Effect of bioirrigation on sediment-water exchange of methylmercury in Boston Harbor, Massachusetts.
Coastal marine sediments are important sites of methylmercury (MMHg) production, and dissolved efflux provides an important source of MMHg to near-shore, and possibly offshore, water columns and food webs. We measured the flux of MMHg across the sediment-water interface at four stations in Boston Harbor that span a range of infaunal population densities and bioirrigation intensities. At each station we carried out total MMHg flux measurements using core incubations and collected near-surface pore waters to establish MMHg gradients for diffusive flux calculations. The flux cores were also imaged by CT scanning to determine the distribution of infaunal burrows, and pore-water sulfide and 222Rn profiles were measured. Total MMHg fluxes, measured using core incubations, ranged from -4 to 191 pmol m(-2) d(-1), and total MMHg fluxes were strongly correlated with burrow densities at the stations. Estimated diffusive fluxes, calculated based on MMHg concentration gradients below the sediment-water interface, were much lower than total fluxes at three of the stations, ranging from 2-19 pmol m(-2) d(-1). These results indicate that MMHg exchange may be significantly enhanced over molecular diffusion in bioturbated sediments. Furthermore, burrow density provides a strong predictor of total MMHg flux. Pore-water exchange of both dissolved MMHg and 222Rn, a naturally occurring pore-watertracer, increased across the range of observed burrow densities, suggesting that the presence of burrows enhances both MMHg production and flux.
Benoit
Janina M
JM
Chemistry Department, Wheaton College, Norton, Massachusetts 02766, USA. jbenoit@wheatonma.edu
Shull
David H
DH
Harvey
Rebecca M
RM
Beal
Samuel A
SA
D001900:Boston; D004058:Diffusion; D004777:Environment; D005740:Gases / Q000737:chemistry; D019015:Geologic Sediments / Q000737:chemistry*; D008404:Massachusetts; D008767:Methylmercury Compounds / Q000032:analysis*; D016062:Porosity; D045483:Rivers / Q000737:chemistry; D012995:Solubility; D013440:Sulfides / Q000737:chemistry; D014867:Water / Q000737:chemistry*
D005740:Gases; D008767:Methylmercury Compounds; D013440:Sulfides; D014867:Water