abstract: Non-native consumers can significantly alter processes at the population, community, and ecosystem level, and they are a major concern in many aquatic systems. Although the community-level effects of non-native anuran tadpoles are well understood, their ecosystem-level effects have been less studied. Here, I tested the hypothesis that natural densities of non-native bullfrog tadpoles (Lithobates catesbeianus) and native Woodhouse's toad tadpoles (Anaxyrus woodhousii) have dissimilar effects on aquatic ecosystem processes because of differences in grazing and nutrient recycling (excretion and egestion). I measured bullfrog and Woodhouse's carbon, nitrogen, and phosphorus nutrient recycling rates. Then, I determined the impact of tadpole grazing on periphyton biomass (chlorophyll a) during a 39-day mesocosm experiment. Using the same experiment, I also quantified the effect of tadpole grazing and nutrient excretion on periphyton net primary production (NPP). Lastly I measured how dissolved and particulate nutrient concentrations and respiration rates changed in the presence of the two tadpole species. Per unit biomass, I found that bullfrog and Woodhouse's tadpoles excreted nitrogen and phosphorus at similar rates, though Woodhouse's tadpoles egested more carbon, nitrogen, and phosphorus. However, bullfrogs recycled nutrients at higher N:C and N:P ratios. Tadpole excretion did not cause a detectable change in dissolved nutrient concentrations. However, the percent phosphorus in mesocosm detritus was significantly higher in both tadpole treatments, compared to a tadpole-free control. Neither tadpole species decreased periphyton biomass through grazing, although bullfrog nutrient excretion increased areal NPP. This result was due to higher biomass, not higher biomass-specific productivity. Woodhouse's tadpoles significantly decreased respiration in the mesocosm detritus, while bullfrog tadpoles had no effect. This research highlights functional differences between species by showing non-native bullfrog tadpoles and native Woodhouse's tadpoles may have different effects on arid, aquatic ecosystems. Specifically, it indicates bullfrog introductions may alter primary productivity and particulate nutrient dynamics. / Dissertation/Thesis / Masters Thesis Biology 2015

I investigated the drivers of nutrient cycling by heterotrophic microbes during leaf decomposition in streams. My research addressed two overarching questions: 1) how do exogenous and endogenous factors interact to drive microbial nitrogen (N) cycling during organic matter decomposition in stream ecosystems, and 2) what affect will the global increase in biologically active N have on these factors and resulting fluxes? I conducted studies in natural streams and laboratory mesocosms to address these questions and used general stoichiometric theory to conceptualize diverse microbial assemblages as a single functional unit within stream ecosystems. First, I described spatial and temporal patterns of N and phosphorus uptake and mineralization by leaf-associated microbial assemblages in five southern Appalachian streams which spanned a gradient of nitrate availability. I found wide variations in nutrient fluxes across time and space, perhaps due to macroinvertebrate-induced changes in microbial assemblage composition. Secondly, I explored the roles of endogenous and exogenous N in meeting microbial requirements. I isolated microbial biomass from leaves that had been labeled with N-15 and incubated in the same five Appalachian streams. The importance of exogenous N increased as decomposition progressed and was particularly important in streams with high N availability. Finally, I tested potential interactions between two exogenous drivers of microbial nutrient cycling: N availability and animal activity. I used mesocosms to test the effects of consumer nutrient recycling (CNR) and grazing by two shredders on microbial uptake under different N regimes. Animals only influenced microbial uptake under low N conditions. Shredder CNR generally stimulated uptake while grazing had a negative effect. My research provides a robust model describing N cycling by detritus-associated microbes over the course of decomposition. According to this model, microbes assimilate endogenous N during the initial stages of decomposition and immobilization of exogenous N becomes more important as decomposition progresses. The labeled substrate technique that I used to generate this model is an elegant way of testing the applicability of this model in other ecosystems. My results also suggest that anthropogenic activities that increase exogenous N availability have implications for N and C cycling in lotic systems. / Ph. D.