Tidal control and mangrove dieback impact on methane emissions from a subtropical mangrove estuary

Mangrove ecosystems with high sediment deposition and active carbon cycling are a source of methane (CH4) to the coastal atmosphere. We investigated diurnal and seasonal variations in CH4 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 CH4 cycling in mangrove creeks. Higher CH4 concentrations were observed during the ebb in spring tides due to the combination of processive export of CH4 along the creeks during ebb tides and the addition of porewater CH4 in upper intertidal sediment under water inundation in spring tides. The annual CH4 emissions offset approximately 0.15% of the carbon stock in normal years, indicating that these mangrove creeks are a weak CH4 source. However, significantly elevated CH4 emissions were observed from mangrove dieback after the extreme cold-freezing event in February 2021. The average CH4 flux from the mangrove creeks (126.1 +/- 128.3 mu mol [m(2)center dot d](-1)) increased 45% in 3 months after mangrove die-off in comparison with the overall average in normal years (87.0 +/- 64.4 mu mol [m(2)center dot d](-1)). It is obvious that the previous small CH4 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.

Automated summary

The study reveals that subtropical mangrove estuaries in southern Texas play a crucial role in methane cycling, influenced by tidal processes, amplitude, and topographic characteristics. Normally, these mangroves are a weak source of methane emissions, but after a severe cold-freezing event, the mangrove dieback resulted in significantly elevated methane emissions. This study highlights the importance of management considerations for blue carbon projects in vulnerable areas.