Wave attenuation by two contrasting ecosystem engineering salt marsh macrophytes in the intertidal pioneer zone

Tidal wetlands play an important role in dissipating hydrodynamic energy. Wave attenuation in vegetation depends on plant characteristics, as well as on hydrodynamic conditions. In the pioneer zone of salt marshes, species co-occur that differ widely in their growth strategies, and it is anticipated that these species act differently on incoming waves. In this field study we investigated, under different hydrodynamic forcing and tidal inundation levels, the wave attenuating capacity of two contrasting pioneer salt marsh species that co-occur in the Yangtze estuary, China. Our study shows that vegetation can reduce wave heights up to 80% over a relatively short distance (<50 m). Our results further indicate that Spartina alterniflora is able to reduce hydrodynamic energy from waves to a larger extent than Scirpus mariqueter, and therefore has a larger ecosystem engineering capacity (2.5× higher on average). A higher standing biomass of S. alterniflora explained its higher wave attenuation at low water depths. Being much taller compared to S. mariqueter, S. alterniflora also attenuated waves more with increasing water depth. We conclude that knowledge about the engineering properties of salt marsh species is important to better understand wave attenuation by tidal wetlands and their possible role in coastal protection.