Graduate Thesis Or Dissertation

Comparative Aquatic Toxicology of Pesticides and Plastics Using the Model Estuarine Fish, Inland Silverside

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  • Based on current trends both pesticide and plastic production are expected to continue to rise in the coming decade. A growing global population coupled with the effects of climate change are the main drivers of increased pesticide use. Throughout this dissertation pesticides refer to herbicides, fungicides, and insecticides. Plastic production is also growing rapidly and likely won’t decrease unless ambitious legislative action is taken, under current trends 53 million tons of plastic waste is expected to be produced annually by 2030. These contaminants end up in waterways and eventually make their way to estuaries. Estuaries are critical habitats for many organisms including humans and provide essential ecosystem services such as breeding grounds and nurseries for developing organisms, filtration of pollutants and sediments, and habitat for many species including those used as sources of human food. Annual variability in freshwater inputs into estuaries under global climate change scenarios is likely to affect estuarine ecosystems. Global sea level rise contributes to increased salinity within estuaries, but different global regions may experience increases or decreases in precipitation which will also impacts the salinity of an estuary. Increased precipitation may decrease salinity despite increase sea levels. Changes in salinity can lead to changes in the partitioning behavior of pesticides and their log KOW. The ‘salting-out’ effect describes this phenomenon as an increase in a compound’s affinity for the lipid phase over the water when salinity increases since salt increases the rigidity of water, decreases the compounds solubility. However, whether these changes in partitioning alter toxicity have still been relatively understudied. To further understand the impact salinity has on the toxicity of pesticides this dissertation presents three studies assessing the toxicity of multiple pesticides at various salinity ranges in an estuarine fish. In Chapter 2, seven pesticides were assessed for LC10 and LC50 at 5 and 15 practical salinity units (PSU) in Inland Silversides from 24 hours post fertilization to 96 hours post hatch (13 days). A number of these pesticides had only been studied in freshwater until this study. This work contributed dose-response curves of seven pesticides and found that only one of the seven pesticides had a significant difference in LC50 at the higher salinity, but all showed a trend towards increased toxicity. Chapters 3 and 4 can be considered sister studies and both investigate the effects of pyrethroids on Inland Silverside following 96-hours of exposure at salinities relevant to the San Francisco Bay. In Chapter 3, growth effects and behavioral toxicity of six pyrethroids were tested at 05, 2, and 6 PSU and three concentrations in the ng/L range. In Chapter 4 the toxicity of three pyrethroids were tested at 6 and 10 PSU at only 1 ng/L. This was a multigeneration study and some F0 larvae were reared in clean water and spawned and the F1 larvae were tested for similar endpoints. We observed decreased toxicity in both of these studies at the higher salinities. One of the main findings from the multigenerational study was that indirectly exposed F1 Inland Silverside larvae appeared to exhibit a compensatory response to the pyrethroids in both salinities. The findings from Chapters 2 - 4 suggest that salinity does influence the toxicity of these pesticides in Inland Silversides but the direction in which toxicity is changed is likely dependent on the pesticide, exposure period, concentration, and endpoint being examined. It is important for future work to examine how the response in Inland Silversides compare to more sensitive species of fish. This has implications for risk assessment as Inland Silversides are used as a model organism for estuarine and marine fish. In addition to soluble contaminants, estuaries also receive plastic pollution. Plastics breakdown into smaller and smaller pieces called micro and nanoplastics. As micro and nanoplastics travel through aquatic ecosystems, they become weathered which can change their shape, size, and chemical composition. Despite this, many studies use unweathered particles to test micro and nanoplastic toxicity. Additionally, there is a bias in the literature for toxicity studies done using polystyrene microsphere, which is not the most relevant plastic type to represent environmental samples. Therefore, we exposed Inland Silverside larvae to 10 different treatments of environmentally relevant particle types, shapes, and sizes for 21 days at 15 PSU. We measured growth, behavior, and changes in gene expression to compare the toxicity of the different treatments. We did not find evidence to suggest weathering necessarily increases or decreases micro and nanoplastic toxicity but instead the change is likely dependent on the particular polymer in question. We found that polyester microfibers likely breakdown into more nano-sized particles when weathered under UV A, B, & C and induce more nanoplastic like effects. Gene expression analysis revealed effects on protein production, processes involved in osmoregulation, sarcomere and muscle function, and response to stimuli. These results provide necessary data that can help future studies more precisely target endpoints related to possible mechanisms of toxicity.
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  • This work was funding by the California Delta Stewardship Council (grant # DSP19206), the Environmental Protection Agency Science to Achieve Results (STAR) program (grant # 83950301), and the Ivan Pratt Marine Hatfield Scholarship (2022-23). The ideas presented in this publication are those solely of the authors and do not necessarily reflect the opinions of the granting agency.
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  • Pending Publication
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  • 2023-03-24 to 2024-04-24



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