The geomorphic impact of mangrove encroachment in an Australian salt marsh

Mangroves are encroaching into salt marshes throughout the world as a result of environmental change. Previous studies suggest mangroves trap sediment more efficiently than adjacent salt marshes, providing mangroves greater capacity to adapt to sea level rise; this may occur by displacing salt marshes. However, sediment transport in adjacent marsh-mangrove systems and its role in mangrove encroachment upon salt marsh remain poorly understood. Here we directly test the hypothesis that mangroves reduce the ability of adjacent marsh to adjust to sea level rise by measuring sediment transport across salt marsh platforms, with and without 6 m of fringing mangroves at the tidal creek edge. We find that salt marshes and mangroves have equivalent sediment trapping efficiencies along the wetland edge. Suspended sediment concentrations, mass accumulation rates, and long-term accretion rates are not lower in salt marshes landward of mangroves than salt marshes without fringing mangroves. Therefore, our work suggests that a relatively narrow zone of mangroves does not impact salt marsh accretion, and activities that limit mangrove encroachment into salt marsh, such as removal of seedlings, will not improve the capacity of salt marsh to trap sediments.