Microcosms In Ecotoxicology

Microcosms are important as research tools in ecotoxicology for understanding the effect of pollutants on ecosystems. Experiments in which treatments are various concentrations of pollutant chemicals can be conducted in microcosms with replication and with containment of environmental impacts due to isolation from the environment. Although this role for microcosms in ecotoxicological research is well established, their potential role within formal regulatory testing or screening protocols in risk assessment is controversial. Challenges for ecological engineering include the design and operation of microcosms that are effective for both research and risk assessment in ecotoxicology. Uses for risk assessment will be emphasized in this section owing to the controversial debate about the role of microcosms and the wide potential applications of microcosm technology that are involved.

Testing or screening of chemicals is regulated by the Environmental Protection Agency (EPA) in the U.S. This regulation is necessary because of the tremendous number of new chemicals that are produced each year for industrial and commercial purposes. Many of these chemicals are xenobiotic or man-made, whose potential environmental effects are unknown. Thus, uncertainty arises because natural ecosystems have never been exposed to them and species have not adapted to them. Special concern is needed for pesticides because they are intentionally released into the environment and are intended to be toxic, at least to target organisms. The primary examples of legislation covering regulatory testing and screening of chemicals are the Toxic Substances Control Act and The Federal Insecticide, Fungicide, and Rodenticide Act, along with several others (Harwell, 1989). An interaction has developed among the EPA, the chemical industry, environmental consulting firms, and academic researchers in relation to risk assessment of new chemicals, which has in turn created opportunities for applications of ecologically engineered microcosm technology.

EPA's risk assessment approach for chemicals (Norton et al., 1995) has evolved over time since early work in the 1940s on methods for measuring the effects of pollutants. The purpose of risk assessment is to evaluate potential hazards in order to prevent damage to the environment and human health. The basis for testing or screening is a hierarchical (tiered) protocol of sequential tests. Physical and chemical properties are tested at the lowest tier, and acute and chronic toxicity data along with estimated exposure data are gathered for several aquatic species at intermediate tiers, followed at least in principle by simulated field testing at the highest tier (Hushon et al., 1979). The intention is to minimize the number of tests required to assess a chemical's hazard and at the same time to include a comprehensive range of tests. Each tier level can trigger testing at higher levels by comparison of test results to established end points which determine whether or not the chemical is considered to be toxic or hazardous. Choice of end points is important because they are the criteria for determining regulatory action. Concern exists at all levels about tests that result in false negatives (results which indicate that a chemical is toxic when it is in fact not toxic) and false positives (results which indicate that a chemical is not toxic when it is in fact toxic). Cairns and Orvos (1989) suggest that the sequential arrangement of tests that were used from simple to the more complex possibly reflects, in a broad, general way, the historical development of the field. As a consequence, tests with which there is a long familiarity are placed early in the sequence and more recent and more sophisticated tests that are still in the experimental stage or development are placed last.

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