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MS thesis defense: Joel Singley

Friday, July 14, 2017, 2:00PM - 4:00PM


Joel Singley


Sievers room, SEEC S228

Thesis title

Hydrological and biological controls on nitrate dynamics under unsteady and intermittent flow in an Antarctic meltwater stream


Low order streams are a primary vector and modulator for the transport of anthropogenically derived reactive nitrogen, especially as nitrate (NO3–). A large proportion of low order streams experience short-term unsteady and intermittent flow conditions, and the prevalence of these dynamics is likely to increase due to climate change and human management. While such hydrologic variability is recognized as an important first-order control on the transport of NO3–, prior reliance on manual sampling has resulted in a disparity between our understanding of coupled physical and biogeochemical dynamics at short time scales. Consequently, a gap exists in our understanding of how unsteady and intermittent sub-daily discharge affects instream NO3– transport patterns. To address this challenge, I used in situ sensors to collect high-frequency (i.e., 15 minute) NO3– concentration and discharge data in an ephemeral, oligotrophic glacial meltwater stream in the McMurdo Dry Valleys, Antarctica. I analyzed concentration discharge relationships using a power-law framework to identify a flow threshold that governed NO3– transport patterns. I observed relative chemostasis of NO3– during large magnitude diel flood pulsing events. In contrast, NO3– concentrations increased up to 4-fold during sustained low flow periods. To explain these patterns, I propose a conceptual model in which short-term flow pulsing and cessation shift sediment redox conditions and microbial processes such that the shallow hyporheic zone temporally becomes a net source and storage zone for a spatially distributed pool of NO3–. The results of this approach will inform understanding of how highly variable hydrological conditions measured at very short time scales interact with instream biogeochemical processes to control N transport.