Design and Assessment of Marsh Ecosystem Response to Increased Temperature (MERIT): an ecosystem warming experiment in a high-energy tidal wetland

Wetlands and other coastal vegetated ecosystems act as carbon (C) sinks and provide crucial ecosystem services, including buffering storm surges and providing habitats for diverse species. Understanding how wetland biogeochemistry and ecosystem function respond to global change drivers separately and in combination is critical for designing and implementing successful climate mitigation practices and policy. Findings from the Marsh Ecosystem Response to Increased Temperature (MERIT) experiment offer critical insights into how wetland processes are currently being altered by climate change and how they may respond in the future. MERIT employs a novel combination of active, feedback-controlled belowground and surface warming with passive aboveground warming using partially-covered domes across the full salt marsh elevational gradient of the Wadden Sea (Germany). Our design was effective at warming this high-energy ecosystem, with mixed models revealing the warming treatment factor driving temperature differences belowground and at the soil surface. Aboveground temperatures showed a weaker warming treatment effect; however, the dome structure minimized some issues seen in open-top chamber (OTC) experiments. The MERIT experimental design achieved whole-ecosystem warming without sacrificing inputs from wind, solar radiation, and tidal inundations, and our methods are adaptable to a wide range of coastal systems. Our work opens a pathway for additional in-situ, manipulative studies and improved collective knowledge of wetland global change processes.