Mangrove ecosystems with high sediment deposition and active carbon cycling are a source of methane (CH 4 ) to the coastal atmosphere. We investigated diurnal and seasonal variations in CH 4 emissions from a subtropical mangrove estuary in southern Texas, northwest Gulf of Mexico. Tidal processes, amplitude (spring vs. neap tides) and topographic characteristics are crucial factors controlling CH 4 cycling in mangrove creeks. Higher CH 4 concentrations were observed during the ebb in spring tides due to the combination of processive export of CH 4 along the creeks during ebb tides and the addition of porewater CH 4 in upper intertidal sediment under water inundation in spring tides. The annual CH 4 emissions offset approximately 0.15% of the carbon stock in normal years, indicating that these mangrove creeks are a weak CH 4 source. However, significantly elevated CH 4 emissions were observed from mangrove dieback after the extreme cold-freezing event in February 2021. The average CH 4 flux from the mangrove creeks (126.1 AE 128.3 μmol [m 2 Ád] À1 ) increased 45% in 3 months after mangrove die-off in comparison with the overall average in normal years (87.0 AE 64.4 μmol [m 2 Ád] À1 ). It is obvious that the previous small CH 4 offset of the healthy mangrove forest was enlarged by the dieback event.Because the mangrove forests in this study live close to the limit of their survival range, our study highlights the important management considerations for blue carbon projects in vulnerable areas.

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