The Effect of Bioirrigation on Sediment-Water Exchange of Methylmercury in Boston Harbor, Massachusetts, USA

Coastal marine sediments are important sites of methylmercury (MeHg) production, and dissolved efflux provides an important source of MeHg to near-shore, and possibly off-shore, water columns and food webs. We measured the flux of MeHg across the sediment-water interface at four sites in Boston Harbor that span a range of infaunal population densities and bioirrigation intensities. At each site we carried out total MeHg flux measurements using core incubations and collected near-surface pore waters to establish MeHg gradients for diffusive flux calculations. The flux cores were also imaged by CT scanning to determine the distribution of infaunal burrows, and they were sieved for infaunal enumeration. Total MeHg fluxes, measured using core incubations, ranged from -8 to 190 pmol/m²d, and total MeHg fluxes were strongly correlated with burrow densities at the sites. Estimated diffusive fluxes, calculated based on MeHg concentration gradients below the sediment-water interface, were much lower than total fluxes at three of the sites, ranging from 2-18 pmol/m²d. These results indicate that MeHg flux may be significantly enhanced over molecular diffusion in bioturbated sediments. Furthermore, burrow density provides a strong predictor of total MeHg flux. Pore-water exchange of dissolved MeHg will be further considered within the context of a bioirrigation model that describes observed Rn-222 pore-water profiles at the sites.