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♤ In aquatic ecosystems oxygen is dissolved in water, where its concentration varies constantly depending on factors that influence the input and output of oxygen in water.
♤ In fresh water the average concentration of dissolved oxygen is 0.0010 per cent (also expressed as 10 parts per million or 10 ppm) by weight, which is 150 times lower than the concentration of oxygen in an equivalent volume of air.
♤ Oxygen enters the aquatic ecosystem through the air water interface and by the photosynthetic activities of aquatic plants.
♤ Therefore, the quantity of dissolved oxygen present in an ecosystem depends on the rate at which the aforesaid two processes occur.
♤ Dissolved oxygen escapes the water body through airwater interface and through respiration of organisms (fish, decomposers, zooplanktons, etc).
♤ The amount of dissolved oxygen retained in water is also influenced by temperature. Oxygen is less soluble in warm water. Warm water also enhances decomposer activity. Therefore, increasing the temperature of a waterbody increases the rate at which oxygen is depleted from water.
♤ When the dissolved oxygen level falls below 3-5 ppm, many aquatic organisms are likely to die.
Other limiting factors which influence on aquatic productivity are