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.