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Role of ecological interactions in saltmarsh geomorphic processes

Bethany L. Williams*, David S. Johnson

*Corresponding author:

ABSTRACT: Accelerated sea-level rise poses a significant threat to coastal habitats, such as salt marshes, which provide critical ecosystem services. Persistence of salt marshes with rising sea levels relies, in part, on vertical accretion. Ecogeomorphic models emphasize the role of plant production in vertical accretion via sediment trapping and belowground organic matter contribution. Thus, changes in plant production can influence saltmarsh persistence with sea-level rise. However, models of marsh accretion do not consider animal-mediated changes in plant production. We tested how two marsh crabs, Minuca pugnax and Sesarma reticulatum, which have contrasting effects (facilitation vs. herbivory) on Spartina alterniflora production, may indirectly influence sediment deposition and belowground production, through observational surveys and field manipulation. Minuca facilitated Spartina biomass in some marshes, but not sediment deposition, and had no effect on belowground organic matter contribution, suggesting that in isolation, Minuca has little indirect impact on saltmarsh geomorphic processes. Sesarma reduced Spartina biomass; however, sediment deposition increased, contrary to ecogeomorphic models, likely due to sediment resuspension by Minuca. When Minuca and Sesarma co-occur, the effect on Spartina production and sediment deposition depended on the amount of grazing. When Sesarma grazing is low, Minuca facilitates Spartina growth and mitigates the effect of grazing. However, when Sesarma grazing is high and vegetation is removed, Minuca can resuspend sediment through bioturbation, suggesting the net effect of these species may depend on their relative abundance. This study demonstrates the effects of plant-animal interactions on marsh resilience against sea level rises, are context dependent.