Ecosystem services can persist in drowning macrotidal salt marshes

Salt marshes can sustainably help to protect our coastlines from high water levels due to sea-level rise (SLR), but they are also vulnerable to increasing water levels. At locations where sediment supply is insufficient or sea levels rise too quickly, marshes will gradually drown - by not maintaining relative elevation - and therefore experience increasing inundation times. Here, we investigated how marsh composition and ecosystem services of drowning macrotidal marshes will change over time. Using a 28-month mesocosm experiment, we assessed i) sediment oxygen conditions, ii) vegetation growth, iii) vegetation traits for wave attenuation, iv) vegetation community and iv) litter decomposition (Teabag Index, indicative of carbon loss), across high marsh (Elymus athericus) and low/pioneer marsh (Spartina anglica) areas under 50 and 100 years of 1 cm y^-1 SLR. Surprisingly, there was almost no plant mortality or impacts on plant traits for wave attenuation across both marsh elevations. The low marsh vegetation remained largely unaffected, while high marsh growth was reduced by 22% under 50-year and 100-year treatments, with a higher community diversity at the end of the second growing season. Wetter treatments typically induced lower rates of litter decomposition and higher stabilisation factors, allowing previously stored carbon to persist. The combination of no plant mortality and predominantly sub-lethal impacts on vegetation indicate that meso- and macrotidal marsh ecosystem services can persist for a long time under gradual sea-level rise, until erosion initiates marsh loss, with sub-lethal drowning effects on vegetation likely occurring slowly enough for ecosystem services to be monitored and managed.