Dissolved oxygen (DO) is one of the most important indicators of water quality. It is essential for the survival of fish and other aquatic organisms. Oxygen dissolves in surface water due to the aerating action of winds. Oxygen is also introduced into the water as a byproduct of aquatic plant photosynthesis. When dissolved oxygen becomes too low, fish and other aquatic organisms cannot survive.
The colder water is, the more oxygen it can hold. As the water becomes warmer, less oxygen can be dissolved in the water. Salinity is also an important factor in determining the amount of oxygen a body of water can hold; fresh water can absorb more oxygen than salt water.
Oxygen levels also may be reduced when there are too many bacteria or algae in water (see Biochemical Oxygen Demand). After the algae complete their life cycle and die, they are consumed by bacteria. During this decay process the bacteria also consume the oxygen dissolved in the water. This can lead to decreased levels of biologically available oxygen, in some cases leading to fish kills and death to other aquatic organisms.
Florida's surface water quality standards include minimum values for dissolved oxygen saturation. The standards for freshwater vary in different bioregions of the state:
Dissolved oxygen standards for marine water bodies are expressed as minimums for daily, weekly, and monthly averages.
Detailed water quality standards for dissolved oxygen can be found in Chapter 62-302.533 of the Florida Administrative Code.