Indicator organisms, sometimes called bioindicators, are plant or animal species known to be either particularly tolerant or particularly sensitive to pollution . The health of an organism can often be associated with a specific type or intensity of pollution, and its presence can then be used to indicate polluted conditions relative to unimpacted conditions.
Tubificid worms are an example of organisms that can indicate pollution. Tubificid worms live in the bottom sediments of streams and lakes, and they are highly tolerant of sewage. In a river polluted by wastewater discharge from a sewage treatment plant, it is common to see a large increase in the numbers of tubificid worms in stream sediments immediately downstream. Upstream of the discharge, the numbers of tubificid worms are often much lower or almost absent, reflecting cleaner conditions. The number of tubificid worms also decreases downstream, as the discharge is diluted.
Pollution-intolerant organisms can also be used to indicate polluted conditions. The larvae of mayflies live in stream sediments and are known to be particularly sensitive to pollution. In a river receiving wastewater discharge, mayflies will show the opposite pattern of tubificid worms. The mayfly larvae are normally present in large numbers above the discharge point; they decrease or disappear at the discharge point and reappear further downstream as the effects of the discharge are diluted.
Similar examples of indicator organisms can be found among plants, fish, and other biological groups. Giant reed-grass (Phragmites australis ) is a common marsh plant that is typically indicative of disturbed conditions in wetlands . Among fish, disturbed conditions may be indicated by the disappearance of sensitive species like trout which require clear, cold waters to thrive.
The usefulness of indicator organisms is limited. While their presence or absence provides a reliable general picture of polluted conditions, they are often little help in identifying the exact sources of pollution. In the sediments of New York Harbor, for example, pollution-tolerant insect larvae are overwhelmingly dominant. However, it is impossible to attribute the large larval populations to just one of the sources of pollution there, which include ship traffic, sewage and industrial discharge, and storm runoff .
The U.S. Environmental Protection Agency (EPA) is working diligently to find reliable predictors of aquatic ecosystem health using indicator species. Recently, the EPA has developed standards for the usefulness of species as ecological indicator organisms. A potential indicator species for use in evaluating watershed health must successfully pass four phases of evaluation. First, a potential indicator organism should provide information that is relevant to societal concerns about the environment , not simply academically interesting information. Second, use of a potential indicator organism should be feasible. Logistics, sampling costs, and timeframe for information gathering are legitimate considerations in deciding whether an organism is a potential indicator species or not. Thirdly, enough must be known about a potential species before it may be effectively used as an indicator organism. Sufficient knowledge regardin! g the natural variations to environmental flux should exist before incorporating a species as a true watershed indicator species. Lastly, the EPA has set a fourth criterion for evaluation of indicator species. A useful indicator should provide information that is easily interpreted by policy makers and the public, in addition to scientists.
Additionally, in an effort to make indicator species information more reliable, the creation of indicator species indices are being investigated. An index is a formula or ratio of one amount to another that is used to measure relative change. The major advantage of developing an indicator organism index that is somewhat universal to all aquatic environments is that it can be tested using statistics . Using mathematical statistical methods, it may be determined whether a significant change in an index value has occurred. Furthermore, statistical methods allow for a certain level of confidence that the measured values repres! ent what is actually happening in nature . For example, a study was conducted to evaluate the utility of diatoms (a kind of microscopic aquatic algae) as an index of aquatic system health. Diatoms meet all four criteria mentioned above, and various species are found in both fresh and salt water. An index was created that was calculated using various measurable characteristics of diatoms that could then be evaluated statistically over time and among varying sites. It was determined that the diatom index was sensitive enough to reliably reflect three categories of the health of an aquatic ecosystem . The diatom index showed that values obtained from areas impacted by human activities had greater variability over time than diatom indices obtained from less disturbed locations. Many such indices are being developed using different species, and multiple species in an effort to create reliable information from indicator organisms. As more is learned about the physiology and life history of indicator organisms and their individual responses to different types of pollution, it may be possible to draw more specific conclusions.
[Terry Watkins ]
Browder, J. A., ed. Aquatic Organisms As Indicators of Environmental Pollution. Bethesda, MD: American Water Resources Association, 1988.
Connell, D. W., and G. J. Miller. Chemistry and Ecotoxicology of Pollution. New York: Wiley-Interscience, 1984.