Dissolved oxygen (often referred to as D.O.) is essential for healthy lakes and impounded rivers. The presence of oxygen in water is a positive sign, while the absence of oxygen is a signal of severe pollution. Rivers range from high to very low levels of D.O. in the water - so low, in some cases, that they are practically devoid of aquatic life.
Most aquatic plants and animals need oxygen to survive. Fish and some aquatic insects have gills to extract the dissolved oxygen from the water. Some aquatic organisms, like pike and trout, require medium-to-high levels of dissolved oxygen to live. Other animals, like carp and catfish, can tolerate low levels of dissolved oxygen. Waters of consistently high levels of dissolved oxygen are usually considered healthy and stable ecosystems, capable of supporting many different kinds of aquatic organisms.
Much of the dissolved oxygen in water comes from the atmosphere. Waves on lakes, slow-moving rivers and tumbling water on fast-moving rivers act to mix atmospheric oxygen with water. Algae and rooted aquatic plants also provide oxygen to the water through photosynthesis.
In general, rooted aquatic plants are more abundant in lakes and impounded rivers than in rivers with significant current. Large daily fluctuations in dissolved oxygen levels are characteristic of bodies of water with extensive plant growth. Dissolved oxygen levels rise from morning through the afternoon as a result of photosynthesis, reaching a peak in the late afternoon. Photosynthesis stops at night, but plants and animals continue to respire and consume oxygen. As a result, dissolved oxygen levels fall to a low point just before dawn. Dissolved oxygen levels may dip below 4 mg/liter in such waters - the minimum amount needed to sustain warmwater fish like bluegill, bass and pike.
Rivers that consistently have dissolved oxygen values of 90 percent or higher are considered healthy, unless the waters are supersaturated due to cultural eutrophication. Rivers below 90 percent saturation may have large amounts of oxygen-demanding materials, i.e. organic wastes.
Water temperature and the volume of water moving down a river (discharge) affect dissolved oxygen levels. Gases, like oxygen, dissolve more easily in cooler water than in warmer water. River discharge is related to the climate of an area. During dry periods, flow may be severely reduced and air and water temperatures are often higher. Both of these factors tend to reduce dissolved oxygen levels. Wet weather, melting snows, and downstream of a dam spillway can increase flow, resulting in higher saturation levels of oxygen.
Depletions in dissolved oxygen can cause major shifts in the diversity of aquatic organisms found in water bodies. Species that cannot tolerate low levels of dissolved oxygen - mayfly nymphs, stonefly nymphs, caddisfly larvae and beetle larvae - will be replaced by a few kinds of pollution-tolerant organisms, such as worms, blood midges and fly larvae.
Why is the amount of dissolved oxygen in the water so low?
What time of the day did you conduct your sampling for dissolved oxygen?
What is the climate of the region where you conducted your water sampling?
Check out your local map - can you find any sewage treatment plants in the watershed?
What sources of runoff can you identify in the watershed? (hint: fertilizers, organic waste)
When conducting biological sampling, did you find a diverse number of aquatic insect and plant species? Do you think these results indicate a relationship between dissolved oxygen levels and the presence of particular aquatic species in the river or lake?
Can you find any food processing plants, meat packinghouses, dairies or other industrial sources with direct discharge into your local river or lake?
What is the cause(s) of your problem? What did you find that led you to this conclusion?
Now is the time to take ACTION!