Marine Biology

Dissolved Gases

The concentration of dissolved oxygen and carbon dioxide are very important for marine life forms. Although both oxygen and carbon dioxide are a gas when outside the water, they dissolve to a certain extent in liquid seawater. Dissolved oxygen is what animals with gills use for respiration (their gills extract the dissolved oxygen from the water flowing over the gill filaments). Dissolved carbon dioxide is what marine plants use for photosynthesis.

The amount of dissolved gases varies according to the types of life forms in the water. Most living species need oxygen to keep their cells alive (both plants and animals) and are constantly using it up. Replenishment of dissolved oxygen comes from the photosynthetic activity of plants (during daylight hours only) and from surface diffusion (to a lesser extent). If there are a large number of plants in marine water mass then the oxygen levels can be quite high during the day. If there are few plants but a large number of animals in marine water mass then the oxygen levels can be quite low. The amount of dissolved gases in seawater depends more on the types of life forms (plants and animals) that are present and their relative proportions.