Master of Science (M.Sc.)
Virginia Institute of Marine Science
David S Johnson
Mark J Brush
Rochelle D Seitz
Matthew L Kirwan
Accelerated sea-level rise poses a significant threat to coastal habitats. Salt marshes are critical coastal ecosystems, providing a host of services such as storm protection, food production, and carbon storage. Persistence of salt marshes in the face of rising sea levels relies, in part, on vertical accretion. Current ecogeomorphic models and empirical studies emphasize the importance of the positive relationship between plant production and vertical accretion via sediment trapping by stems aboveground and belowground organic matter production. Thus, changes in plant production influence salt marsh persistence with sea-level rise. However, studies and models of marsh accretion do not consider the effects of animal-mediated changes in plant production. Here, I tested how two co-occurring marsh crustaceans, Uca pugnax (marsh fiddler crab) and Sesarma reticulatum (purple marsh crab), which have contrasting effects on smooth cordgrass (Spartina alterniflora) production, indirectly influence sediment deposition and belowground organic matter contribution, through observational surveys and field manipulation. S. reticulatum feeds directly on S. alterniflora, while U. pugnax facilitates S. alterniflora production through burrowing and biodeposits. I found that U. pugnax facilitated S. alterniflora biomass in some marshes, but not others. However, this facilitation of S. alterniflora biomass did not enhance sediment deposition. U. pugnax had no effect on belowground components of vertical accretion (i.e. root production and decomposition). These results suggest that in isolation, U. pugnax has little impact on saltmarsh geomorphic processes. S. reticulatum reduced S. alterniflora above- and belowground biomass; however, sediment deposition increased as S. alterniflora biomass decreased, contrary to models of ecogeomorphology. This trend was likely due to sediment being resuspended by crab bioturbation, as U. pugnax abundances were higher in S. reticulatum-grazed areas than in non-grazed areas. When U. pugnax occurred in areas of low S. reticulatum grazing, S. alterniflora biomass and sedimentation was similar to areas with only U. pugnax. I suggest that the negative impacts of S. reticulatum are exaggerated when intense grazing results in completely unvegetated areas and subsequent increases in U. pugnax density, where bioturbation erodes sediments. Thus, while S. reticulatum can increase the susceptibility of marsh sediments to physical erosion by removing vegetation, it may also do so by facilitating U. pugnax bioturbation. However, when S. reticulatum grazing intensity is low, facilitation of S. alterniflora growth by U. pugnax can mitigate the negative effect of grazing, which suggests that the net effect of these species may depend on their relative abundance. This study demonstrates that ecological interactions, in addition to physical processes, have significant effects on marsh persistence as sea level rises, and merit incorporation into ecogeomorphic models and empirical studies of marsh accretion.
© The Author
Williams, Bethany Lynn, "The Role of Ecological Interactions in Saltmarsh Geomorphic Processes" (2018). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1530192504.