Fish Kill - Causes - Oxygen Depletion

Oxygen Depletion

See also: Hypoxia (environmental) and Hypoxia in fish

Oxygen enters the water through diffusion. The amount of oxygen that can be dissolved in water depends on the atmospheric pressure, the water temperature and whether the water is salty. For example, at 20 °C (68 °F) and one atmosphere of pressure, a maximum of 8 mg/l of oxygen can dissolve in sea water (35 mg/l salinity) while a maximum of 9 mg/l of oxygen can dissolve in fresh water. The amount of oxygen that can be dissolved in the water decreases by about 1 mg/l for each 10 °C increase in water temperature above 20 °C.

Many cold water fish that have evolved to live in clean cold waters become stressed when oxygen concentrations fall below 8 mg/l whilst warm water fish generally need at least 5 ppm (5 mg/l) of dissolved oxygen. Fish can endure short periods of reduced oxygen. Depleted oxygen levels are the most common cause of fish kills. Oxygen levels normally fluctuate even over the course of a day and are affected by weather, temperature, the amount of sunlight available, and the amount of living and dead plant and animal matter in the water. In temperate zones oxygen levels in eutrophic rivers in summertime can exhibit very large diurnal fluctuations with many hours of oxygen supersaturation during daylight followed by oxygen depletion at night. Associated with these photosynthetic rhythms there is a matching pH rhythm as bicarbonate ion is metabolised by plant cells. This can lead to pH stress even when oxygen levels are high.

Additional dissolved organic loads are the most common cause of oxygen depletion and such organic loads may come from sewage, farm waste, tip leachate and many other sources

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