Gas is more soluble with increasing pressure. The quantity of gas that dissolves in water at a given temperature, pressure and salinity is the saturation value . Since gas enters seawater at the ocean's surface, saturation values depend on the temperature and salinity of the water at one atmosphere of pressure. However some gases, such as oxygen, can be produced or consumed within the ocean, and this complicates their chemical cycles. Oxygen is one of the major dissolved gases in seawater and it is necessary for the respiration of marine animals and aerobic bacteria.
Respiration consumes oxygen and produces carbon dioxide.
Marine plants also sometimes consume, but they produce more oxygen than they consume. Indeed, they are the source of much of the oxygen in the atmosphere, since some of the oxygen they produce escapes to the air. All plants need light to grow and in the oceans the maximum depth of light penetration is typically about 100 meters. Well-mixed water in contact with the atmosphere generally has enough oxygen to meet the respiratory requirements of aquatic organisms. The metabolic processes of many aquatic organisms are seriously impaired at values below 3 ppm.