When fish are observed dying because of oxygen depletion, it is often too late to stop the kill. However, oxygen depletion can often be anticipated and detected before fish begin to die and measures can be taken to prevent a fish kill. Causes of Oxygen Depletion Oxygen depletion in a pond is the result of demand exceeding supply. Aquatic animals, plants, and decaying organic matter consume oxygen. Aquatic plants are primary producers of oxygen.
Plants produce oxygen as a by-product of photosynthesis.
The rate of photosynthesis is dependent upon light. Because of the effect of sunlight on photosynthesis, the amount of oxygen in pond water fluctuates daily. Oxygen levels are usually highest at midday and lowest just before sunrise. Fish kills usually occur when more oxygen is consumed during the night hours than is produced during daylight hours. Warm water does not contain as much oxygen as cold water.