|Dissolved Oxygen in Lakes|
Is Dissolved Oxygen Important?
Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), microscopic bubbles of oxygen gas in the water, called dissolved oxygen (DO), are transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Oxygen also is needed by virtually all algae and all macrophytes, and for many chemical reactions that are important to lake functioning.
Reasons for Natural Variation
Oxygen is produced during photosynthesis
and consumed during respiration and decomposition. Because it requires
light, photosynthesis occurs only during daylight hours. Respiration
and decomposition, on the other hand, occur 24 hours a day. This difference
alone can account for large daily variations in DO concentrations. During
the night, when photosynthesis cannot counterbalance the loss of oxygen
through respiration and decomposition, DO concentration may steadily
decline. It is lowest just before dawn, when photosynthesis resumes.
Other sources of oxygen include the air
and inflowing streams. Oxygen concentrations are much higher in air,
which is about 21% oxygen, than in water, which is a tiny fraction of
1 percent oxygen. Where the air and water meet, this tremendous difference
in concentration causes oxygen molecules in the air to dissolve into
the water. More oxygen dissolves into water when wind stirs the water;
as the waves create more surface area, more diffusion can occur. A similar
process happens when you add sugar to a cup of coffee - the sugar dissolves.
It dissolves more quickly, however, when you stir the coffee.
Another physical process that affects DO concentrations is the relationship between water temperature and gas saturation. Cold water can hold more of any gas, in this case oxygen, than warmer water. Warmer water becomes "saturated" more easily with oxygen. As water becomes warmer it can hold less and less DO. So, during the summer months in the warmer top portion of a lake, the total amount of oxygen present may be limited by temperature. If the water becomes too warm, even if 100% saturated, O2 levels may be suboptimal for many species of trout.
Dissolved oxygen concentrations may change dramatically with lake depth. Oxygen production occurs in the top portion of a lake, where sunlight drives the engines of photosynthesis. Oxygen consumption is greatest near the bottom of a lake, where sunken organic matter accumulates and decomposes. In deeper, stratified, lakes, this difference may be dramatic - plenty of oxygen near the top but practically none near the bottom. If the lake is shallow and easily mixed by wind, the DO concentration may be fairly consistent throughout the water column as long as it is windy. When calm, a pronounced decline with depth may be observed.
Seasonal changes also affect dissolved oxygen concentrations. Warmer temperatures during summer speed up the rates of photosynthesis and decomposition. When all the plants die at the end of the growing season, their decomposition results in heavy oxygen consumption. Other seasonal events, such as changes in lake water levels, volume of inflows and outflows, and presence of ice cover, also cause natural variation in DO concentrations.
Expected Impact of Pollution
To the degree that pollution contributes oxygen-demanding organic matter (like sewage, lawn clippings, soils from streambank and lakeshore erosion, and from agricultural runoff) or nutrients that stimulate growth of organic matter, pollution causes a decrease in average DO concentrations. If the organic matter is formed in the lake, for example by algal growth, at least some oxygen is produced during growth to offset the eventual loss of oxygen during decomposition. However, in lakes where a large portion of the organic matter is brought in from outside the lake, oxygen production and oxygen consumption are not balanced and low DO may become even more of a problem.
The development of anoxia in lakes is most pronounced in thermally stratified systems in summer and under the ice in winter when the water mass is cut-off from the atmosphere. Besides the direct effects on aerobic organisms, anoxia can lead to increased release of phosphorus from sediments that can fuel algal blooms when mixed into the upper euphotic (sunlit) zone. It also leads to the buildup of chemically reduced compounds such as ammonium and hydrogen sulfide (H2S, rotten egg gas) which can be toxic to bottom dwelling organisms. In extreme cases, sudden mixing of H2S into the upper water column can cause fish kills.
Dissolved oxygen concentrations are most often reported in units of milligrams of gas per liter of water - mg/L. (The unit mg/L is equivalent to parts per million = ppm).
DO - % saturation
Oxygen saturation is calculated as the percentage of dissolved O2 concentration relative to that when completely saturated at the temperature of the measurement depth. Recall that as temperature increases, the concentration at 100% saturation decreases. The elevation of the lake, the barometric pressure, and the salinity of the water also affect this saturation value but to a lesser extent. In most lakes, the effect of dissolved solutes (salinity) is negligible; but the elevation effect due to decreased partial pressure of oxygen in the atmosphere as you go up (recall the breathing difficulties faced by Mt. Everest climbers) is about 4% per 300 meters (1000 feet).
The DO concentration for 100% air saturated water at sea level is 8.6 mg O2/L at 25°C (77°F) and increases to 14.6 mg O2/L at 0°C. Use the chart below for nomograms for calculating saturation.
DETERMINING PERCENT SATURATION THE "QUICK AND EASY" METHOD
Michaud, J.P. 1991. A citizen's guide to understanding and monitoring lakes and streams. Publ. #94-149. Washington State Dept. of Ecology, Publications Office, Olympia, WA, USA (360) 407-7472. Moore, M.L. 1989.
NALMS management guide for lakes and reservoirs. North American Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443, USA (http://www.nalms.org).