Diffusive and ebullitive methane emissions under tidal fluctuation and climate warming in a nutrient-rich subtropical estuary

Estuaries are important sources of methane (CH₄) emissions under human activities and global warming. However, diffusive and ebullitive CH₄ emissions in response to nutrients enrichments and tidal fluctuations remain largely constrained. Here, we investigated seasonal and monthly CH₄ emissions in a nutrient-rich subtropical estuary and evaluated relative contribution of diffusion and ebullition CH₄. Dissolved CH₄ concentrations were in a range of 0.18–11.2 µmol L⁻¹, corresponding to diffusive fluxes of 3.08–375 μg m⁻² h⁻¹ along the estuary. Diffusive CH₄ fluxes were observed to decrease significantly from the upper to lower estuary and were higher in the cold than warm seasons. Turbidity, suspended particulate, NH₄⁺, NO₃⁻, DOC/NO₃⁻ and NO₂⁻ were the crucial factors affecting CH₄ fluxes, collectedly explaining 43 % of CH₄ emissions variability. Ebullitive and diffusive fluxes were significantly higher under ebbing than flooding tides, with an ebullition contribution of 82–98 %. Ecosystem-level temperature sensitivity and apparent activation energy of CH₄ emissions were higher for ebullitive than diffusive pathways and were also higher under the ebbing than flooding tides, indicating that ebullitive pathway is high temperature and low water depth dependent. It is conservatively projected that CH₄ emissions would increase by 17 ± 9 % and 40 ± 22 % under the 2 and 4 °C warming scenarios, respectively. These results suggest that enhanced CH₄ emissions under human activities and climate warming can be refined in further CH₄ projections and highlight the importance of estuaries in global carbon cycles and climate change.