Seagrass Impact on Sediment Exchange Between Tidal Flats and Salt Marsh, and The Sediment Budget of Shallow Bays

Seagrasses are marine flowering plants that strongly impact their physical and biological surroundings and are therefore frequently referred to as ecological engineers. The effect of seagrasses on coastal bay resilience and sediment transport dynamics is understudied. Here we use six historical maps of seagrass distribution in Barnegat Bay, USA, to investigate the role of these vegetated surfaces on the sediment storage capacity of shallow bays. Analyses are carried out by means of the Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) numerical modeling framework. Results show that a decline in the extent of seagrass meadows reduces the sediment mass potentially stored within bay systems. The presence of seagrass reduces shear stress values across the entire bay, including unvegetated areas, and promotes sediment deposition on tidal flats. On the other hand, the presence of seagrasses decreases suspended sediment concentrations, which in turn reduces the delivery of sediment to marsh platforms. Results highlight the relevance of seagrasses for the long-term survival of coastal ecosystems, and the complex dynamics regulating the interaction between subtidal and intertidal landscapes. Plain Language Summary Seagrasses influence the resilience of coastal wetlands to external agents, such as sea level rise, by altering the velocity field and sediment transport dynamics of coastal environments. In many areas worldwide seagrass habitats are declining. This paper studies how seagrasses influence the sediment budget of shallow bays using a computer model, and Barnegat Bay, New Jersey, as test case. Specifically, we used computer models to simulate velocity and sediment transport dynamics in Barnegat Bay with historical seagrass maps for the period 1968-2009. These maps show that for Barnegat Bay seagrasses have decreased in time. We found that seagrasses are important for the retention of sediments within bay systems, and when seagrasses are present less sediments are lost in the ocean, which is relevant for the long-term survival of coastal wetlands as an abundance of sediments generally corresponds to more resilient wetlands. The presence of seagrasses mainly increases the storage of sediments on tidal flats, while it decreases the delivery of sediments to the marsh platforms during high tide. Our results highlight the importance of seagrasses and are relevant for coastal communities and coastal managers worldwide as they could aid the design of coastal protection schemes.