O₂ in the same way. However, the diversity of the pigment antenna systems is high. This is because the photosynthetic organism have evolved to utilise light in aquatic environments. Due to the filtration of light (loss red wavelengths initially then green) the light colour changes with depth. Plants and algae therefore have evolved different pigment antenna complexes to cope with these changes in light "colour". This means specific adaptations are found at specific depths.

Most photosynthesis takes place on the surface waters 0-20 m depth. The general feeling was that the lower limit was ~200 m, however, photosynthesis has been recorded down to 269 m on a seamount. (Littler, MM et al, (1985) Science 227, 57-59.)