Why Is It 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.

Mid-summer, when strong thermal stratification develops in a lake, may be a very hard time for fish. Water near the surface of the lake - the epilimnion - is too warm for them, while the water near the bottom - the hypolimnion - has too little oxygen. Anoxia forces the fish to spend more time higher in the water column where the warmer water is suboptimal for them. This may also expose them to higher predation, particularly when they are younger and smaller. Eutrophication exacerbates this condition by adding organic matter to the system which accelerates the rate of oxygen depletion in the hypolimnion. Urban, and other forms of runoff, can also add to this problem very suddenly and dramatically by causing fish kills after excess soils and road hydrocarbons are washed in from intense rainstorms. Conditions may become especially serious during a stretch of hot, calm weather, resulting in the loss of many fish. You may have heard about summertime fish kills in local lakes that likely results from this problem. In eutrophic and hypereutrophic lakes, summertime fish kills can happen most easily during periods with high temperatures, little wind and high cloud cover. The clouds reduce daytime photosynthesis with its oxygen production and so the DO in the mixed layer. Or even throughout the water column of a shallow unstratified lake, can become critical for fish and other aquatic organisms. The same basic phenomenon can occur in winter (winterkill) when ice cover removes re-aeration from the atmosphere and snowcover can light-limit algal and macrophyte photosynthesis under the ice. Many lakes in the upper midwest are mechanically re-aerated or injected with air, oxygen or even liquid oxygen to keep ice off of some of the lake and to add oxygen directly to prevent winterkills.

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 8.26 mg O₂/L at 25°C (77°F) and increases to 14.6 mg O₂/L at 0°C. Use the chart below for nomagrams for calculating saturation.

DETERMINING PERCENT SATURATION THE "QUICK AND EASY" METHOD For a quick and easy determination of the percent saturation value for dissolved oxygen at a given temperature, use the saturation chart above. Pair up the mg/l of dissolved oxygen you measured and the temperature of the water in degrees C. Draw a straight line between the water temperature and the mg/l of dissolved oxygen. The percent saturation is the value where the line intercepts the saturation scale. Streams with a saturation value of 90% or above are considered healthy, but this of course is only one measure of "health". Read the rest of this section and the Lake Ecology Primer for more about dissolved oxygen in lakes. Note that this nomogram assumes that the lakes are at sea level whereas the Minnesota WOW lakes vary from 928 to 1400 feet elevation. Since gas pressures decrease with elevation, the true values will be about 5% lower for these "higher" lakes. The saturation value can also vary slightly depending on barometric pressure with lower values expected when a storm front moves through as compared to bright and sunny "high pressure" days. The RUSS and ancillary manual data in the WOW website are all corrected for this effect. DETERMINING PERCENT SATURATION THE "NOT SO QUICK AND EASY" METHOD There is also a series of equations you can use to calculate percent saturation. You begin by determining the equilibrium oxygen at nonstandard pressure, Cp, using the equation shown below. But even before you can do that you first need to determine the atmospheric pressure at your lake's altitude (h in kilometers) using equation 1: Equation 1 where P = pressure (atm) at altitude h (km) relative to standard partial pressure (Pst) at 760 mm Hg or 101.325 kpa at sea level. Now you can dive into equation 2 below. Oh, by the way, temperature in Kelvin (K) is equal to temperature in degrees C + 273.15 degrees and 1 atmosphere = 760 mm Hg. Equation 2

Note: C* = exp[7.7117 - 1.31403 • ln(t + 45.93)] Now that you have solved for Cp you can finally determine %saturation based on your DO concentration (mg/L) by going one more step: Equation 3 (where DO is your measured value).

You can use this Javascript utility to perform the calculations. It uses the equations from above, which were obtained from the 1956 Mortimer, C.H. article referenced below.

The following formula is an Excel version of Equation 2 -- you can use it to calculate Cp in a spreadsheet: =((EXP(7.7117-1.31403*LN(B7+45.93))) * $C$3 * (1-EXP(11.8571-(3840.7/(B7+273.15))-(216961/((B7+273.15)^2)))/$C$3) * (1-(0.000975-(0.00001426*B7)+(0.00000006436*(B7^2)))*$C$3)) / (1-EXP(11.8571-(3840.7/(B7+273.15))-(216961/((B7+273.15)^2)))) / (1-(0.000975-(0.00001426*B7)+(0.00000006436*(B7^2)))) Enter P (atm) at your altitude in spreadsheet cell "C3", and enter the water temperature (degrees C) in cell "B7". Copy the above formula (in a single line) to a spreadsheet cell -- it will calculate Cp. Once you have determined Cp you can use equation 3 (from above) to determine % saturation from your DO concentration. Here is a sample spreadsheet that makes use of this formula.

REFERENCES

APHA.1995. Standard methods for the examination of water and wastewater. Amer. Publ. Health Assoc.

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).

Mortimer, C.H. 1956. The oxygen content of air-saturated fresh waters, and aids in calculating percentage saturation. Intern. Assoc. Theoret. Appl. Commun. No 6.

Mortimer, C.H. 1981. The oxygen content of air-saturated fresh waters over ranges of temperature and atmospheric pressure of limnological interest. Mitteilungen der IVL, Number 22, 23 p.