Oxygen is as important to life in water as it is to life on land. Most aquatic plants and animals require oxygen for survival and the availability of oxygen affects their growth and development. The amount of oxygen found in water is called the dissolved oxygen concentration (DO). DO is a very important measure of the health of a stream--the presence of oxygen in water is a positive sign, the absence of oxygen in the water is often a sign that the stream is polluted. Dissolved oxygen levels vary from stream to stream, as do organisms' tolerance for dissolved oxygen levels. Factors which affect Dissolved Oxygen (DO): 1. Temperature: Oxygen, like all gases, is more soluble at lower temperatures, so generally the colder the water, the more oxygen it can hold!

2. Altitude/ Atmospheric Pressure: The higher the air pressure, the more oxygen that water can hold. Water at sea level can hold more oxygen than water found in a lake in the Cascade Mts. You can see this by pouring a cold soda pop into an open glass. If you let the soda pop sit on the table, you begin to see bubbles leave the pop and enter the air because there is less pressure to keep it in the water. Eventually, the soda pop goes flat.....in other words, it has lost all of its carbonation (carbon dioxide).

3. Water Turbulance:

4. The growth of plants: As plants are exposed to sunlight, they perform photosynthesis, which converts carbon dioxide and water into sugar and oxygen. The oxygen is released directly into the water, which raises the amount of oxygen in the water.

5. The amounts of decaying organic materials (dead things) in the water: The bacteria, molds, and other critters that decompose dead and decaying materials use oxygen to break the dead things down into simpler things. As they use this oxygen, the oxygen levels in the water decrease.

6. Physical, chemical, and biochemical activities in the water:

For short periods of time water may become supersaturate, holding more oxygen or other gases than it is supposed to. Supersaturation can also be harmful to aquatic organisms, causing a condition called gas bubble disease, which is similar to the bends disease that deep sea divers may get if they surface too fast.

For meaningful interpretation of dissolved oxygen levels two pieces of information are required, the amount of oxygen dissolved in the water sample, measured in mg/L, and the temperature of the stream at the time DO was measured. With these, the perecent saturation of the oxygen in the water can be determined. Other factors, such as biochemical oxygen demand (BOD) affect the interpretation of percent saturation levels, but for general purposes, the following guidelines may be useful:

125% or more Supersaturation at levels dangerous to fish

If oxygen levels in the water are to low, then aquatic organisms have difficulty respiring (breathing). Some organisms have adapted to low oxygen in water (like carp) or are able to ingest air directly (like bettas).

First the current barometric pressure and mean water temperature (ūC) are compared to a special table (available from your instructor) to find the solubility of oxygen in water at that air pressure and temperature. The mean of the sampled Dissolved Oxygen as measured in mg/L is then divided by the oxygen solubility and multiplied by 100 to get an answer which is in percent.

In rivers there is usually adequate mixing of water from the surface to the river bottom. However, in very slow moving or very deep rivers there may be little mixing of the water. This could cause differences in DO measurements from the surface to the river bottom. This is usually the worst in the hot summer, where rivers are shallow, slow moving and warm.

Type of Water Standard (mg/L DO)

Trout & Salmon Spawning Ž 11.0

Cold Water fisheries Ž 8.0

Cool Water fisheries Ž 6.5

Warm Water fisheries Ž 5.5

1. Rapid decomposition of organic materials, including dead algae, shoreline vegetation, manure or wastewater sources, all decrease oxygen levels.

2. High ammonia concentrations in the stream use up oxygen in the process of oxidizing ammonia (NH4) to nitrate (NO3).

3. At higher temperatures, less oxygen can dissolve in water.

4. Lack of turbulence (too many pools, not enough riffles) or mixing to expose water to atmospheric oxygen results in low dissolved oxygen concentrations in the stream.