Changing sediment and surface water processes increase CH₄ emissions from human-impacted estuaries

Coastal waters are known to emit globally significant quantities of CH₄, a potent greenhouse gas, but the potential of the rapid and ongoing human alterations to coastal areas to alter these emissions remains undefined. Here we addressed this gap by quantifying water-to-air CH₄ fluxes and δ¹³C-CH₄ values in sub-tropical estuaries at Low (n = 3), Moderate (n = 2), and High (n = 3) levels of human modification (agricultural land use, wastewater discharge), and sediment-to-water CH₄ fluxes from the major benthic habitats in representative Low, Moderate, and High systems. An increase in water-to-air CH₄ fluxes from 9.7 µmol m⁻² d⁻¹ (Low) to 28 µmol m⁻² d⁻¹ (Moderate) to 47 µmol m⁻² d⁻¹ (High) was accompanied by a shift from hydrogenotrophic to acetoclastic production pathways. Unexpectedly, benthic CH₄ production, which ranged from −48 µmol m⁻² d⁻¹ to +180 µmol m⁻² d⁻¹ between habitats, estuaries, and seasons, was not the primary driver of this shift. Sediments produced more CH₄ (∼600%) than emitted from the Low estuary, ∼90% of CH₄ emitted from the Moderate estuary, but only 9% of CH₄ emitted from the High estuary. Instead, a combination of wastewater, groundwater, and apparent water column production caused a ∼ 3-fold increase in estuary CH₄ emissions. Our findings indicate that human alterations to the source, rate, and pathways of CH₄ production are driving a net increase in emissions from estuaries, demonstrating a need to redefine how we quantify ‘anthropogenic’ CH₄ emissions.