Seawater has many different gases dissolved in it, especially nitrogen, oxygen and carbon dioxide. It exchanges these gases with the atmosphere to keep a balance between the ocean and the atmosphere. This exchange is helped by the mixing of the surface by wind and waves.
Gases and life:
Dissolved oxygen and carbon dioxide are vital for marine life. Marine plants use dissolved carbon dioxide, sunlight and water to make carbohydrates through the process of photosynthesis. This process releases oxygen into the water. All marine organisms use oxygen for respiration, which releases energy from carbohydrates and has carbon dioxide and water as byproducts. Marine animals with gills, such as fish, use these organs to extract oxygen from the seawater.
Variation in dissolved gases
Some of the properties of seawater affect how much gas can be dissolved in it:
Cold water holds more gas than warm water.
Seawater with low salinity holds more gas than high salinity water.
Deep water, which has a high pressure, holds more gas than shallow water.
The use and creation of dissolved gases by living things can over-ride the effect of these properties. For example, warm water with lots of plankton in it can hold more carbon dioxide than cold water with few living things in it.
Carbon dioxide:
Carbon dioxide is one of the most important gases that dissolve in the ocean. Some of it stays as dissolved gas, but most reacts with the water to form carbonic acid or reacts with carbonates already in the water to form bicarbonates. This removes dissolved carbon dioxide from the water.
Many plants and animals use the bicarbonate to form calcium carbonate shells. When these organisms die, some of the bicarbonate is returned to the water, but a lot of it settles down to the sea bed. This process locks up, for long periods of time, carbon that originated in carbon dioxide in the atmosphere.
If the ocean and atmosphere stayed the same, there would be a balance between the concentrations of carbon dioxide in each, but carbon dioxide levels in the atmosphere are rising, so more of the gas is dissolving in the ocean.