USE OF BIOLOGICAL INDICATORS IN TMDL ASSESSMENT AND IMPLEMENTATION

Abstract

Most states in the U.S. have a general water quality standard intended to protect water from all potential pollutantsnot specifically named or identified in other standards. Biological indicators are used, in part, to assess the level of waterquality with respect to this general standard. Under EPAs Total Maximum Daily Load (TMDL) program, impaired watersbased on a biological assessment require an additional step compared with non-biological TMDLs. In non-biological TMDLs,the pollutant is typically the parameter being monitored, with a direct link to the impairment. In biological TMDLs, causeand effect must first be established between one or more pollutants and the impacted biological community. This articlepresents examples of approaches taken in different states to monitor and assess the biological health of our streams basedon varying combinations of algal, macroinvertebrate, and fish communities. While fish are the ultimate integrator of lowerecological organisms, their occurrence and abundance has been greatly manipulated by humankind. Periphytic algae areperhaps the fastest responding biological population and can be used for some pollutant-specific diagnoses, but most stateslack the expertise required for detailed taxonomic classification. Macroinvertebrates, the most commonly monitoredbiological community, are abundant in most streams, but most metrics are not diagnostic of specific stressors. Within theTMDL framework, issues are discussed related to setting TMDL targets, linking biological impairments with pollutants, anddefining biological target endpoints. Although surrogate measures are often used for setting TMDL target loads, biologicalrecovery is measured against biological endpoints. The use of biological indicators for assessment and development ofbiological TMDLs can be improved through modeling procedures that better define cause-and-effect relationships, througha better understanding of the limits of restoration, and through a more unified national policy that focuses on restoration.