High spatiotemporal variability of methane concentrations challenges estimates of emissions across vegetated coastal ecosystems

Coastal methane (CH(4)) emissions dominate the global ocean CH(4) budget and can offset the “blue carbon” storage capacity of vegetated coastal ecosystems. However, current estimates lack systematic, high‐resolution, and long‐term data from these intrinsically heterogeneous environments, making coastal budgets sensitive to statistical assumptions and uncertainties. Using continuous CH(4) concentrations, δ(13)C‐CH(4) values, and CH(4) sea–air fluxes across four seasons in three globally pervasive coastal habitats, we show that the CH(4) distribution is spatially patchy over meter‐scales and highly variable in time. Areas with mixed vegetation, macroalgae, and their surrounding sediments exhibited a spatiotemporal variability of surface water CH(4) concentrations ranging two orders of magnitude (i.e., 6–460 nM CH(4)) with habitat‐specific seasonal and diurnal patterns. We observed (1) δ(13)C‐CH(4) signatures that revealed habitat‐specific CH(4) production and consumption pathways, (2) daily peak concentration events that could change >100% within hours across all habitats, and (3) a high thermal sensitivity of the CH(4) distribution signified by apparent activation energies of ~1 eV that drove seasonal changes. Bootstrapping simulations show that scaling the CH(4) distribution from few samples involves large errors, and that ~50 concentration samples per day are needed to resolve the scale and drivers of the natural variability and improve the certainty of flux calculations by up to 70%. Finally, we identify northern temperate coastal habitats with mixed vegetation and macroalgae as understudied but seasonally relevant atmospheric CH(4) sources (i.e., releasing ≥ 100 μmol CH(4) m(−2) day(−1) in summer). Due to the large spatial and temporal heterogeneity of coastal environments, high‐resolution measurements will improve the reliability of CH(4) estimates and confine the habitat‐specific contribution to regional and global CH(4) budgets.