Dissolved Oxygen, also known as Oxygen Saturation, is a measure of the amount of gaseous oxygen that is present in natural bodies of water – ponds, lakes, rivers, streams – as well as stormwater runoff, wastewater, ﬁsh farms and aquariums. It gives an indication of the health of the water and its ability to sustain a balanced aquatic ecosystem. DO levels are also important in drinking water and many other industries.*
*Our drinking water tastes better with high DO levels; however, industry prefers low DO in its water supply as the oxygen promotes corrosion in piping.
All About Balance
Oxygen enters water by diffusion from the atmosphere, plant photosynthesis, and from the increased surface area of churning water. In turn, oxygen is consumed by respiration of aquatic animals (and plants, too, at night), decomposition of organic matter and pollutants, and various chemical reactions. In a healthy ecosystem, oxygen creation and oxygen use remain in balance.
DO levels do ﬂuctuate over a 24-hour period, and seasonally. They vary with the depth of water, with water temperature, and with altitude. Some imbalance can be tolerated over short periods of time, but signiﬁcant imbalance can be disastrous to aquatic life, making it imperative to know and control DO levels.
When To Measure
Unlike streams and rivers, still waters in ponds and lakes experience little churning; therefore, photosynthesis plays a larger role in the production of dissolved oxygen. DO levels are at their highest in the late afternoon of very hot, sunny days. Once the sun goes down, aquatic plants stop producing oxygen and join with ﬁsh and other organisms in the water to consume oxygen through respiration. If the demand for oxygen is too heavy during the night, DO levels may get dangerously low by early morning. Levels below 1-2mg/L for several hours can cause a ﬁsh kill.
Know Your DO
Dissolved oxygen meters measure in parts per million (equals mg/L) or as a percentage of saturation, where saturation is the maximum amount of oxygen that can be dissolved in water at a given altitude and temperature. Most meters also measure temperature, in Celsius, and may include an auto compensation feature.The probe is ﬁlled with a salt solution and has a selectively permeable membrane that allows DO to pass from the water being tested into the salt solution. The DO that has diffused into the salt solution changes the electrical potential of the solution. This change is sent from the probe to the meter where it is displayed in mg/L and %.
Some meters feature auto calibration, while less expensive meters must be manually calibrated. This is an easy, open-air calibration requiring no calibration solution. You can ﬁnd open-air calibrations on the TECH TIPS section of this website.
DO Plays a Key Role in Pond Management
A common cause of ﬁsh kills is oxygen depletion. This condition usually occurs during summer in very fertile ponds as a result of pond turnover or the die-off of an algal bloom. During hot weather most ponds have a layer of water near the bottom that contains little or no dissolved oxygen. When high winds or cold rain cause this water to mix with the upper pond water, oxygen levels often drop low enough to kill ﬁsh. Oxygen depletion also occurs when dead algae or other plants decay in the pond after herbicides have been applied to control weeds.