The interactions between parasites, hosts, and the ecosystems they live in are complex and diverse. This can make predicting how they will respond to pressures like nutrient enrichment difficult. However, this endeavor is important, due to the far-reaching effects it has on ecosystems functioning, industries like aquaculture, and even human health. Even with the significance of this topic, there are key parts of the picture that have received little research attention. For this reason, I chose to study the effects of nutrient enrichment on parasite-host interactions at two different scales, first investigating the interaction directly between parasites and their host, and then manipulating experimental ponds to look at trophic effects. Using the theory of ecological stoichiometry as a conceptual framework allowed for comparison across the two investigations. In both studies, I used largemouth bass and the macro parasites of their digestive tract as my study system, due to the importance of largemouth bass as sport fish, their high trophic status, and diverse parasite assemblages. My first study examined the effects of eutrophication on parasite-host interactions occurring within the host. It centered on four hypotheses: 1) the nutrient content of different host tissues (infection sites) varies within and across hosts, 2) the nutrient contents of parasite genera differ from that of their host tissue(s), 3) the nutrient contents of parasite genera differ from one another, and 4) nutrient availability within specific host tissues is related to the nutrient content and abundance of parasite genera within tissues. I quantified the nutrient content (carbon (C), nitrogen (N), and phosphorous (P) and their ratios) of parasitic infracommunities in the tissues of the liver, stomach, pyloric caeca, intestines, and visceral mesentery of 30 largemouth bass (Micropterus salmoides). I found stoichiometric variation across host tissues and that fish collection site explained variation in pyloric caeca N:P ratios. Parasitic genera differed in their nutrient content with actively feeding parasite forms having higher %N and lower C:N ratios than encysted/non-reproductive forms. In addition, the %C of both actively feeding parasites varied across organs, and, for one genus, this variation reflected differences in host tissue %C. Finally, I found that the total number of actively feeding parasites in the pyloric caeca increased with that tissue’s N:P ratio. My results suggest that parasites encounter significant variation in nutrient availability within and across hosts, and that this variation can influence the nutrient content and abundance of some actively feeding parasites within specific tissues. To determine if the effects from my first study were a result of nutrient changes to host tissues or if they were an artifact of changes occurring at other trophic levels caused by nutrient addition, I conducted an experimental manipulation of a short trophic system. I applied two treatments, a fertilized treatment and a control, to 14 ponds stocked with largemouth bass. I hypothesized that adding a low N:P fertilizer to the ponds would result in fewer parasites in the pyloric caeca of the bass in accordance with findings from my previous work. Alternatively, I hypothesized that indirect changes to intermediate hosts would be responsible for changes in parasite abundance. To measure the effects of nutrient additions on multiple trophic levels, samples of fine particulate organic matter (FPOM), benthic invertebrates, and vertebrate prey were taken from each pond. Stocked bass were retrieved and dissected to obtain samples for nutrient analysis from their pyloric caeca, and to quantify parasite abundances. My treatment was able to affect the %P, C:N, C:P, and N:P of the FPOM in the ponds. Benthic invertebrate biomass was negatively correlated with FPOM %P. I did not find a direct effect of benthic invertebrates on vertebrate prey species, but fertilized ponds had a smaller decrease in the number of vertebrate prey over the course of the experiment. Counterintuitively, the %N of the pyloric caeca of fish in the fertilized ponds decreased, and the %C and C:N increased in relation to the number of vertebrate prey, but %P did not change. The treatment status of the pond and the nutrients of the pyloric caeca did not affect the number of parasites that were found, but parasite numbers were positively associated with prey fish. This result would suggest the number of intermediate hosts, in this case, prey fish, is the determining factor for parasitic loads in the bass I collected. The indirect effects of trophic interaction and the direct effects of changes to host and parasite dynamics both play important roles in predicting the outcome of eutrophication on disease. The conflicting results of these two studies highlight the need for in-depth knowledge of the systems being affected by nutrient additions.

Glacial lakes, such as those in the Midwest region of the United States, are ecologically and economically important, and they provide a wide range of ecosystem services, such as habitat for wildlife and fishes, flood control and recreational boating. Glacial lakes often support locally important sport fisheries, such as largemouth bass <i>Micropterus salmoides</i> and bluegill <i>Lepomis macrochirus</i>,<i> </i>which are partially dependent on suitable habitat within lakes. Nearshore vegetation is often removed by lakeshore landowners for perceived aesthetics and boat access, or by area managers as a form of indirect fisheries management and invasive species control. The connection between nearshore vegetation and fish population health, though widely studied is somewhat unclear. In the two research chapters of this thesis we attempted to further understanding of the environmental factors that influence vegetation abundance and distribution, how vegetation abundance and distribution influences fish population abundance and size structure, and how young-of-year (YOY) largemouth bass utilize habitats within the nearshore environment. In the first research chapter, we used structural equation modeling (SEM) and data collected by the Indiana Department of Natural Resources to quantify the complexity of relationships among catchment characteristics (e.g., catchment size), lake morphology, water quality, vegetation abundance and distribution, and fish population abundance and size structure. Across multiple lakes, lake productivity was more influential in explaining cross-lake variation of largemouth bass and bluegill proportional stock density (PSD) and largemouth bass catch per unit effort (CPUE) than vegetation. This may be a result of the feedback between phytoplankton production and rooted vegetation production. The models we constructed provide insights into the complexity of environmental variables that influence nearshore vegetation and fish populations. In the second research chapter we used stable isotopes (δ¹³ Carbon, δ¹⁵ Nitrogen, δ¹⁸ Oxygen and δ² Hydrogen) to examine the consistency of habitat use and foraging of YOY largemouth bass within Indiana glacial lakes. We observed spatial variation in stable isotope ratios of YOY largemouth bass between habitat types and sites. Additionally, there were significant, positive relationships between δ¹³C of locally collected potential prey items and δ¹³C of YOY largemouth bass suggesting localized foraging patterns. Later in the summer, as young bass grew in size and likely switched to piscivory, we did not observe similar spatial variation in young bass stable isotopes or spatial relationships between prey and the young bass suggesting more homogeneous foraging patterns. Understanding the habitat use patterns of young bass may allow for more efficient and effective management of the nearshore environment. Overall, a greater consideration for the complexity of relationships between nearshore habitat and fish populations may facilitate more effective management.