THE IMPORTANCE OF PHYSICAL MIXING AND SEDIMENT CHEMISTRY IN MERCURY AND METHYLMERCURY BIOGEOCHEMICAL CYCLING AND BIOACCUMULATION WITHIN SHALLOW ESTUARIES

The objective of this study was to examine, using mesocosm experiments, the long-term effects of sediment resuspension on the fate, transport, and bioaccumulation of Hg and MeHg in shallow ecosystems. A bioenergetic-based model including sediment resuspension was developed to assess MeHg bioaccumulation into benthic and pelagic organisms under the experimental conditions. In addition, this study examined the spatial distribution of Hg and MeHg in the sediments from the Chesapeake Bay and used the model developed to examine the important factors in Hg and MeHg distribution and bioaccumulation in the Bay. Using STORM (high bottom Shear realistic water column Turbulence Resuspension Mesocosm) mesocosms, two 4-week experiments were conducted in July and October of 2001 (experiments 1 and 2) with Baltimore Harbor sediments. Tidal resuspension (4 h-on and 2 h-off cycles) was simulated with three replicates of the resuspension (R) and no-resuspension (NR) tanks. In experiment 1, there was no benthic macrofauna. In experiment 2, hard clams, Mercenaria mercenaria, were added to the sediment in the mesocosm tanks. Water, sediment, and biota (zooplankton and clams) samples were collected and analyzed for Hg and MeHg. Using Hg stable isotopes, Hg methylation and MeHg demethylation rates were determined. The STORM experiments showed that during sediment resuspension there was a significantly higher suspended particulate total Hg (THg) (on a mass basis), while particulate MeHg was significantly lower, as sediment particles with relatively poor MeHg were dominant in the water column. The results suggested that equilibrium partitioning between the dissolved and particulate phases for THg and MeHg was not occurring. It appeared that resuspension enhanced Hg methylation in the top sediment layer, especially in summer. Concentrations of THg and MeHg in biota showed that resuspension had a complex effect of system productivity and bioaccumulation. It appeared that organic matter content played an important role in the distribution of THg and MeHg in sediments and bioaccumulation into benthic and pelagic organisms. The modeling studies demonstrated that sediment resuspension played a role in transporting the enhanced MeHg to the water column and ultimately in increasing the MeHg burden into biota