Drainage Ditches (“Hot Spots”) and Storms (“Hot Moments”) Define Aquatic Greenhouse Gas (CO2, CH4, N2O) Emissions From the Land-to-Ocean Aquatic Continuum

Humans are altering coastal regions directly (land-use, drainage) and indirectly (climate change). Alterations potentially create positive climate feedback loops by enhancing production and emission of aquatic greenhouse gases (GHGs) CO₂, N₂O, and CH₄. We tested this hypothesis by measuring dissolved CO₂, N₂O, and CH₄ concentrations across the anthropogenic aquatic continuum (farm ponds, ditches, irrigation drains, streams, tidal rivers, and estuaries) and continuously during a winter storm. Combining measurements with hydrodynamic modeling enabled us to parameterize physical gas transfer uncertainties, revealing artificial waterways contributed disproportionately to emissions. Ditches and drains cover 5% of water surface area but produced >50% of emissions (2–11 Mmol d⁻¹ CO₂-equivalents). But storms inverted this pattern by increasing estuary emissions 16-fold (5.0 Mmol d⁻¹ CO₂-equivalent), suggesting storm patterns could control both sources and magnitudes of aquatic GHG emissions. Findings show overlooked artificial drains and hard-to-measure storms will increasingly define the aquatic offsets of landscape carbon budgets.