In aquatic habitats, the amount of oxygen, carbon dioxide and other atmospheric gases dissolved in water and therefore accessible to organisms varies greatly in time. In water bodies with a high content of organic substances, oxygen is the primary limiting factor. The oxygen content in the surface layer of water depends mainly on the exchange of gas between air and water and varies between 6-12 million ^ – 1. The solubility of the gas decreases with increasing temperature and increasing salinity of the water. At great depths, where there is no contact with the atmosphere, this exchange depends on the oxygen consumption for biological processes and the replenishment of its reserves when mixed with oxygen-rich water. The rate of diffusion of dissolved oxygen into a mass of water depleted of oxygen is a local characteristic
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