Dissolved Oxygen (DO)Status and TrendsDissolved oxygen is a measure of the amount of oxygen freely available in water. It is commonly expressed as a concentration in terms of milligrams per liter (mg/L) or ppm, or as a percent saturation which is temperature dependent (DO is inversely related to WATEMP). Percent saturation is a measure of the potential capacity of the water to hold oxygen in its dissolved state.. The DO for surface water ranges from 0 in extremely poor water conditions to a high of 15 mg/L at 0 degrees Celsius (freezing) water. USEPA regulatory standards have not been established with respect to drinking water. |
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Oxygen is the single most important gas for most aquatic organisms as it is needed for respiration. DO levels below 3 ppm are stressful to most aquatic organisms. No life stages of any fish species will survive when DO levels are less than 1 ppm; levels of 5 to 6 ppm are usually required for all life stages of most fish (see table below). Oxygen gets into the river when water crashes over rocks, through photosynthesis by algae or by atmospheric diffusion, a process enhanced by turbulent mixing of wind and water. Based on this process, fast moving rivers often have more oxygen content that slow moving rivers. If oxygen is plentiful (at the high end of the range), fish that require a lot of oxygen, like trout, may thrive. If the river is slow and oxygen is not abundant (at the low end of the range), only carp and other bottom feeding fish that require less oxygen will thrive.
Median DO ranged from a low of 8.6 mg/L on the Choptank River to 10.95 mg/L on the Anacostia River. Highest median DO concentrations occurred at the Anacostia River station located at the bridge on Bladensburg Road, on Cabin John Branch and on the upper Gunpowder River. Forty-six percent of non-tidal stations had median DO concentrations in the middle 1/3 of the benchmark data set and 48% had median DO concentrations less than 9.9 mg/L. All trends in DO concentration were decreasing. These occurred at 59% of all stations. Decreases in DO concentration at stations with highest DO could be related to a decrease in algal production (algae release O2 during photosynthesis) or an increase in water temperature. Neither appears to be the case at the station on the upper Gunpowder as there were no trends in CHLA or WATEMP at this station. Water temperatures increased and DO concentration decreased at the station on Georges Creek at Franklin, MD and on the Susquehanna River below the Dam. Chlorophyll, also, significantly decreased at the Susquehanna station suggesting that increased temperature and not algal decomposition is the dominant factor affecting the DO trend at this station. Chlorophyll is not sampled at stations west of Hancock, MD (map # 13) so it is not possible to say if a downward trend in chlorophyll concentration played a role in the observed decrease In DO. Dissolved Oxygen data for sampled locations are summarized in these plots. |
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