Temperature: Temperature changes can affect all aquatic life. For example, warmer water holds less dissolved oxygen (D.O.) than cold water and triggers higher plant growth and respiration rates. The lowered oxygen levels of warmer waters are further reduced when more plants die and decay. Although most aquatic life has adapted to survive within a range of water temperatures, some fish species (trout, for example), require cooler waters. Temperatures over 78 degrees F, for example, are usually fatal to brook trout, which need waters in the range of 55-65 degrees F. The metabolic rate of organisms, or the rate at which they convert food into energy, also increases with higher water temperatures, resulting in even greater demands on oxygen.

Turbidity: Water clarity is one of the most obvious measures of water quality. Turbidity can be a useful indicator of runoff from construction sites, fields, logging activity, industrial discharges, stream bank erosion, and other sources. All streams have background turbidity, or a baseline standard for a natural amount of turbidity. Fish and aquatic life that are native to streams have evolved over time to adapt to varying levels of background turbidity. Turbidity is measured in NTU's or Nephelometeric Turbidity Units, which is a measure of the amount of light scattered by suspended material in the sample. The more turbid the water is the higher the turbidity level and NTU's will be. For example, many of our local streams will run on the average of <10 NTU or > 21.5" of visibility. A water sample having visibility of 9.6" will have a value of 30 NTU's. Very turbid water with visibility of 2.5" would have a value of 240 NTU.

Relational Trends of Freshwater Fish Activity to Turbidity Values and Time: With values exceeding 10 NTUs for a few hours, fish start to show signs of stress. For a few days, we see reduced feeding rates and coughing rates. For a few weeks, long-term reduction in feeding success. With values exceeding 100 NTUs for a few hours, fish begin to show stress. For a few days, we see reduced feeding rates and increased respiration. For a few weeks, delayed hatching rates and long term reduction in feeding success. For a few months, death usually occurs.

Dissolved Oxygen and Percent Oxygen Saturation: Both aquatic plants and animals depend on dissolved oxygen (D.O.) for survival. D.O. concentrations are influenced by many factors including water temperature, the rate of photosynthesis, the degree of water turbulence or wave action, and the amount of oxygen used by respiration and decay of organic matter. Both plants and animals depend on dissolved oxygen for survival. Lack of dissolved oxygen can cause aquatic animals to quickly leave the area or face death. Under low-oxygen conditions, the aquatic animal community changes quickly. Under extreme conditions, lack of oxygen can kill aquatic plants and animals. Measuring dissolved oxygen is probably the most significant water quality test to determine the suitability of a stream for fish and many other aquatic organisms.

Factors affecting oxygen levels: Oxygen is removed from the water by respirations of living organisms including fish, bacteria, fungi, and protozoans; by the decay process of organic matter; and by chemical reactions. Water temperature and atmospheric pressure affect the capacity of water to hold dissolved oxygen. Cold water at high atmospheric pressure holds more dissolved oxygen than warm water at low atmospheric pressure. Oxygen levels also are affected by the degree of light penetration (turbidity, color, and water depth) and the degree of water turbulence or wave action. D.O. is reported as milligrams of oxygen per liter of water (mg/L) which can be called parts by weight per million (ppm). Different aquatic organisms have different oxygen needs. Trout and stoneflies, for example, require high oxygen levels. Trout need water with at least 6 mg/L D.O. Warm water fish like bass and bluegills survive nicely at 5 mg/L D.O. and carp and bloodworms can survive on less than 1 mg/L D.O.

Oxygen Saturation: Saturation is the limit of how much oxygen water can hold under certain conditions.