Oxygen

It should be remembered that oxygen is a natural element forming water together with hydrogen. However, the “oxygen in sea water” is actually oxygen dissolved from the atmosphere through vigorous stirring and turbulence cause by wind and waves, and through precipitation. The equatorial and mid-latitude regions of the earth have a lower oxygen content at the surface than the high-latitude regions (see Figure).

www.seafriends.org.nz/oceano/seawater.htm