How are living organisms adapted to the lack of oxygen in water?

Oxygen in the aquatic environment is an important environmental factor. Oxygen is produced in water as a result of plant photosynthesis. The oxygen content is mainly influenced by two factors: mixing of water, such as in mountainous and fast-flowing rivers, and cooling of the water. Therefore, the species of fish that are most demanding on the oxygen content mainly live in cold, torrential bodies of water. Fish that are unpretentious to the oxygen content in the water live at the bottom of deep water bodies and are adapted to live in warm waters. Most aquatic insects are unpretentious in the oxygen content of water, because they periodically rise to the surface of the water to breathe fresh air.