Nowlin, WestonLeBoeuf, Crystal Marie2020-06-152020-06-152010-05LeBoeuf, C. M. (2010). The effects of invasive species and eutrophication on riverine nutrient dynamics (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.https://hdl.handle.net/10877/11655Invasion of exotic species has been identified as one of the greatest threats to aquatic ecosystems. The invasion of aquatic ecosystems by herbivorous and detritivorous fishes is especially concerning because they can alter trophic pathways and nutrient cycling. Increased nutrient loading and productivity may allow for more successful invasion of ecosystems and may also modify the effects of these fishes on ecosystem properties. Herbivorous suckermouth catfish have invaded spring-fed ecosystems in North America and can have profound impacts on ecosystem function. I present the results of a two-part study in which I investigated (1) whether the spatial variation in the abundance of suckermouth catfish in the San Marcos River (Texas, USA) was related to variation in productivity, and (2) if the effects of catfish on ecosystem processes were influenced by nutrient enrichment. To examine the first question, I conducted a field survey examining the habitat associations of catfish in the river and found that catfish densities were highest at the most upstream site, which was characterized by deeper depths, lower flows, smaller substrates, and lower canopy cover and turbidity. However, spatial variation in catfish densities was not related to productivity (measured as periphyton biomass). To examine the second question I conducted a stream channel experiment in which I cross-classified the presence and absence of catfish and nutrient additions. Catfish reduced periphyton biomass, reduced periphyton N:P, altered the severity of periphyton P limitation, and altered detrital processing. The presence of nutrient enrichment altered leaf litter decomposition rates and the nutrient stoichiometry of decomposing leaf litter. I did not find an interaction between the effects of catfish and nutrient enrichment. The results of this study indicate that the spatial variation in population density of catfish in the San Marcos River are not strongly associated with variation in productivity and that the effects of this invasive herbivore on ecosystem dynamics are not dependent upon nutrient loading.Text71 pages1 file (.pdf)encatfisheseutrophicationintroduced fisheswatershed ecologyThesis