Determining the Linkages for a Nutrient TMDL in a Stream Listed as Use-Impaired for Endangered Species

The Cahaba River located in Jefferson County, Birmingham, Alabama has been placed on the 303(d) list by USEPA Region IV and the Alabama Department of Environmental Management as use-impaired from nutrients. In a letter to the County, USEPA stated “The listing of the Cahaba River for nutrients is based on the inability of endangered species to survive or adequately reproduce as a result of alterations of habitat substrate caused by excessive nutrients…”. The premise by USEPA and the US Fish and Wildlife Service is that excessive nutrient levels have caused an algal community shift, from diatoms to filamentous green algae within the river, which has resulted in habitat impairment. Further, the agencies contend that this habitat impairment has led to the extirpation of eight endangered species (2 fishes and 6 molluscs). According to USEPA's TMDL flowchart, if nutrient impairment occurs in rivers dominated by periphytic algae, simple empirical relationships can be used to relate nutrient levels to the cause of impairment, excessive periphytic algae growth. However, in the case of the Cahaba River, a valid empirical relationship between total phosphorus and periphytic chlorophyll a does not exist (r2 = 0.12). We have demonstrated that this is due to various in-stream limiting factors such as light, turbidity, scour, herbivory and possibly herbicide toxicity. In order to model this complex system, we have applied USEPA's AQUATOX model. AQUATOX accounts for additional in-stream processes not available in other water quality models and resulted in a verifiable linkage between total phosphorus and algal biomass. AQUATOX also provided the linkages to ecosystem trophic levels which are important in determining the target level of total phosphorus which is protective of endangered species' habitats. We will describe the data used to define the issues on this TMDL, the process used in the calibration of AQUATOX, and the resulting linkages between total phosphorus and periphytic growth that are key to determining the ultimate target level of total phosphorus used for TMDL development.