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

Abstract

• CH 4 fluxes increased significantly from minimally to highly impacted estuaries. • Water δ 13 C-CH 4 values increased from minimally to highly impacted estuaries. • Human activity drove CH 4 production from hydrogenotrophic to acetoclastic pathways. Nutrient loading caused by human activities can drive changes in methane (CH 4 ) production pathways, yet the mechanisms and magnitudes of estuary CH 4 emissions remain poorly understood. Here we investigated CH 4 fluxes and δ 13 C-CH 4 values in wet and dry seasons from five subtropical estuaries with varying human activity intensity and hydrologic conditions. Water dissolved CH 4 concentrations varied from 0.19 to 0.51 µmol CH 4 L –1 in these studied estuaries, resulting in coincident water–air CH 4 fluxes ranging from 5.46 to 29.6 µg CH 4 m –2 h −1 . Water CH 4 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 CH 4 emissions. The ratios of C-CH 4 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 CH 4 emissions. In addition, an increase in δ 13 C-CH 4 and a declining in fractionation factor ( α app ) from the low to high human-impacted estuaries were accompanied by a switch of CH 4 production from hydrogenotrophic to acetoclastic pathways. These results suggest that human activity intensity can drive the alterations in the pathways and magnitudes of CH 4 production and that hydrologic conditions can modify CH 4 emissions, which have important implications for estimates of CH 4 emissions from global estuaries under increasing human activity intensity.