Fish, invertebrates, plants and aerobic bacteria require oxygen. Temperature influences DO greatly. Water with a temperature of 31 degrees Celsius will only hold about half as much oxygen as water with a temperature of 1 degree Celsius. The amount of DO in an estuary depends on wind, water flow and thermal upwelling and mixing. It also depends on the type and amount of aquatic plants, algae and phytoplankton that produce oxygen as a by-product of photosynthesis.
Hypoxia refers to water with low DO levels and anoxia refers to water with no DO levels.
Too many aquatic plants and animals can consume most of the oxygen. Too many aquatic plants or increased alga growth can result form the elevated concentrations of nitrogen and phosphorus in the water. These two nutrients may come from run-off from farms, golf courses, fertilized yards and sewage plant effluent. Since DO is dependent on temperature, standards for DO in estuaries are also tied to temperature measurements. A measurement of pH is also used to determine estuary water quality.