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Why is dissolved oxygen important to fish and other aquatic organisms?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — oxygen group element Modern version of the periodic table of the elements (printable).

Dissolved oxygen is essential for the survival of fish and other aquatic organisms because it is required for their respiration. Unlike terrestrial animals that breathe air, fish utilize gills to extract oxygen directly from the water.123

The process works as follows:
* Gas Exchange: As water passes over the feathery, branched structure of the gills, oxygen diffuses into the fish's bloodstream, while carbon dioxide is released into the water.3
* Efficiency: Fish often employ a process called countercurrent exchange, where blood in the gill capillaries flows in the opposite direction of the water, ensuring that oxygen continues to diffuse into the blood along the entire length of the gill filament.3
* Metabolic Demand: Because water contains significantly less oxygen than air and is much denser and more viscous, aquatic animals must expend considerable energy—sometimes up to 20 percent of their total oxygen consumption—just to pump water over their respiratory surfaces.4

The availability of dissolved oxygen in natural waters is influenced by factors such as temperature, salinity, and atmospheric pressure, all of which can limit the amount of oxygen accessible to aquatic life.45