have been increasingly aware of the need to bring biology back into the water quality equation. In some cases, chemical monitoring has actually exceeded the ability to detect biological impacts of chemical contaminants, so that large sums of money have been spent to remove contaminants that do not even affect aquatic organisms. On the other hand, reliance on chemical criteria or laboratory-derived toxicological information taken out of the environmental context has often allowed levels of toxicants or other materials that are harmful to aquatic populations.

The return of biology to environmental assessments has brought with it a need for knowledge about whole-organism biology, the study of which has become increasingly neglected in academic institutions over the past several decades. At the same time, many exciting developments, in fields ranging from molecular biology to landscape ecology, have potential application to the study and management of inland aquatic resources. This paper reviews the historical basis for the application of biological methods to water quality assessment and discusses factors that need to be considered in evaluating the biological integrity of inland aquatic ecosystems.

HISTORICAL BACKGROUND

Modern bioassessment of inland aquatic ecosystems has given rise to several terms and concepts regarding protection or restoration of aquatic environments (Steedman, 1994). Foremost among these are the ideas of integrity, which relates to whether biological systems are intact or restorable, and health, management, and sustainability, which relate to modification of sites by human activity.

The idea of biological, and subsequently ecological, integrity is traceable at least as far back as the writings of Aldo Leopold (1949), but its emergence as a formal ecosystem concept did not occur until the mid-1970s (e.g., Cairns, 1977a,b). The Water Quality Act Amendments of 1972 (P.L. 92-500) formalized the term ''biological integrity" under the directive to restore and maintain the "chemical, physical, and biological integrity of the nation's waters."

Initially, the primary focus was on chemical and physical aspects of the environment and on toxicity tests performed in the laboratory on both individual contaminants and complex mixtures of waste effluents from industry and other sources. The idea of biological integrity gradually evolved to include naturalness, sustainability, and ecosystem balance, structure, and function (Jackson and Davis, 1994). Karr and Dudley (1981) defined biological integrity as the "ability of an aquatic ecosystem to support and maintain a balanced, adaptive community of organisms having a species composition, diversity, and functional organization comparable to that of natural habitats within a region." Others have refined,



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