Nitrate Dynamics Under Unsteady and Intermittent Flow in an Antarctic Stream

• Main Author: Singley, Joel Greene
• Format: Text
• Language: unknown
• Published: CU Scholar 2017
• Subjects: Antarctica; Chemostasis; Intermittent Flow; Nitrate; Stream; Biogeochemistry; Environmental Sciences; Hydrology; Antarctic; McMurdo Dry Valleys; Antarc*
• Online Access: https://scholar.colorado.edu/envs_gradetds/50; https://scholar.colorado.edu/cgi/viewcontent.cgi?article=1050&context=envs_gradetds

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 orders 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 physical and hydrochemical dynamics at short-timescales, such that a large 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 dynamics. I observed relative chemostasis of NO3– during large magnitude diel flood pulsing events. This suggests that biological and physical processes controlling the transport and transformation of NO3–, and N more generally, are likely to exhibit spatial and temporal variability at very short timescales in response to extreme hydrologic variability. Such spatiotemporal variability in N processing dynamics has not been included in prior conceptual models of N cycling in MDV streams. As such, 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 timescales interacts with instream biogeochemical processes to control N transport.