Frost, Paul

We studied the physiological and molecular responses of lab-grown Daphnia pulex to shifts in dietary (carbon, nitrogen, phosphorus, and cyanobacteria) and growth media (calcium) nutrient supply using a set of potential biomarkers via qPCR. Each examined nutrient had a strong effect on Daphnia mass-specific growth rate, and we found significant dose-dependent effects of treatment level (medium and low) on the gene expression of selected indicator genes. Furthermore, linear discriminant analysis models using different combinations of treatment levels could separate the animals between nutritional treatments with 86-100% prediction accuracy. This would suggest that Daphnia appear able to respond to nutrient limitation by adjusting their growth rate and associated molecular pathways to deal with an insufficient supply of nutrients. While this study provides valuable information regarding Daphnia's ability to adjust physiological and molecular processes under controlled laboratory conditions, more validation needs to be conducted before applying these potential biomarkers to wild populations to assess the type and intensity of nutritional stress.

As an emerging contaminant, the antimicrobial agent silver nanoparticles (AgNPs) have been receiving considerable attention to determine their potential effects to aquatic ecosystems. However, estimates of aquatic consumer survivorship and other toxicological endpoints vary considerably among experiments, largely due to the environment in which the test takes place. Throughout this thesis I aim to understand which natural environmental variables impact toxicity to the common aquatic consumer Daphnia. I focus on the effects of particulate matter as it may play a role in animal nutrition as well as interact with AgNPs. I explore particulate matter's effect on survival in the complex matrices including other natural variables that could impact toxicity. I conduct a series of complimentary field and laboratory studies to understand how particles impact AgNP toxicity and how those interactions vary within whole lake ecosystems. Using laboratory studies, I establish that algal particles mitigate the toxic effects of AgNPs on Daphnia survival through removing Ag from the water column and that phosphorus increases this effect. Using wild Daphnia and lake water, I demonstrate the ability of particulate matter to mitigate toxicity in complex natural settings. It was also one of the major predictors of AgNP toxicity to Daphnia along with dissolved organic carbon and daphnid seasonal health. Finally, using a whole lake AgNP addition experiment, I demonstrate that particles and AgNPs interact variably in the lake. Silver from AgNPs binds to particles and is removed to the sediments through the actions of settling particles without impacting the dynamics of living communities. Overall, I am able to demonstrate that the natural components of lake ecosystems, especially particulate matter, are able to mitigate the effects of AgNPs in lake ecosystems to a point where they likely will be never pose a threat to the survivorship of aquatic consumers such as Daphnia.

Author Keywords: Daphnia, ecotoxicity, particulate matter, Silver nanoparticles, whole lake experiment