ETBC: Interacting Hydrological and Biogeochemical Controls on Nitrogen Transformation Hot Spots and Hot Moments in a Eutrophic Reservoir

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To meet the food and energy demands of a growing global population, humans have more than doubled terrestrial nitrogen (N) fixation. Resulting elevated nutrient concentrations have damaged many freshwater and coastal ecosystems. Meanwhile, a global increase in the number of dams has caused a 7-fold increase in the average standing stock of continental surface waters. Together, these anthropogenic changes interact to modify key processes in the nitrogen cycle, including denitrification (the microbialy mediated removal of biologically available N) and associated production of nitrous oxide, a greenhouse gas capable of depleting stratospheric ozone. N processing in reservoirs is likely critical in controlling downstream N transport, nitrous oxide production, and ecosystem function. However, reservoir N processing is poorly understood, in part because denitrification has proven difficult to measure. To address this gap in understanding, this research program will: 1) develop and test novel, interdisciplinary methods to quantify sediment-to-water N fluxes, 2) use these novel methods, in conjunction with well-established approaches, to identify hot spots and hot moments for microbial N removal and nitrous oxide production in a small polluted reservoir, and 3) relate these hot spots and hot moments to biogeochemical and physical processes. To achieve these aims, the program will integrate hydrological measurements (including reservoir-wide temperature stratification and highly-resolved near-bed currents) and biogeochemical measurements (including reservoir-wide and near-bed N accumulation and gradients, as well as intact core incubations). Established mass-balance and intact core incubation approaches for quantifying dinitrogen and nitrous oxide production will be complimented with more novel hypolimnion gas accumulation and flux gradient approaches. The flux gradient approach aims to resolve in situ N fluxes on scales of weeks and tens of meters, thereby resolving the ?hot moments? and ?hot spots? of rapid denitrification and nitrous oxide production. Sampling will be conducted to resolve seasonal variability in N processing, in addition to variability between shallow, intermediate, and deep regions of the reservoir. Preliminary measurements indicate that an autumn dam release is a period of particularly rapid transformation, so special effort will be made to characterize N dynamics during this time. The novel flux estimation techniques could, if proven successful in this project, be applied in future to other systems and other chemical compounds that cycle between the water column and sediments (e.g. phosphorus, sulfur, and iron), and may eventually be incorporated into deterministic models of reservoir biogeochemical cycling. Our capacity to understand, predict, and mitigate the impacts of anthropogenic acceleration of the global N cycle has been hampered in-part by an inability to measure denitrification and nitrous oxide production at appropriate temporal and spatial scales. This study will address this pressing need by developing broadly applicable new methods for quantifying sediment-water N fluxes. Results will also 1) lend insight into the fundamental hydrologic and biogeochemical controls on N cycling within a reservoir system, 2) quantify the importance of hot spots and hot moments for N removal in this system, and 3) help pinpoint times of year when water release from reservoirs could enhance system N-removal efficiency, thereby reducing downstream N transport and subsequent effects on downstream ecosystems. Finally, this project will promote teaching, training, and learning by supporting the professional development of graduate and undergraduate students in an interdisciplinary context.