Dissolved Oxygen

Oxygen is essential for aquatic life as well as terrestrial animals. The amount of oxygen dissolved in water affects the number and kind of animals found there.

There are several causes of reduced dissolved oxygen levels in streams. Problems most often occur during the summer low flow period. Warm water holds less oxygen than cold water and shallow water heats up more quickly than deep water. Slow flowing water has little surface turbulence, so little oxygen mixes into the water. Organic wastes such as sewage and agricultural runoff consume oxygen as they decompose. Fertilizers from gardens and farmlands provide nutrients for algae and, initially, the algae grow quickly and add oxygen to the water; however, later they decompose and consume oxygen.

During the summer low flow period, dissolved oxygen concentrations often fluctuate during the day. When water temperature increases during the day, the oxygen level decreases by late afternoon. Algal photosynthesis during blooms can increase the oxygen level during the day. Animal and plant respiration can depress the oxygen level during the night.

Planting stream bank vegetation helps to increase oxygen levels in streams. Foliage provides shade and roots absorb nutrients. Controlling sources of sewage, manure, and fertilizers also helps. Fencing keeps livestock away from streams. Building structures in streams helps to aerate the water.

Dissolved oxygen is indicated in hundredths of parts per million (ppm), in mg/L or in % air saturation. Healthy streams are saturated with oxygen (90 to 100% saturation) during most of the year, at 25° C at sea level.

Minimum concentrations should be not less than 11.0 mg/L (11 ppm) when fish larvae/alevin are present, minimum of 8.0 mg/L (8 ppm) other periods.

pH

The pH scale measures the relative acidity or alkalinity of any substance. The scale ranges from very strong acid, at pH 0, to very strong base, at pH 14. Pure water has a neutral pH of 7, with an equal concentration of H+ (hydrogen) and OH-(hydroxyl) ions. Acidic water has a high concentration of hydrogen ions and a low concentration of hydroxyl ions. The reverse occurs in alkaline water. The scale is logarithmic, so a one unit difference in pH reflects a tenfold change in acid or alkaline concentration. For example, vinegar (pH 3) is ten thousand times more acidic than distilled water (pH 7).

Most aquatic organisms are sensitive to small pH changes and prefer a pH of 6.0 to 8.5. Waters with a pH beyond this range usually do not have enough species to maintain a food web.

Stream pH level depends on the geology of the surrounding area, and usually falls between 6.5 and 8.0. Streams that drain soils with high mineral content usually are alkaline, whereas streams that drain coniferous forests usually are acidic. Algal photosynthesis during a bloom can cause increased pH.