Dissolved oxygen (DO) is the oxygen in the water available for use by aquatic organisms. It can be a very good indicator of aquatic health, and is affected by factors such as temperature, salinity, photosynthesis, and pollution. DO levels change throughout a 24-hour cycle, and are lower at night and early morning and higher in the day. This is because at night, plants are not photosynthesizing and organisms are consuming oxygen. Conversely, in the day, plants are photosynthesizing, so DO levels go back up. When DO levels meet or exceed 5 mg/L, animals can generally grow and reproduce. Dissolved oxygen levels from 3-5 mg/L usually result in animals becoming stressed. Levels under 2 mg/L mean the water is hypoxic (does not contain enough oxygen to support animal life). Currently, there is a hypoxic zone at the mouth of the Mississippi River that measures approximately 8,000 kilometers across, roughly the size of Rhode Island.
The normal range for DO in Galveston Bay is 5-11 mg/L.
This is the instrument Artist Boat students use for measuring DO: