Performance of different landscape classifications for aquatic bioassessments: introduction to the series

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Bioassessment is the process of determining if human activity has altered the biological properties of an ecosystem. To quantify assessments we must be able to specify those biological properties that are either expected to occur in the absence of human alteration (the pristine condition) or that are attainable given current ecological, economic, and political constraints. Because we almost always lack knowledge about the biota that existed at sites prior to human alteration, we must usually infer the biological potential of a site from other information. Such inferences are typically derived from a classification of sites that relate variation in biological properties of interest to class membership. Our ability to detect impairment is therefore largely a function of how precisely and accurately expected conditions can be inferred from the classification used (Karr and Chu 1998). Many classification systems have been developed for freshwater habitats (reviews by Hawkes 1975, Cowardin et al. 1979, Busch and Sly 1992, Maxwell et al. 1995), but it is those with the ability to predict biological properties that are of primary interest to water resource managers who must quantify the biological condition of a site. As many countries move toward the use of biological assessments as a primary means of measuring the ecological health of their surface waters, it is imperative that the most robust classification systems possible be developed and implemented. All predictive classifications are based on the idea that biological properties can be inferred from readily measured environmental or historical features. Classifications based on regional landscape descriptors have received considerable attention from water resource managers