Linking sea–air and benthic methane fluxes across seasons in a tropical seagrass meadow of Taiwan

Seagrass meadows are important blue carbon ecosystems that sequester large amounts of organic carbon but can also emit methane (CH ₄ ), potentially offsetting part of their climate mitigation benefits. However, the seasonal and diurnal dynamics of CH ₄ production and emissions in tropical seagrass ecosystems remain poorly understood. This study presents the first integrated assessment of CH ₄ cycling in the Haikou seagrass meadow, southern Taiwan. Field campaigns conducted between May 2022 and June 2023 combined surface water sampling, in situ benthic chamber incubations, and porewater profiling in a Thalassia hemprichii–Halophila pinifolia seagrass meadow to quantify CH ₄ dynamics at both the sediment-to-water and sea-to-air interfaces. The seagrass meadow exhibited higher dissolved CH ₄ concentrations and sea-to-air CH ₄ fluxes than adjacent coastal waters, confirming that it functions as a localized atmospheric CH ₄ hotspot. Nighttime dissolved CH ₄ concentrations generally exceeded daytime values, reflecting diurnal variations in oxygen availability, whereas seasonal variability in sea-to-air CH ₄ fluxes was primarily driven by wind speed. Mean sediment-to-water and sea-to-air CH ₄ fluxes were 88.2±41.2 and 4.9±2.2µmol m ⁻²  d ⁻¹ , respectively. Despite substantial benthic CH ₄ production, atmospheric emissions remained comparatively low, indicating a marked decoupling between sediment-to-water and sea-to-air CH ₄ fluxes. This decoupling suggests that much of the sediment-derived CH ₄ was transported laterally by tidal exchange and coastal circulation before atmospheric release, with microbial oxidation acting as an additional sink. Compared with adjacent bare sand sediments, seagrass sediments maintained higher baseline CH ₄ production rates and benthic fluxes owing to greater organic matter availability and favorable microbial conditions, whereas unvegetated sandy sediments generally exhibited low CH ₄ production but produced episodic CH ₄ pulses following storm-induced sediment disturbance. Elevated porewater CH ₄ concentrations during summer and autumn further indicated enhanced methanogenesis associated with organic matter accumulation under calmer hydrodynamic conditions. Overall, these findings demonstrate that CH ₄ cycling in tropical seagrass meadows is jointly regulated by biological processes and hydrodynamic conditions, highlighting the need to consider both CH ₄ emissions and carbon sequestration when evaluating the climate benefits of blue carbon ecosystems.