Impact of crab bioturbation on benthic flux and nitrogen dynamics of Southwest Atlantic intertidal marshes and mudflats

The effect of the SW Atlantic intertidal burrowing crab Neohelice (Chasmagnathus) granulata on benthic metabolism, benthic flux, and benthic N cycling processes was investigated through field experiments and in situ benthic chambers incubations. Our experimental results show that the presence and activity of N. granulata and its burrows may affect the direction and magnitude of nutrient benthic fluxes. Bioturbation enhanced ammonium efflux at mudflats, and influx at marshes. The flux of nitrate toward the sediment was stimulated by crabs at light and dark conditions in marshes, but only under light exposure in mudflats. Crab bioturbation stimulated benthic metabolism, N mineralization, nitrification and denitrification potentials in both sites. Crabs seem to have contrasting effects on dissolved inorganic nitrogen (DIN) availability between marshes and mudflats, as reflected on benthic DIN flux. This different effect on DIN availability and also the possible different effects of crabs on N ₂-fixers organisms could explain the opposite N ₂ fixation pattern found for both habitats, since crabs promoted N ₂ fixation in marshes, but diminished its rate in mudflats. Thus, the results obtained here through manipulative field experiments using benthic chambers suggest that macrofauna may influence the N benthic cycle and DIN fluxes in estuarine sediments. Besides, these macrofauna effects could be context-dependent, being many of them opposite between mudflats and marshes. We concluded that the above mentioned effects and the bioturbation-macrophytes interaction may be affecting the dissolved nutrient exportation from marshes to open waters.