Role of ecological interactions in saltmarsh geomorphic processes

Bl Williams,Ds Johnson

doi:10.3354/meps13554

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 2 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 that the effects of plant-animal interactions on marsh resilience against sea-level rise are context dependent.

Highlights

Salt marshes are among the most productive ecosystems in the world (Mendelssohn and Morris 2002) and provide important ecosystem services such as storm protection, carbon storage, food production, and tourism (Barbier et al 2011)
Effect of Uca on Spartina alterniflora production and sediment deposition Uca density and site interacted to affect Spartina biomass (P = 0.006, Figure 4), indicating that there is a site-specific response to Uca burrows
When Sesarma grazing intensity was high, Uca bioturbation likely increased marsh erosion. These results suggest that Sesarma and Uca have a density-dependent impact on components of vertical accretion, and their relative population size may influence the ability of salt marshes to keep pace with sealevel rise

Summary

Introduction

Salt marshes are among the most productive ecosystems in the world (Mendelssohn and Morris 2002) and provide important ecosystem services such as storm protection, carbon storage, food production, and tourism (Barbier et al 2011). Habitat loss due to accelerated sea-level rise is a major concern for salt marshes, especially in regions where accelerations of sea-level rise rates are higher than the global average, like Atlantic coast of the United States. The rate sea-level rise is increasing 3-4 times faster than the global average (Sallenger et al 2012). Salt marsh persistence in the face of sea-level rise relies on landward migration and vertical accretion (Kirwan et al 2016). Landward migration is often inhibited by anthropogenic structures such as roads, sea walls, and houses, causing a coastal squeeze (Pontee 2013). Since 14% of the United States shoreline has been hardened (Gittman et al 2015), understanding the factors that influence accretion will be important to predict the vulnerability of salt marshes to accelerated sea-level rise

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