High spatiotemporal variability of methane concentrations challenges estimates of emissions across vegetated coastal ecosystems. (12th April 2022)

• Record Type: Journal Article
• Authors: Roth, Florian; Sun, Xiaole; Geibel, Marc C.; Prytherch, John; Brüchert, Volker; Bonaglia, Stefano; Broman, Elias; Nascimento, Francisco; Norkko, Alf; Humborg, Christoph
• Is Part Of: Global change biology. Volume 28:Number 14(2022)
• Page Start: 4308
• Page End: 4322
• Publication Date: 2022-04-12
• Subjects: blue carbon -- carbon cycle -- climate change -- coastal greenhouse gas emissions -- methane fluxes; Climatic changes -- Environmental aspects -- Periodicals; Troposphere -- Environmental aspects -- Periodicals; Biodiversity conservation -- Periodicals; Eutrophication -- Periodicals; 551.5
• DOI: 10.1111/gcb.16177
• Languages: English
• ISSNs: 1354-1013

Abstract

Abstract: Coastal methane (CH4 ) emissions dominate the global ocean CH4 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 CH4 concentrations, δ 13 C‐CH4 values, and CH4 sea–air fluxes across four seasons in three globally pervasive coastal habitats, we show that the CH4 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 CH4 concentrations ranging two orders of magnitude (i.e., 6–460 nM CH4 ) with habitat‐specific seasonal and diurnal patterns. We observed (1) δ 13 C‐CH4 signatures that revealed habitat‐specific CH4 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 CH4 distribution signified by apparent activation energies of ~1 eV that drove seasonal changes. Bootstrapping simulations show that scaling the CH4 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 CH4 sources (i.e., releasing ≥ 100 μmol CH4 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 CH4 estimates and confine the habitat‐specific contribution to regional and global CH4 budgets. Abstract : Shallow coastal waters are hotspots for ocean methane emissions, but the fluxes remain poorly constrained in space and time due to the heterogeneous and dynamic nature of these environments. Using continuous measurements, we identify unprecedented spatiotemporal variability in water‐column methane distribution that range orders of magnitude, with habitat‐specific production and consumption pathways, seasonal temperature dependencies, and short‐term fluctuations. We discuss how failing to include spatially and temporally resolved assessments in future studies will result in a continued systematic bias of emission estimates and propose sampling intensities that help determine the scale and drivers of this variability across the coastal seascape.