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Biota (living organisms) can become contaminated with toxic substances by ingesting contaminated water, food, or other substances; inhaling toxic substances in the atmosphere; respiring contaminated water; or bathing or swimming in contaminated waters. Biota often become contaminated when they are exposed to substances that they can not readily metabolize or excrete at high enough concentrations or frequencies. The contamination of biota is often manifested in altered metabolic or endocrine processes, developmental or reproductive abnormalities, or the rapid and aberrant growth of certain cells (as in tumors or cancers). In many cases, the contamination of terrestrial, freshwater, or marine organisms can be an indicator of the impacts of human activities on the environment, as well as a warning sign of the potential contamination of human populations. An upward trend would not be sustainable indefinitely.
Extensive monitoring of the contamination of freshwater fish and birds by persistent pesticides and other synthetic organic compounds has been conducted since the early 1970s. Figure 4.21 demonstrates that the overall concentrations of dichloro-diphenyl-trichloroethane (DDT) and its derivatives and polychlorinated biphenyls (PCBs) have declined in freshwater fish since the 1970s. In the 1960s and early 1970s, DDT (a pesticide) and PCBs (a family of industrial compounds) were widely used, and they eventually washed into rivers and lakes and accumulated in fish and other organisms. Contaminated fish were eaten by eagles, which in turn accumulated the DDT, impairing their ability to produce viable eggs. As the use of DDT and PCBs was curtailed during the 1970s, fish became less contaminated and the eagle populations grew. We have made major progress in reducing the exposure of humans and other organisms to DDT and PCBs in North America.
The contamination of marine organisms is an important concern for growing coastal populations. Today more than 50% of the U.S. population lives within 80 miles of an ocean or Great Lake, and population densities are more than five times the national average. Since 1986, the Mussel Watch Project of the National Status and Trends Program has conducted measurements on surface sediments and on mussels and oysters from about 250 coastal and estuarine sites. Samples are analyzed to determine the levels and trends of synthetic chlorinated compounds (e.g., DDTs), PCBs, polycyclic aromatic hydrocarbons (PAHs), and trace metals such as mercury (Hg), lead (Pb), and cadmium (Cd). It should be noted that not all 250 sites are sampled every year, that the sites selected for sampling vary from year to year, and that mussels are collected at some sites and oysters at other sites. At least four years of data are needed to determine whether statistically significant trends in concentrations are occurring, and it is overall trends that matter--not year-to-year variations.
Since 1986, the overall levels of chlorinated hydrocarbons and cadmium in coastal biota have decreased; these changes were not unexpected since chlorinated hydrocarbons have been banned for use in the United States, and the use of cadmium in electroplating, rust-proofing, stabilizers in plastics, and pigments has declined. Chlordane and tributyltin have also decreased due to bans or restrictions on their use. The levels of most other chemicals (e.g., PAHs, mercury, and lead) have remained about the same since 1986, and no significant chemicals show an increasing trend in concentration. Based on the overall data from 1986 through 1995, we can say that coastal contamination has generally decreased for those monitored persistent contaminants whose use has been curtailed.
It should be noted that despite improvements in a number of persistent contaminants, there is some concern that other substances, such as chlorine-containing compounds, released through a variety of human activities may disrupt the normal functioning of endocrine systems in humans and animals. Additional measures may be needed to track their use and effects on biota.
References:
• O’Connor, T.P. “Trends in Chemical Concentrations in Mussels and Oysters Collected along the U.S. Coast from 1986-1993,” Marine Environmental Research, Vol. 41, pp. 183-200: 1996.
• Schmitt, C.J. and C.M. Bunck. “Persistent Environmental Contaminants in Fish and Wildlife” Our Living Resources, National Biological Survey, U.S. Dept. of the Interior, Washington DC, 1995.
Link(s) to be added, when feasible, to data at level of detail suitable for use at the community level.
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http://www.sdi.gov/indicators/lc_contam.htm Last Modified: May 13, 2002 |