| Summary: | Snowmelt spring floods regulate carbon transport from land to streams. However, these coupled processes are rarely documented through high-resolution measurements focused on water-carbon interactions. We collated a state-of-the-art high-frequency data set throughout a snowmelt and early post snowmelt period, alongside regular samples of stream water, precipitation, and snowmelt isotopes (d(18)O). Our study was conducted during the 2019 snowmelt and initial post snowmelt season in a subarctic, peatland influenced headwater catchment in Pallas, Northern Finland. We measured high-frequency dissolved organic carbon (DOC), and in-stream carbon dioxide (pCO(2)). We identified a change in hydrological processes as the snowmelt season progressed and the post snowmelt season began. We found (a) Overland flow dominated stream DOC dynamics in early snowmelt, while increased catchment connectivity opened new distal pathways in the later snowmelt period; (b) CO2 processes were initially driven by rapid bursts of CO2 from the meltwaters in snowmelt, followed by dilution and source limitation emerging post snowmelt as deep soil pathways replaced the snowpack as the main source of CO2; (c) stream carbon concentration shifted from being relatively balanced between CO2 and DOC during the early snowmelt period to being increasingly DOC dominated as snowmelt progressed due to changes in DOC and CO2 source supply. The study highlights the importance of using high-frequency measurements combined with high-frequency data analyses to identify changes in the processes driving water-carbon interactions. The degree to which water-carbon interactions respond to the continuation of Arctic water cycle amplification is central to delineating the evolving complexity of the future Arctic. Peer reviewed
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