Methane emissions offset atmospheric carbon dioxide uptake in coastal macroalgae, mixed vegetation and sediment ecosystems.

Coastal ecosystems can efficiently remove carbon dioxide (CO ₂ ) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH ₄ ) emissions from these ecosystems may counterbalance atmospheric CO ₂ uptake. Still, knowledge of mechanisms sustaining such CH ₄ emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH ₄ in the range of 0.1 - 2.9 mg CH ₄  m ^-2  d ^-1 to the atmosphere, revealing in situ CH ₄ emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO ₂ -equivalent CH ₄ emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO ₂ uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO ₂ release of unvegetated sediments by 57%. Accounting for CH ₄ alongside CO ₂ sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.
(© 2023. The Author(s).)