A bioindicator is a plant or animal species that is known to be particularly tolerant or sensitive to pollution . Based on the known association of an organism with a particular type or intensity of pollution, the presence of the organism can be used as a tool to indicate polluted conditions relative to unimpacted reference conditions. Sometimes a set of species or the structure and function of an entire biological community may function as a bioindicator. In assessing the impacts of pollution, bioindicators are frequently used to evaluate the "health" of an impacted ecosystem relative to a reference area or reference conditions. Field-based, site-specific environmental evaluations based on the bioindicator approach generally are complemented with laboratory studies of toxicity testing and bioassay experiments.
The use of individual species or a community structure as bioindicators involves the identification, classification and quantification of biota in the affected area. While many species are in use, the most widely used biological communities are the benthic macroinvertebrates. These are the sedentary and crawling worms and insect larvae that reside in the bottom sediments of aquatic systems such as lake and river bottoms. The bottom sediments usually contain most of the pollutants introduced into an aquatic system. Since these macroinvertebrates have limited mobility, they are continually exposed to the highest concentrations of pollutants in the system. Therefore, this benthic community is an ideal bioindicator: stationary, localized and exposed to maximum pollutant concentrations within a specific location.
Often, alterations in community structure due to pollution include a change from a more diverse to a less diverse community with fewer species or taxa. The indicator community may also be composed mostly of species that are tolerant of or adapted to polluted conditions and pollution-sensitive species that are present upstream may be absent in the impacted zones. However, depending on the type of pollutant, the abundance of the pollution-tolerant species may be very high and, therefore, the size of the benthic community may be similar to or exceed the reference community upstream. This is common in cases where pollution from sewage discharges adds organic matter that provides food for some of the tolerant benthic species. In the case of toxic chemical (e.g., heavy metals , organic compounds) pollution, the benthic community may show an overall reduction both in diversity and abundance.
Tubificid worms are an example of pollution-tolerant indicator organisms. These worms live in the bottom sediments of streams and lakes and are highly tolerant of the kind of pollution that results from sewage discharges. In a river polluted by wastewater discharge from a sewage treatment plant, it is common to see a large increase in the number of tubificid worms in the stream sediments immediately downstream of the discharge. Upstream of the discharge, the number of these worms is much lower, reflecting the cleaner conditions. Further downstream, as the discharge is diluted, the number of tubificid worms again decreases to a level similar to the upstream portions of the river. Large populations of these worms dramatically demonstrate that pollution is present, and the location of these populations may also indicate the general area where the pollution enters the environment .
Alternatively, 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 show a pattern opposite to that of the tubificid worms. The mayfly larvae are normally present in large numbers above the discharge point, decrease or disappear at the discharge point (just where the tubificid worms are most abundant) and reappear further downstream as the effects of the discharge are diluted. In this case, the mayflies are pollution-sensitive indicator organisms and their absence serves as the indication of pollution. Similar examples of indicator organisms can be found among plants, fishes and other biological groups. Giant reedgrass (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 unquestionable but limited. While their presence or absence provides a reliable general picture of polluted conditions, it is often difficult to identify clearly the exact sources of pollution, especially in areas with multiple 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 numerous possible sources of pollution in this area which include ship traffic, sewage discharge, industrial discharge, and storm runoff . As more is learned about the physiology and life-history of an indicator organism and its response to different types of pollution, it may be possible to draw more specific conclusions.
Although the two terms are sometimes used interchangeably, indicator organisms should not be confused with monitor organisms (also called biomonitors) which are organisms that bioaccumulate toxic substances present in trace amounts in the environment. For example, when it is difficult to measure directly the low concentrations of a pollutant in water, chemical analysis of shellfish tissues from that location may show much higher, easily detected concentrations of that pollutant. In this case, the shellfish is used to monitor the level of the long-term presence of that pollutant in the area.
In the environmental field, bioindicators are commonly used in field investigations of contaminated sites to document impacts on the biological community and ecosystem. These studies are then followed up with focused laboratory tests to pinpoint the source of toxicity or stress. After clean-up and remedial actions have been implemented at a site, bioindicators are also used to track the effectiveness of the remediation activity. In the future, bioindicators may be used more widely as investigative and decision-making tools from the initial pollution and impact assessment stage to the remediation and post-remediation monitoring stages.
[Usha Vedagiri ]
Connell, D. W., and G. J. Miller. Chemistry and Ecotoxicology of Pollution. New York: Wiley-Interscience, 1984.