Potential ecological impacts of trace metals on aquatic biota within the Upper Little Tennessee River Basin, North Carolina

Abstract

The Upper Little Tennessee River (ULTR) possesses one of the most diverse assemblages of aquatic biota in North America, including the endangered Appalachian elktoe mussel (Alasmidonta raveneliana). Populations of the Appalachian elktoe significantly declined along with other species following an extreme flood in 2004. This paper examines the potential role that four toxic trace metals (Cu, Cr, Ni, and Zn) played in the population declines. Dissolved and total-recoverable concentrations of Cr and Ni measured during three flood events were below USEPA and North Carolina freshwater guidelines for potential impacts on aquatic biota, respectively. In contrast, 58% of the samples exceeded NC guideline values for total-recoverable concentrations of Cu and Zn. In general, metal concentrations increased with increasing discharge and suspended sediment concentrations (SSC). These relationships, combined with sequential extraction data from sediments, suggest that most metals were transported in the particulate form and were not readily bioavailable. During individual events, metal concentrations for a given discharge were influenced by a “first flush” hysteresis effect. Rapid increases in metal concentrations during the early stages of an event appear to be related to the entrainment of fine sediment, including particulate Fe to which the metals are sorbed. Instantaneous metal loads calculated for nine tributaries to the ULTR, combined with previously collected data, suggest that the majority of the metals were derived from the erosion of sediment and particulate Fe from subsurface soil horizons developed in bedrock containing sulfidic layers. The erosion was particularly pronounced in tributary basins in poor to moderate ecological condition. While a fraction of the Cu may have been derived from Cu-based pesticides and was periodically elevated above guideline values in river waters, the data in total suggest that toxic trace metals were unlikely to play a major role in the decline of elktoe populations.