Emerging trade-offs in saltmarsh ecosystem services under sea-level rise

Coastal defence and carbon storage by coastal wetlands provide an increasingly recognised means of adaptation and mitigation in the face of climate change, yet these same services are threatened by the impacts against which they protect. Our understanding to date tends to consider impacts of rising sea levels on individual marsh services but fails to consider trade-offs and potential interactions between them. For instance, sea-level rise has the potential to enhance carbon storage but may decrease soil stability, erosion resistance and wave-attenuation capacity by creating sub-optimal growth conditions, causing plants to be weaker and smaller. As such, we aim to answer the question: will rising sea levels drive unidirectional impacts, or contrasting ‘trade-offs’, between saltmarsh carbon storage and coastal defence? We conducted a field experiment across increasing inundation durations beyond the usual seaward limit of Spartina anglica growth, mimicking SLR at established vegetation patches, and measured (1) carbon decomposition rates using the teabag index as proxy, (2) sediment stability, and (3) biomechanical traits of S.anglica as proxy for wave-attenuation capacity. With increasing inundation duration, rates of carbon decomposition slowed, supporting carbon storage, and therefore positively impacted climate change mitigation. In contrast, both sediment stability and vegetation stiffness decreased and hence negatively impacted climate change adaptation. Our findings point to a short-term trade-off in climate ecosystem services with SLR, where positive impacts on carbon storage may offset negative impacts on coastal defence services. However, since lower sediment stability increases the chance of marsh erosion under extreme events, this will increase over time the likelihood of sediment resuspension and the ultimate rerelease of previously stored carbon into the marine carbon cycle. Thus, the interaction between ecosystem services may lead to a net negative impact of SLR on climate mitigation services. Overall, we highlight the necessity of taking a multi-service approach to considering the impact of global change effects on ecosystem services.