Development of a Biological Condition Gradient for the Upper Mississippi River and 'Synthetic' Historical Fish Community
The "'Biological Condition Gradient'" (BCG) was originally developed by U.S. EPA to overcome difficulties in effectively communicating about ecological condition measured by fundamentally different biological assessment models and indices. It was also developed to articulate the conceptual underpinnings that are common to all biological assessment methods. The BCG is a scientiﬁcally based conceptual model of biological responses to the increased effects of stressors. The BCG encompasses the complete range, or gradient, of aquatic resource condition from "as naturally occurs", i.e., undisturbed or minimally disturbed, through increasing levels of alteration to severely degraded conditions. The BCG is divided into six condition levels, with Level 1 representing natural, undisturbed conditions, to Level 6 representing severely degraded conditions, and the interceding levels representing the gradient of conditions in between. Ten ecological attributes are described for each BCG level as to how each is expected to change as biological condition transitions along a gradient of increasing stress. The intent is to tailor the make-up and response of the ecological attributes of the BCG to the system in question, which is the main channel of the Upper Mississippi River. This part of the UMR project developed a BCG for the UMR main channel. While natural conditions are the conceptual "anchor", such conditions likely no longer exist in the UMR due to legacy alterations of the landscape and river system. Setting reasonably protective and attainable CWA attainment thresholds depends on being able to quantify both the biological and stressor gradients that exist within the UMR. The task of identifying potential impairment thresholds for the UMR main channel demanded an understanding of the historical "anchor" or the "as naturally occurs" condition. A lack of minimally disturbed reference sites required a supplementary approach, the development of a historically accurate "synthetic" fish assemblage. Having this as the "anchor" broadened the environmental gradient along which to measure biological performance and better understand the influence of stressor gradients. The original GRFIn was calibrated separately for the Impounded and Open River reaches of the UMR which will result in a different response to stressors and different thresholds. The BCG supported synthetic fish assemblage results suggest that the characteristics of the historical fish assemblage were similar between the two reaches, thus indicating that a single GRFIn calibrated for the Impounded reach is wholly applicable to the Open River reach. Relying on the GRFIn calibrated to the Open River reach stressor gradient under-estimates the true potential of the fish assemblage.