Assessing the effects of restoration and conservation on gaseous carbon fluxes and climate mitigation potential across six European coastal wetlands

Coastal wetlands play a substantial role in regulating Earth's climate through exchanges of greenhouse gases (GHGs). Current European policies promote widespread coastal wetland restoration to reverse historical losses and ongoing pressures. However, substantial uncertainty remains regarding how carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes respond to restoration across different coastal wetland types and whether these responses translate into net climate mitigation in terms of CO 2 equivalents (CO 2 -eq). We measured simultaneous CO 2 and CH 4 fluxes using static chambers across four seasons at multiple locations spanning preserved, altered and restored sites within six European coastal wetlands of different ecological types. By comparing GHG exchanges and resulting CO 2 -eq balances across wetlands, we identified dominant biogeochemical drivers of CO 2 and CH 4 dynamics and assessed the climate mitigation potential of conservation and restoration actions. CO 2 fluxes were primarily controlled by landscape-scale vegetation cover and inundation, whereas CH 4 emissions responded to less evident changes in water quality, salinity and wetland hydrodynamics. Comparisons of CO 2 -eq balances between altered and restored sites showed that seagrass replantation was associated with significant mitigation potential under both 100-year and 20-year CH 4 warming scenarios, while eutrophication reversal through improved water treatment was only significant under the 20-year scenario. In contrast, other restoration measures modified CO 2 and CH 4 fluxes in opposing directions, resulting in no significant net change in combined CO 2 -eq balances. Overall, our results demonstrate that climatic responses to coastal wetland restoration are both GHG-specific and wetland-type dependent, underscoring the need for tailored restoration strategies and robust, multi-GHG monitoring to detect and accurately quantify potential climatic benefits.