Understanding Controls on Stream Ecosystem Function Using Experiments Across a Range of Spatial Scales from Mesocosms to Watersheds

Global environmental changes such as climate and land use change alter the quantity of energy and nutrient inputs to aquatic ecosystems. My dissertation examines how organic matter and nutrient inputs influence stream biogeochemistry and ecosystem function under changing conditions, using experiments conducted across spatial scales. In my first chapter, I assessed how stream warming affects nutrient uptake across different substrates using indoor experimental mesocosms. Warming increased the rate of nutrient uptake, particularly by biofilms on inorganic substrates. Specifically, ammonium (NH4+-N) removal was more sensitive to elevated temperatures than soluble reactive phosphorus (SRP). These findings suggest that on a warming planet, nutrient cycling may be altered. In my second chapter, I compared ecosystem responses to a novel form of organic matter input, periodical cicadas, and standard leaf litter using outdoor experimental streams. Cicadas decomposed rapidly, releasing pulses of dissolved nutrients and carbon that enhanced metabolism and nutrient uptake compared to leaves, highlighting their role as a unique resource subsidy. In my third chapter, I quantified reach-scale denitrification, a process that removes nitrogen (N) from streams, in a tributary and mainstem river across three seasons, by measuring nitrogen gas (N2) with a membrane inlet mass spectrometer (MIMS). Denitrification rates were higher per unit area streambed in the tributary, particularly in spring when nitrate (NO3--N) concentrations peaked. However, when expressed per km of channel length, rates were more similar between systems, indicating that human-impacted freshwaters of all sizes play a role in N removal. In my fourth chapter, I evaluated how winter cover crops affect sediment export in two agricultural watersheds. Cover crops reduced field-scale sediment export by 26–65% and, when fully implemented, could decrease watershed-scale export by 6-67%. Therefore, cover crops may limit sediment export during winter and spring. In my fifth chapter, I conducted a synoptic sampling of 105 stream and river sites across four river basins in Michigan and Indiana to examine how land use and stream size influence water chemistry signatures. Concentrations of NO3--N, NH4+-N, and SRP were higher in agricultural basins and did not differ between tributary and mainstem sites, and N2 and nitrous oxide (N2O) disequilibrium was more strongly associated with stream discharge than NO3--N concentrations, highlighting the complexity of nutrient dynamics in human-impacted systems. As human disturbances and climate change reshape stream ecosystems, understanding the drivers of biogeochemical processes is critical. My research advances knowledge of nutrient and energy dynamics in fluvial systems, informs conservation efforts, predicts aquatic responses to climate warming, and highlights the critical role of streams and rivers in mitigating nutrient pollution on a changing planet.