Dissolved oxygen concentrations of 0.0 to 1.5 mg/l can be lethal depending upon exposure time and other conditions. Best survival and growth is obtained at dissolved oxygen levels between 3.5 mg/l and saturation. High levels of dissolved oxygen supersaturation are potentially harmful (Boyd & Fast, 1992).

The major factors affecting dissolved oxygen solubility in water are temperature, salinity, and pressure. Shrimp ponds normally are at sea level and variation in pressure is slight. Shrimp pond water is seldom at saturation, because of the combined effects of oxygen production by photosynthesis and oxygen consumption by respiration. Biological processes can change dissolved oxygen concentrations so quickly that diffusion of oxygen between air and water cannot maintain saturation (Boyd & Fast, 1992).

Pond dissolved oxygen concentrations are dynamic. They exhibit both diel cycles and vertical stratification. Concentrations are greatest in the afternoon and least near dawn as a result of community photosynthesis and respiration. During daylight hours, oxygen concentrations are greatest near the surface because of light extinction with depth and thermal stratification. The magnitude of dissolved oxygen fluctuation during a 24-hour period and the degree of vertical stratification increases with increasing phytoplankton abundance. Fertilization and feeding increase phytoplankton abundance. Therefore, problems with low dissolved oxygen increase with increased shrimp production per unit area (Boyd & Fast, 1992).

Dissolved oxygen monitoring is most important in semi-intensive, intensive, and ultra-intensive ponds. Since shrimp live near or on the pond bottom and dissolved oxygen concentrations normally are lowest at the bottom, dissolved oxygen measurements made a few centimeters above the bottom are most useful for management purposes. With intensive culture, dissolved oxygen concentrations should be measured at 2 to 3 hour intervals during the night. These and other data may be used to predict whether aeration or water exchange will be necessary before dawn. If concentrations less than 3 mg/liter are projected, emergency measures to increase oxygen concentrations should be taken (Boyd & Fast, 1992).

Dissolved oxygen concentrations can be measured with water analysis kits or with polarographic oxygen meters. The kits employ traditional wet chemistry and provide sufficiently accurate data. It takes considerable time, however, to measure dissolved oxygen by wet chemistry techniques. Where there are many ponds and many measurements, it is more convenient to use oxygen meters (Boyd & Fast, 1992).