Biophysical Feedbacks Mediate Tidal Creek Formation in Salt Marshes



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How organisms and ecological systems respond to global change is of great interest to ecologists. These responses may or may not be beneficial because there might be positive or negative feedbacks that would make the impacts larger or smaller than expected. I conducted laboratory and field experiments to explore the mechanisms driving tidal creek formation in southeastern US salt marshes, focusing on three topics. 1) The effect of crabs on creek growth via four potential mechanisms: sediment excavation, plant removal, subterranean erosion, and decomposition. I found that Sesarma reticulatum (henceforth Sesarma) is the primary crab species mediating creek growth. Sesarma is concentrated at creek heads where plants are dying and creeks eroding. Sesarma excavated larger amounts of sediment through burrowing than other crabs, and is unique in creating burrow networks that likely increase belowground erosion and decomposition. Sesarma also is the only crab species that directly kills vegetation. Thus, Sesarma negatively impacts the marsh plant Spartina alterniflora and alters marsh geomorphology by engineering creek growth. 2) The factors mediating crab feeding preferences. I discovered that Sesarma did less damage to its food plant S. alterniflora in the presence of predators. Sesarma prefers and grows better consuming rhizomes than leaves; however, the cost of accessing rhizomes leads to higher mortality if rhizomes are the only diet option. A choice in feeding location allows Sesarma flexibility to balance the risks of predation, the nutritional benefit of feeding below-ground, and the survival costs of below-ground feeding. 3) The factors driving the aggregation and movement of Sesarma at creek heads. Creek heads are cooler, have higher dissolved oxygen levels, and lower hydrogen sulfide concentrations than elsewhere on the marsh. These superior conditions drive Sesarma aggregation. Additionally, I found that hydrology drives Sesarma aggregations when creek conditions were mimicked on the marsh platform. The results of this dissertation suggest that there is a biophysical feedback loop in marsh creek formation: creek growth is driven by a positive feedback between Sesarma crabs, which accelerate erosion, and creeks, which create abiotic conditions favorable for Sesarma crabs.



Crab burowing, Crab herbivory, Creek growth, Marsh geomorphology, Marsh hydrology, Sesarma, Soil erosion, Above-ground herbivory, Below-ground herbivory, Panopeus, Eurytium, Non-consumptive effects, Predato-prey interactions, Spartina, Asymmetrical movement, Consumer front, Tidal creek, Hydrology