CADDIS Volume 2: Sources, Stressors & Responses
Ways to Measure DO
The concentration of oxygen in water is often reported either as the concentration in mg/L or as the percent saturation. DO concentrations and percent saturation are related, but not equivalent. Saturation level varies naturally, as water can contain more DO at lower temperatures, higher pressures, and lower salinities. For example, 100% saturation occurs at low oxygen concentrations at high elevations compared to low elevations (Hem 1985).
The Winkler titration procedure was the first recognized method for determining DO concentrations in natural waters (Winkler 1988, cited in Mitchell 2005). More recently, this method was found prone to over-reporting DO under hypoxic conditions and under-reporting DO under nearly anoxic conditions. Fairly simple and reliable DO measurements now can be obtained with DO meters or field test kits. The electronic meter does not measure oxygen directly; rather, it uses electrodes to measure the partial pressure of oxygen in the water, expressed as a concentration (usually mg/L of water) [see APHA (1998), Mitchell and Stapp (1992), and USGS (1998)]. Percent saturation is calculated by dividing the measured DO concentration by the saturation level and multiplying by 100. Saturation levels can be obtained from U.S. Geological Survey solubility tables based on water temperature and corrected for different salinities and pressures. Equations for calculating percent saturation are available from Water on the Web.
Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are measures of the potential consumption of oxygen by microbial respiration and the oxidation of chemicals in the water, respectively. The actual rate of oxygen consumption in a stream is affected by a number of variables including temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.
The lowest concentrations of DO are usually measured before photosynthesis begins for the day (i.e., just before dawn), and just above the sediments, where most decomposition occurs. Documentation of DO concentrations over a 24-hour period may be useful for identifying diurnal patterns and may reveal information about DO depletion.