Ocean oxygen (overview)

　Marine plants use a small portion of the vast amount of carbon dioxide and water present in the ocean for photosynthesis to synthesize organic matter, and in return they exhale oxygen molecules (O₂). On the other hand, plants and animals consume oxygen and organic matter through respiration and exhale carbon dioxide and water. Also, keep in mind that most of the oxygen in seawater is absorbed from the atmosphere.

Oxygen is produced during photosynthesis only near the surface where sunlight reaches. On the other hand, bacteria that decompose organic matter live from the surface to the deep layers, so oxygen is consumed throughout the body through respiration. The layer where oxygen production (photosynthesis) exceeds oxygen consumption (respiration) is called the photic layer. Additionally, since the ocean surface mixed layer is in contact with the atmosphere, oxygen and carbon dioxide are quickly exchanged between the atmosphere and the ocean. Therefore, the oxygen concentration in mixed layer water is considered to be approximately in equilibrium with the atmosphere. The amount of oxygen contained in water in the surface mixed layer, which is in equilibrium with the atmosphere, is at most 0.3 mmol kg-1, which is not as large as total carbon dioxide. This raises concerns that if oxygen continues to be consumed through respiration in the mid-deep layer, which is deeper than the photic layer, oxygen will eventually become depleted. In reality, oxygen does not become depleted in the deep layers of most open oceans, but in the bottom layers of closed water bodies where there is a large supply of organic matter, there are reports of poor oxygen conditions, causing damage to fisheries such as the death of benthic organisms.