What are they?
Biological indicators are aquatic plant and animal life that are susceptible to specific types and levels of pollutants. Many organisms require a specific range of physical and chemical parameters to flourish in a surface water.
Why test for them?
Fish, algae and benthic macroinvertebrates are sometimes used when determining the biological integrity of a surface water.
Benthic (bottom dwelling) macroinvertebrates, primarily immature insects, are useful organisms because (1) many are sensitive to physical and chemical changes in their habitat; (2) many live in the water more than a year; (3) they cannot easily "escape" pollution; and (4) they are easily collected in many streams and rivers. The presence or absence of these "indicator" species can reveal the overall health of the water body. Biosurveys or bioassays use the collection and classification of aquatic organisms and changes in their populations as indicators of environmental change.
Typically, unpolluted water will contain a greater diversity of organisms than polluted water. Polluted water will support larger numbers of tolerant organisms and have less diversity.
What affects aquatic organisms?
Factors that affect the aquatic community will vary greatly as you travel from the source of a river to its mouth. The most significant factors are water temperature, volume and velocity and instream habitat. Instream habitat includes stream bottom composition (substrates) and food quantity and quality.
In addition to raising the water temperature, suspended solids can settle on eggs and suffocate them, while other types of pollution will eliminate the most sensitive organisms, reducing the diversity of the community, yet an increase in pollution-tolerant organisms may occur. Pollution-intolerant species include mayflies, stoneflies, rock bass and some dace. Examples of tolerant species are leeches, midge larva, bullheads and carp.
How can we help strengthen the community?
Closely monitoring the riparian area and watershed associated with a river or stream for erosion, sources of pollution, and changes in the diversity and population of aquatic organisms are the most important tools.