Silva, R.; van Tussenbroek, B.I.; Escudero-Castillo, M.; Mariño-Tapia, I.; Dijkstra, H.A.; van Westen, R.M.; Pietrzak, J.D.; Candy, A.S.; Katsman, C.A.; van der Boog, C.G.; Riva, R.E.M.; Slobbe, C.; Klees, R.; Stapel, J.; van der Heide, T.; van Katwijk, M.M.; Herman, P.M.J. & Bouma, T.J. (2019). Maintaining tropical beaches with seagrass and algae: a promising alternative to engineering solutions. BioScience, 69, 136-142 is available online at: https://dx.

Background and Aims It is important to have an in-depth mechanistic understanding of tidal marsh establishment and dynamics to ensure the long-term persistence of these valuable ecosystems. As wave forcing may be expected to impact seedling establishment, we studied the effect of water-imposed drag forces on seedling survival, morphology and biomechanical properties of three marsh pioneer species that are dominant along the salinity gradient in many areas around the world: Spartina anglica (salt to brackish), Scirpus maritimus (brackish) and Phragmites australis (brackish to fresh). Methods Using a newly developed plant-shaking mesocosm (PSM) that mimicked water-imposed wave drag forces, the effect of wave stress on seedling survival was examined, together with impacts on morphology and biomechanical properties. Key Results After 7 weeks of exposure to wave stress, lowered seedling survival and growth for all species was revealed. Wave treatments increased the root/shoot biomass ratio to enhance anchorage and made seedlings more flexible (i.e. reduced flexural rigidity), which might be regarded as a mixed outcome between a stress avoidance and stress tolerance strategy. Conclusions The different biomechanical responses between the three dominant marsh pioneer species, overall, make them less resistant to external stress. Therefore, our results indicate that the likelihood of marshes becoming established is reduced if wave energy increases. Despite the different biomechanical response of these three pioneer species to waves, the seedlings of all species were found to have low resistance to external stresses.

Sea-level rise poses severe threats to coastal and low-lying regions around the world, by exacerbating coastal erosion and flooding. Adequate sea-level projections over the next decades are important for both decision making and for the development of successful adaptation strategies in these coastal and low-lying regions to climate change. ocean components of climate models used in the most recent sea-level projections do not explicitly resolve ocean mesoscale processes. Only a few effects of these mesoscale processes are represented in these models, which leads to errors in the simulated properties of the ocean circulation that affect sea-level projections. Using the Caribbean Sea as an example region, we demonstrate a strong dependence of future sea-level change on ocean model resolution in simulations with a global climate model. the results indicate that, at least for the Caribbean Sea, adequate regional projections of sea-level change can only be obtained with ocean models which capture mesoscale processes. The ongoing increase of global sea level threatens both coastal and low-lying regions. The combination of future sea-level rise with increasing storm occurrence and intensity may exacerbate beach erosion 1. This can have severe consequences for areas which are highly dependent on their beaches, either for flood safety or economically for local tourism 2,3. Higher sea levels can also lead to changes in coastal ecosystems, and permanent submergence of land and human settlements 4. For making adequate decisions on the development of successful adaptation strategies to sea-level rise, skillful projections over the next decades are crucially important 5. By decomposing the components contributing to the satellite-observed global mean sea-level (GMSL) rise between 1993 and 2014, it was shown that the dominant contributor to GMSL is thermal expansion of the ocean 6. Over the same period, the contribution of mass loss of both glaciers and large ice sheets to GMSL rise became more important over time 6,7. On a regional scale, sea-level rise may deviate from GMSL rise 8 and can be caused by other processes than thermal expansion. For example, the dominant contributor to sea-level rise in the Caribbean is thermal expansion (40% = 0.8 mm year −1) while east of the Caribbean 9 this is ocean mass redistribution (50% = 1.7 mm year −1). Regional sea-level change induced by variations in the gravitational contribution due to large ice sheets and glacial isostatic adjustment is very homogeneous over these two regions 9. Hence, the difference in magnitude of sea-level rise in the different regions is caused by ocean sterodynamic effects (i.e. mass redistribution and thermal expansion) 10. Ocean volume conserving climate models, such as those used in the sixth coupled model inter-comparison projects (CMIP) of the Intergovernmental Panel on Climate Change (IPCC) Assessment Report, provide projections of the dynamic sea level (DSL) and sterodynamic sea level (SDSL) 22. Due to their low spatial resolution, the ocean component ...