Shining light on Arctic river biogeochemical dynamics: Using in-situ high-frequency optical sensors to constrain carbon and nitrogen exports and stoichiometry in two headwaters

Abstract:

Climate change is rapidly altering hydrological processes and the structure and functioning of Arctic ecosystems, yet predicting how these alterations will shape biogeochemical responses in rivers remains a major challenge. Ultimately, the long-term trajectory of Arctic riverine nutrient balance remains an important biogeochemical puzzle. To address this puzzle, we measured carbon (C) and nitrogen (N) concentrations continuously from two Arctic watersheds capturing a wide range of flow conditions. Our study watersheds represent low-gradient, high-productivity landscapes typical of the headwaters on the North Slope of Alaska and are part of the Arctic Long-Term Ecological Research (LTER) site: the Kuparuk River and Oksrukuyik Creek. In both watersheds, we deployed high-frequency sensors that optically measure dissolved organic carbon (DOC) and nitrate (NO₃^-) for three consecutive thaw seasons (2017-2019). To assess hydrochemical relationships for varying N species, we also developed robust time series of total Kjeldahl N (TKN), the sum of dissolved organic N (DON) plus ammonium (NH₄⁺). We used these data to further explore C and N concentration-discharge (CQ) behavior and changing stoichiometric ratios (C:NO₃^-, C:TKN, and TKN:NO₃).Consistent with previous studies from the same watersheds, as discharge increased, NO₃^- was largely diluted (CQ slope = -0.32 to -0.40), while DOC was enriched (CQ slope = 0.35 - 0.46). TKN represented a large proportion (>70%) of total N exports, and largely enriched during storms (CQ slope = 0.11 to 0.25). Our analyses emphasize that storms can alter N speciation and potentially exacerbate inorganic N-limitation. For example, C:NO₃^- and TKN:NO₃ always increased during high flows and remained elevated for several days following a flow event, revealing a delay in stoichiometric recovery after a hydrologic perturbation. Further, the enrichment of TKN at high flows, even in watersheds with relatively high N-demand, represents a potential “leak” or “shunt” of hydrologically-available N to downstream environments. Our analysis of optical sensor spectra provides a high-precision solution for estimating N budgets and export behavior, which could be incorporated into our ecosystem conceptualizations in a changing Arctic.