Sediment–water exchange of methylmercury determined from shipboard benthic flux chambers

Benthic mobilization is an important pathway by which organisms in aquatic systems may be exposed to monomethylmercury (MMHg) produced in sediments. We have developed a shipboard method for measuring MMHg effluxes from undisturbed sediment cores. This approach was used at multiple locations in a temperate coastal system, New York/New Jersey Harbor, during summer (August 2003) and winter conditions (February 2004). MMHg fluxes at the sediment–water interface were determined with shipboard benthic flux chambers (SBFCs), an intact core of sediment and overlying water that is stirred inductively and incubated under in situ conditions. For all locations and each period, sediments were a net source of MMHg; levels in water overlying sediments increased as a linear function of incubation time, and associated fluxes varied less than 10% relative standard error among replicate chambers with sediment from the same sampling location and period. Seasonality was evident with measured MMHg fluxes (pmol m− 2 d− 1) ranging from 39 to 55 among locations in February and 71–92 in August. Fluxes determined with the SBFCs were compared with diffusional effluxes estimated from gradients of MMHg in pore water of the same sediments (SBFC:diffusional flux ratio). SBFC:diffusional flux ratios ranged from 0.9 to 1.4 in August and 2.3–4.3 in February. SBFC:diffusional flux ratios, which correlate positively with dissolved oxygen (DO) in bottom waters, suggest that MMHg mobilization is largely diffusional when DO is less than about 80% saturation and enhanced at greater levels. Further, increased MMHg fluxes are likely due to increased bioirrigation. This is consistent with the observed positive relationship between macrofauna abundance and DO. Thus, benthic MMHg fluxes, which are enhanced relative to diffusion when DO in bottom waters is near saturation, may be linked to increased abundance and/or activity of infauna that irrigate sediment. Improving oxygen conditions, therefore, could increase sediment–water effluxes of MMHg in NY/NJ Harbor and comparable coastal marine systems.