Interactions Between Water Chemistry and Waterborne Lead Exposure to Freshwater Organisms

This dissertation characterizes the influences of water chemistry on the acute toxicity of lead (Pb) to two of the long-standing sentinel test organisms commonly employed by the United States Environmental Protection Agency (USEPA), the fathead minnow (Pimephales promelas) and daphnid (Ceriodaphnia dubia), for parameterization of an acute Pb Biotic Ligand Model (BLM). In addition, a toxicogenomic approach was employed to identify genes that might serve as molecular markers of Pb exposure and long-term effects, as well as provide new insights as to the underlying toxic mechanisms of chronic Pb exposure in P. promelas. The endpoints of growth, reproduction, Pb accumulation, prey capture ability, and swimming performance of P. promelas were examined to assess the influences of water chemistry during chronic Pb exposures and to potentially link microarray-identified genes to outcomes of ecological significance. Importantly, this work revealed that calcium does not protect against acute toxicity to C. dubia or chronic Pb accumulation by P. promelas, indicating that current hardness-based regulations are inappropriate and provide further support for the need for alternative approaches to setting environmental regulations for Pb. The findings reported herein should facilitate the arrival of such an approach in the form of a new acute Pb BLM. However, different responses with respect to the influences of water chemistry on the acute toxicity of Pb were exhibited by these species suggesting that development of separate BLMs for P. promelas and C. dubia should be considered to ensure adequate protection for both species. Furthermore, the influences of water chemistry were found to be inconsistent during acute and chronic Pb exposures to P. promelas and thus caution against inferring chronic effects from acute exposures. A number of Pb-responsive genes were identified that exhibited a strong potential for serving as robust indicators of Pb exposure and accumulation in P. promelas. While these genes also provided insight as to the likely toxic mechanisms of Pb, additional work will be necessary to firmly link these genes to chronic outcomes of ecological relevance in the context of ambient water chemistry.