Dissolved Oxygen (DO) in a stream may vary from 0 mg/l to 18 mg/l. Readings above 18 mg/l are physically impossible at Earth’s surface. Dissolved oxygen gets into the water by diffusion from the atmosphere, aeration of the water as it tumbles over falls and rapids, and as a waste product of photosynthesis.
Reduced DO levels in stream water may be because the water is too warm. As the temperature of the water increases, the maximum amount of dissolved gases, like oxygen, that the water can hold within solution, decreases. The higher temperatures may also cause other chemical or biological reactions to occur that "consume" or use up the existing oxygen content of the water faster than it can be reintroduced to the solution through diffusion or mixing.
Decreased DO levels may also be indicative of too many bacteria and an excess amount of biological oxygen demand - BOD (untreated sewage, partially treated sewage, organic discharges, anoxic discharges) which use up DO.
A third reason for decreased DO may be fertilizer runoff from farm fields and lawns. The same fertilizer which was meant to make land plants grow better, now makes the aquatic plants do the same. If the weather becomes cloudy for several days, respiring plants will use much of the DO while failing to photosynthesize. When the increased numbers of aquatic plants eventually die, their organic matter supports greater amounts of bacteria which use up large amounts of the dissolved oxygen.
Water Quality Index Chart - Based on Dissolved Oxygen
For the surface water quality index calculator, the chart for Dissolved Oxygen is actually based on the percent saturation of Dissolved Oxygen in water. For example, the field-measured Dissolved Oxygen concentration is 5.0 mg/L, but the saturated concentration of Dissolved Oxygen, as a function of temperature elevation and salinity, is 8 mg/;, the percent saturation 62.5%. Therefore, using the graph below, the Q-value would be approximately 54. The Q-value is a weighing factor in the Water Quality Index Calculator for Surface Water. The major benefit of the web-based Water Quality Index Calculator is that it uses a more uniform and accurate way to interpret the Q-value from the historic graph below.
Water Quality Index and BOD - Biological Oxygen Demand
For the surface water quality index calculator, the chart for BOD is actually based on a laboratory test to determine the 5-day BOD. The 5-day BOD is how much oxygen will be consumed or used by the naturally-occurring organism over a 5-day period of time. This test relates to the amount of potential decomposable organic material or food in the water. If the 5-day BOD is ten, the corresponding Q-value would be approximately 34.
Students should be aware that plants, in general, only produce oxygen when light is available for photosynthesis. Rooted aquatic plants are more abundant in lakes and impounded rivers than in rivers with significant current or in streams. Large daily fluctuations in dissolved oxygen are characteristic of bodies of water with extensive plant growth. DO levels rise from morning through the afternoon as a result of photosynthesis, reaching a peak in late afternoon; cloudy days will also reduce photosynthesis. Photosynthesis stops at night, but plants and animals continue to respire and consume oxygen. As a result, DO levels fall to a low point just before dawn. Dissolved oxygen levels may dip below 4 mg/l in such waters - the minimum amount needed to sustain warm water fish like bluegill, bass, and pike.
The amount of DO an aquatic organism needs depends upon its species, the temperature of the water, pollutants present, and the state of the organism itself (adult or young, active or dormant).
A trout needs five to six times more DO when the water temperature is 24 °C (75 °F) as compared to when the water temperature is 4 °C (41 °F). The increased DO is needed to support an increase in metabolic activity - a phenomenon shared by other cold-blooded aquatic animals.
The generally accepted minimum amount of DO that will support a large population of various fish is from 4 to 5 mg/l. When the DO drops below 3 mg/l, even the hardy fish die. Keep in mind that even though there may be enough DO to keep an adult alive, reproduction may be hampered by the need for higher DO for eggs and immature stages. Depletion in DO can cause major shifts in the kinds of aquatic organisms found in water bodies. Species that cannot tolerate low levels of DO - mayfly nymphs, stonefly nymphs, and beetle larvae - will be replaced by a few kinds of pollution-tolerant organisms, such as worms and fly larvae. Nuisance algae and anaerobic organisms (that live without oxygen) may also become abundant in waters with low levels of DO.
The following will give you some idea of how various fish species differ in their DO requirements:
Northern Pike - 6.0 mg/L
Black Bass - 5.5 mg/L
Common Sunfish - 4.2 mg/L
Yellow Perch - 4.2 mg/L
Black Bullhead - 3.3 mg/L
Certain pollutants interfere with oxygen uptake and metabolism so that some species of aquatic animals may need higher DO levels when these pollutants are present.