Marsh crab impacts on hydrology and biogeochemistry alter coastal carbon cycling

Carbon sequestration in tidal marshes is a valuable carbon sink on the global scale, yet these ecosystems are threatened by climate change and anthropogenic influences. Large uncertainties in the present-day salt marsh carbon budget and mechanisms mediating the magnitude and direction of carbon fluxes limit the efficacy of conservation efforts, investment in tidal wetlands for long-term carbon storage, and our ability to predict carbon budget feedbacks with anthropogenic and climatic change. In an effort to mechanistically link ecosystem components and enhance future carbon budget predictions, we assess the interactions between crab activity, water movement, and biogeochemical conditions in a mid-Atlantic salt marsh. Here we show that crab burrows increase the permeability of the marsh platform in the summer by an order of magnitude, resulting in an increase in the volume of groundwater-surface water exchange between the carbon-rich marsh and creek and in the depth of the oxic zone where carbon oxidation more readily occurs - enhancing vertical and horizontal carbon exports through physical and chemical changes. Thus, we identify a positive feedback mechanism between crab activity and carbon efflux. On the global scale, we calculate that crab burrows are responsible for a 0.3-5.5% decline in salt marsh carbon burial, a number that could grow to 11% in the coming century.