Switches of methane production pathways and emissions with human activity intensity in subtropical estuaries

Nutrient loading caused by human activities can drive changes in methane (CH₄) production pathways, yet the mechanisms and magnitudes of estuary CH₄ emissions remain poorly understood. Here we investigated CH₄ fluxes and δ¹³C-CH₄ values in wet and dry seasons from five subtropical estuaries with varying human activity intensity and hydrologic conditions. Water dissolved CH₄ concentrations varied from 0.19 to 0.51 µmol CH₄ L^–1 in these studied estuaries, resulting in coincident water–air CH₄ fluxes ranging from 5.46 to 29.6 µg CH₄ m^–2h⁻¹. Water CH₄ concentrations and fluxes were significantly lower in the wet than dry season, and were higher in the high human-impacted estuaries, indicating that large discharge and water flow could slow down areal CH₄ emissions. The ratios of C-CH₄ to organic carbon (OC) were much higher and showed a positive response to increasing nitrogen concentrations in high human-impacted estuaries, suggesting that eutrophic estuaries trigger high CH₄ emissions. In addition, an increase in δ¹³C-CH₄ and a declining in fractionation factor (α_app) from the low to high human-impacted estuaries were accompanied by a switch of CH₄ production from hydrogenotrophic to acetoclastic pathways. These results suggest that human activity intensity can drive the alterations in the pathways and magnitudes of CH₄ production and that hydrologic conditions can modify CH₄ emissions, which have important implications for estimates of CH₄ emissions from global estuaries under increasing human activity intensity.