Decision process for identification of estuarine benthic impairments

摘要: To meet the requirements of the Clean Water Act, the States of Maryland and Virginia are in the process of developing biological criteria for evaluating estuarine waters. The Chesapeake Bay Benthic Index of Biotic Integrity (B-IBI) is the basis for these biological criteria. As an initial step, a Workgroup developed an interim framework for the application of the B-IBI to the States’ water quality inventory report (305b report) and list of impaired waters (303d list). The Workgroup included representatives of the Maryland Department of the Environment (MDE), Maryland Department of Natural Resources (MDNR), Virginia Department of Environmental Quality (DEQ), USEPA Region III Office of Watersheds, Old Dominion University Department of Biological Sciences, and Versar, Inc.Two alternative methods were initially proposed for 303 (d) impairment decisions based on the BIBI:(1) a weighted (stratified) mean approach, and (2) comparisons of cumulative frequency distributions and proportions. In the first method, the mean benthic condition of a Chesapeake Bay segment (as determined by the stratified mean B-IBI value) would be compared against the mean of the reference condition (based on the original index development reference sites) using a t-test. Because a segment may be represented by more than one habitat for which reference conditions may differ, weights are applied to combine means for each habitat type (strata)(Cochran 1977). In the second method, the cumulative frequency distribution of B-IBI values for a segment would be compared to the cumulative distribution of B-IBI values for the reference condition, and a test conducted to …