One of the defining characteristics of our current epoch—the Anthropocene—is modification of nutrient cycles. At regional to global scales, humans have fundamentally reshaped the availability of carbon, nitrogen, and phosphorus. These changes are particularly apparent in freshwater ecosystems, which receive surface and groundwater inputs of nutrients from agriculture, fossil fuel use, and wastewater. In this thesis, I investigated how the addition of nutrients affects microbial community and biogeochemistry in two extreme environments: the hypereutrophic shallow Utah Lake and nutrient-limited Arctic permafrost streams. In my first chapter, I used bioassay and dilution bioassay experiments to identify what factors control harmful algal blooms in Utah Lake. Specifically, I measured phytoplankton and cyanobacteria growth, cyanotoxin production, and aquatic N-fixation potential. I included physical factors, such as temperature, light, nutrient concentrations, and pH, as well as biological factors, such as top-down control by zooplankton grazers. Phytoplankton showed a threshold behavior at 0.005 mg/L for soluble reactive phosphorus and 0.14 mg/L for dissolved inorganic nitrogen. Surprisingly, nitrogen fixation rates were only high in active bloom samples and were augmented by the addition of both nitrogen and phosphorus. Also contrary to our hypothesis, zooplankton preferentially grazed cyanobacteria over total phytoplankton. In my second chapter, I investigated how permafrost degradation might influence dissolved organic matter (DOM) in Arctic stream networks. Specifically, I used nutrient and labile carbon additions to simulate the effects of permafrost thaw DOM degradation and microbial community in three distinct permafrost-covered catchments on the North Slope of Alaska. The alpine catchment had higher biodegradability but lower DOM concentration across seasons compared with the lake-influenced and tundra catchments. For all catchments, there were strong seasonal changes in microbial community and distinct responses to nutrient addition. The addition of nutrients stimulated DOM biodegradation in the late season—the period of the year when permafrost DOM release typically occurs. Microbial communities differed by catchment type, but overall diversity was similar. Together, these experiments highlight the diverse downstream consequences of human alteration of global carbon, nitrogen, and phosphorus cycles. Even in extreme systems, alteration of the microbial community regulating many of these cycles has potential to exacerbate ecosystem and climate change, so understanding our influence over biogeochemical cycles and microbial interactions is vital for informing future management practices and planetary boundaries.



College and Department

Life Sciences; Plant and Wildlife Sciences



Date Submitted


Document Type





nitrogen, phosphorus, eutrophication, permafrost, zooplankton, cyanobacteria, dissolved organic carbon



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Life Sciences Commons