Fiddler crab burrowing increases salt marsh greenhouse gas emissions

Salt marshes are globally important sites of carbon burial, but microbial processes in their soil produce potent greenhouse gases such as carbon dioxide (CO2) and methane (CH4) that can be released back to the atmosphere. Although a great deal is known about how shifting seasons, tides, and salinity influence salt marsh greenhouse gas emissions, little effort has been devoted to understanding how emissions respond to bioturbation and pollution. In this study, we evaluated how fiddler crab bioturbation and oil pollution altered CO2 and CH4 fluxes from Louisiana salt marsh soil through a series of microcosm incubation experiments. CO2 and CH4 fluxes did not consistently increase as a function of crab burrow depth even though soil CO2 and CH4 concentrations increased with depth. Instead, regardless of burrow depth, CO2 and CH4 fluxes were highest (536.7 ± 41.6 and 6.35 ± 0.59 nmol g−1 h−1, respectively) immediately after burrowing activity ceased and within ~ 2 h decreased to a low background level (117.4 ± 17.1 and 0.53 ± 0.26). These background CO2 fluxes persisted through the end of our experiments, but background CH4 fluxes were not detectable after < 1 day, suggesting an important role for methanotrophy. Therefore, crab burrow size had a strong influence on CO2 fluxes, while the rate of burrow excavation was more important for CH4 fluxes. Low to moderate oil concentrations (up to 25.55 mg cm−2) did not alter greenhouse gas fluxes. However, severe oil contamination that decreases fiddler crab abundance and burrowing activity will also indirectly decrease salt marsh greenhouse gas emissions. These findings illuminate the importance of fiddler crab bioturbation to salt marsh carbon cycling and export.