A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 2 - Lakes

Dissolved Oxygen in Lakes

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 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 concentrations steadily decline. They are 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 percent 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. Rivers and streams also deliver oxygen to lakes, especially if thy are turbulent and thus well aerated when they reach the lake. Consequently, natural variation of DO concentration in lakes is also caused by weather and changes in inflowing streams (e.g., higher, more turbulent flow during winter months).

Another physical process that affects DO concentrations is the relationship between water temperature and gas saturation. Cold water can hold more gas – that is DO – 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 or in the warmer top portion of a lake, the total amount of oxygen present may be limited by temperature.

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 decomposes. In deeper, stratified, lakes, this difference may be acute – plenty of oxygen near the top but practically none near the bottom. If the lake is shallow and easily mixed by the wind, the DO concentration may be fairly consistent throughout the water column.

Putting the Squeeze on Fish

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 water near the bottom – the hypolimnion has too little oxygen. Conditions may become especially serious during a spate of hot, calm weather, resulting in the loss of many fish. You may have heard about summertime fish kills in local lakes that likely result from this problem.

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 or lawn clippings) 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 algae 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, the balance between oxygen production and oxygen consumption becomes skewed and low DO may become even more of a problem.

Dissolved oxygen concentrations are reported in units of milligrams of gas per liter of water – mg/L. (The unit mg/L is equivalent to parts per million – ppm.) There is no numerical Washington State water quality standard for oxygen in lakes. The standard reads - there will be "no measurable decrease from natural conditions." DO concentrations for three Western Washington lakes are shown in the table above to provide values for comparison.

The next section talks about pH in lake water.

Chapter Four provides information on how to collect and analyse samples for dissolved oxygen.

Temperature | Oxygen | pH | Secchi | Nutrients | Turbidity | Chlorophyll | Fecal Coliforms

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Last updated on April 01, 2008