Dissolved Oxygen: What It Is, Why It Matters

The term “dissolved oxygen” (or DO) refers to microscopic oxygen gas bubbles in water. DO levels, measured in parts per million or milligrams per liter, naturally vary with water temperature and pressure. Concentrations of DO can vary widely within a water column and even from day to night. The DO level of your lake or pond is important because fish and other aquatic organisms depend on dissolved oxygen to live; too little can cause suffocation, but too much can be toxic. Dissolved oxygen in a water body is the result of the photosynthesis of the algae population; DO is consumed by the respiration of fish and other aquatic organisms, as well as by decomposition of vegetation and other organic matter in the water. During the warm-weather growing season, which can extend well into October here in Georgia, both photosynthesis and decomposition accelerate. Warm temperatures also cause thermal stratification (warmer, oxygen-rich water at the top and cooler, oxygen-poor water at the bottom). Periods of very hot, calm weather exacerbate these conditions and summertime fish kills resulting from plummeting DO levels can occur. Cooler temperatures will eventually result in fall turnover, when the stratified water layers mix and DO levels optimize throughout the water column. Dangerously low DO levels can occur again in the winter if prolonged ice cover prevents oxygenation of the water, but we don’t typically experience the protracted freezes that cause such conditions. Spring brings another period of turnover as the water warms. In addition to natural seasonal fluctuations, many other factors can affect DO levels in a lake or pond, including: the introduction of organic matter from outside the water body, such as frequent dumping of grass clippings into the water; influx of sewage, animal waste or other pollutants; stormwater runoff; significant water level changes; and changes to inflowing streams.