Porewater‐Surface Water Interface Mediates Methanogenesis, Oxidation, and Large Methane Emissions in Mixed Seagrass‐Macroalgae Ecosystems

The porewater‐surface water interface influences methane (CH 4 ) dynamics in mixed seagrass‐macroalgae ecosystems, but the mechanisms governing methanogenesis, oxidation suppression, and emission amplification remain poorly understood. Here, in situ high‐resolution monitoring at such a mixed system, combined with 222 Rn mass balance models, revealed that CH 4 fluxes primarily from porewater exchange are strongly regulated by tidal pumping and large‐scale seasonal bioturbation. Compared with global data sets, measured CH 4 emissions (200 μmol m −2 d −1 at surface water‐air interface and 1,375 μmol m −2 d −1 at porewater‐surface water interface) from the seagrass‐macroalgae system significantly exceed global median values (26.5 μmol m −2 d −1 at surface water‐air interface and 44.0 μmol m −2 d −1 at porewater‐surface water interface) reported for seagrass meadows at both porewater‐surface water and surface water‐air interfaces. To elucidate the underlying mechanisms, laboratory incubations with 13 C‐labeled substrates were conducted and revealed that methylotrophic methanogenesis dominated CH 4 production, with negligible anaerobic oxidation of CH 4 . The presence of macroalgae amplified CH 4 emissions through enhanced labile organic matter inputs and microbial priming effects. These results highlight porewater exchange driven CH 4 emissions in mixed seagrass‐macroalgae ecosystems as a key mechanism making them potential coastal CH 4 hotspots, underscoring the necessity of incorporating seagrass‐macroalgae interactions into coastal blue carbon budgets.