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The seawater oxygen content depends primarily on factors such as salinity and temperature. Relationships have been derived from which the equilibrium concentration of dissolved oxygen can be calculated if the absolute temperature T (K) and salinity S(‰) are known:
ln [O2] (ml/L) = A1 +A2(100/T)+A3 ln (T/100)+ A4(T/100)+S[B1 + B2(T/100)+ B3(T/100)2]
where:
A1 = -173.4292
A2 = 249.6339
A3 = 143.3483
A4 = -21.8492
B1 = -0.033096
B2 = 0.014259
B3 = -0.0017000
The primary source of the dissolution of oxygen is the air-sea exchange with oxygen in the atmosphere, leading to near saturation. However, due mainly to biological processes, deviations may occur with seasons. The normal profile of corrosion of unprotected steel, as in the case of piling or the supporting legs for offshore oil drilling structures is shown here, based on the measurements of the distribution of corrosion of test pilling exposed in a partially enclosed basin at Kure Beach, NC, USA.
At any location there are seasonal variations in salinity, temperature and other parameters. There are also variations with the depth of water, as illustrated here in a figure representing data collected during studies at U.S. Naval Engineering test sites in the Pacific Ocean. It should not be assumed that the variations found in these studies can be extrapolated to other oceanographic sites. For example, observations within the same depth range of depth in the Atlantic Ocean showed a much higher concentration of dissolved oxygen to the bottom, even approaching the concentration found at the surface. (Reference)
