Importance of seasonality and watershed size on carbon dioxide evasion in Arctic streams

Résumé fourni par la source

Abstract Arctic headwater streams play a crucial role in the carbon cycle, contributing disproportionately to carbon dioxide (CO 2 ) evasion despite their small size and surface area. In this study, we examined how seasonal hydrologic patterns, such as spring snowmelt (freshet), mid-summer low-flows (baseflow) and the late summer period in August and early September (late-season), influence CO 2 evasion in Arctic headwater streams (1–9 km 2 basin size; 1–4 stream orders) near Utqiagvik, Alaska. During three summers, we measured key environmental parameters (CO 2 partial pressure, water temperature, discharge, dissolved organic carbon (DOC), gas transfer velocity) and CO 2 evasion using a floating chamber. CO 2 evasion followed distinct seasonal patterns with the highest evasion rates during freshet, followed by the late-season, and the lowest during baseflow. Despite its short duration (10 d), freshet accounted for 22% more total CO 2 evasion than the 36 d late-season period. The descending peakflow within freshet had the highest median evasion rate (2486 mg C–CO 2 m −2 d −1 ) of the entire season, while the ascending portion of the peakflow had the lowest (448 mg C–CO 2 m −2 d −1 ). These two freshet periods showed a statistically significant difference ( p < 0.05). Water temperature, DOC, and p CO 2 explained 35%, 30% and 67% of freshet evasion variability, respectively. No measured parameters reliably predicted CO 2 evasion for the baseflow or late-season periods, suggesting increased hydrologic and biological complexity. The stream showed supply-limitation during freshet but transitioned to chemostatic behavior thereafter. By synthesizing our measurements with previously published data, we show that small streams (<1 km 2 watersheds) exhibit ∼10-times the CO 2 evasion rates than large waterways (>1000 km 2 ). We also find that across the Arctic, only ∼15% of riverine CO 2 measurements capture the freshet period. Together, the omission of small streams and the underrepresentation of freshet likely lead to a systematic underestimation of CO 2 losses from Arctic surface waters to the atmosphere.

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