Dynamic contribution by aquatic metabolism to carbon dioxide emissions from a low-land mixed land-use catchment

Abstract

Carbon dioxide (CO₂) emissions from streams are globally significant despite their limited areal extent and rival those from terrestrial ecosystems. Agricultural land represents 40% of the world's landmass; still, the magnitude and source dynamics of CO₂ emissions from streams draining agricultural-influenced landscapes are poorly understood. We used high-frequency measurements of CO₂ and dissolved oxygen in a temperate low-land mixed land-use catchment (21% agricultural) to estimate emissions and the contribution of in-stream metabolism. Although CO₂ concentrations were high (median 3520 μatm), emissions were relatively low (median 4.0 g C m² day⁻¹) due to consistently low gas transfer velocities (k₆₀₀). In-stream metabolism accounted for 46% of the emissions on average, which was higher than expected for a third-order stream. Emissions peaked during late summer despite low k₆₀₀. Emissions were instead driven by high CO₂ concentrations, highlighting the importance of CO₂ supply over physical drivers. We show that low-elevation human-impacted streams exhibit distinct emission dynamics, underscoring the need to account for biological drivers in carbon budgets for these landscapes.