Infaunal burrow densities and sediment monomethyl mercury distributions in Boston Harbor, Massachusetts

We investigated the relationship between infaunal population density and monomethyl mercury (MMHg) accumulation in Boston Harbor sediments. Two hypotheses were addressed: 1) the depth of maximum MMHg concentration is positively correlated with infaunal burrow density, and 2) maximum MMHg inventories occur at intermediate burrow densities. These hypotheses were based on a model for MMHg production in which Hg methylation occurs primarily in a narrow zone characterized by relatively low concentrations of pore water sulfide. The presence of burrows indirectly influences MMHg production and accumulation by controlling the geometry of this Hg methylation zone. The observed patterns of burrow density, bulk sediment MMHg and pore water oxygen and sulfide concentration profiles were consistent with both hypotheses. The observed decrease in MMHg inventory at very high burrow density could also be explained by enhanced efflux of MMHg from the most densely populated site.