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Abstract Climate change is rapidly altering hydrological processes and consequently the structure and functioning of Arctic ecosystems. Predicting how these alterations will shape biogeochemical responses in rivers remains a major challenge. We measured [C]arbon and [N]itrogen concentrations continuously from two Arctic watersheds capturing a wide range of flow conditions to assess understudied event‐scale C and N concentration‐discharge (C‐Q) behavior and post‐event recovery of stoichiometric conditions. The watersheds represent low‐gradient, tundra landscapes typical of the eastern Brooks Range on the North Slope of Alaska and are part of the Arctic Long‐Term Ecological Research sites: the Kuparuk River and Oksrukuyik Creek. In both watersheds, we deployed high‐frequency optical sensors to measure dissolved organic carbon (DOC), nitrate (), and total dissolved nitrogen (TDN) for five consecutive thaw seasons (2017–2021). Our analyses revealed a lag in DOC: stoichiometric recovery after a hydrologic perturbation: while DOC was consistently elevated after high flows, diluted during rainfall events and consequently, recovery in post‐event concentration was delayed. Conversely, the co‐enrichment of TDN at high flows, even in watersheds with relatively high N‐demand, represents a potential “leak” of hydrologically available organic N to downstream ecosystems. Our use of high‐frequency, long‐term optical sensors provides an improved method to estimate carbon and nutrient budgets and stoichiometric recovery behavior across event and seasonal timescales, enabling new insights and conceptualizations of a changing Arctic, such as assessing ecosystem disturbance and recovery across multiple timescales. 

Rapid climate change and intensifying disturbance regimes in the Arctic are altering lateral fluxes of carbon and nutrients from permafrost landscapes to Arctic rivers. However, the seasonal dynamics and landscape characteristics that regulate patterns of solute flux in Arctic watersheds remain poorly understood. To characterize potential drivers of change in solute fluxes across Arctic watersheds, we implemented a spatially extensive synoptic sampling framework within four Arctic watersheds: Upper Kuparuk River, Oksrukuyik Creek, Trevor Creek, and Putuligayuk River. We collected water grab samples at 31-50 nested subcatchments within each watershed up to four times between late May and early September in 2021, 2022, and 2023. We also sampled watershed outlets weekly. We analyzed all samples for a broad suite of biogeochemical constituents including dissolved organic carbon (DOC), organic and inorganic nutrients, dissolved organic matter optical properties, and trace elements and heavy metals, which are provided in this datafile along with sampling date, time, and site coordinates. 

As environmental change in the Arctic accelerates, there is a growing need to accurately quantify the response of Arctic ecosystems throughout the year. To assess the temporal coverage of observations of carbon and nutrient fluxes, we used literature synthesis, quantitative meta-analysis, and exploration of a novel biogeochemical dataset from one of the best-documented Arctic ecosystems: the headwaters of the Kuparuk River in Northern Alaska. The meta-analysis of 204 peer-reviewed studies revealed a strong temporal gap in observations of biogeochemistry and hydrology of the Kuparuk River, with substantially fewer observations from the early and late "shoulders" of the thaw season (defined as the period before snowmelt or after plant senescence). To test and illustrate how much this bias might influence fundamental ecosystem level measurements, such as riverine carbon and nutrient fluxes, we used high-frequency, in-situ water chemistry sensors to estimate riverine export budgets across the thaw season for dissolved organic carbon (DOC) and nitrate (NO3-) in the Kuparuk headwaters. With this novel dataset, we found that a large proportion (~30%) of the annual export of DOC and NO3 - occurred during the shoulder seasons, which are not well characterized even for this well-documented Arctic system. These analyses raises the broader question: What ecological information are we missing by giving these seasons the "cold shoulder"? As climate change alters seasonality, filling this major data gap in the shoulder seasons is crucial to understand the response of Arctic ecosystems.