Date of Award


Document Type

Campus Access Thesis

Degree Name

Master of Science (MS)


Environmental Sciences

First Advisor

Helen Poynton

Second Advisor

William Robinson

Third Advisor

Luis De León


As pesticide use increases, concern for non-target species impacted by agricultural runoff grows. Resistance to the organophosphorus (OP) pesticide, chlorpyrifos, has been associated with an amino acid substitution in the target site acetylcholinesterase (AChE) gene within the freshwater amphipod, Hyalella azteca. Since resistant organisms are able to survive in contaminated habitats there is potential for them to accumulate higher insecticides, increasing the risk for trophic transfer, especially in the San Francisco Bay Delta. In this study, we estimate the uptake and elimination of chlorpyrifos in non-resistant and resistant H. azteca population by conducting toxicokinetic experiments with 14C-chlorpyrifos for 24 hours of uptake and 48 hours of elimination. Our results indicate that non-resistant H. azteca has a larger uptake clearance coefficient (1467 mL g-1 h-1) compared to resistant animals (557 mL g-1 h-1). The half-life derived from the toxicokinetic models also estimates that steady state is met at 13.5 and 32.5 hours for US Lab and ULC, respectively. Our elimination rate constants are similar between the two populations. Bioaccumulation of OP was then compared between non-resistant and resistant H. azteca by exposing animals to six different concentrations relevant to concentrations detected in California for 28 hours. Non-resistant animals did not survive the four highest concentrations while resistant animals did. Detection of OP in animal tissue indicate that resistant animals exposed to high concentrations of chlorpyrifos are capable of accumulating the insecticide up to 58 times higher compared to non-resistant animals. Metabolite analysis from the 28-hour concentration experiments showed that between 20 to 50% parent compound is detected in H. azteca. These results infer that bioaccumulation potential can be more significant in chlorpyrifos resistant H. azteca and may be an essential factor in assessing the full impacts of toxicants on critical food webs, especially in the face of increasing pesticide and chemical runoff.


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