Atmospheric mercury is the dominant Hg source to fish in northern Minnesota and elsewhere. However, atmospherically derived Hg must be methylated prior to accumulating in fish. Sulfate-reducing bacteria are thought to be the primary methylators of Hg in the environment. Previous laboratory and field mesocosm studies have demonstrated an increase in methylmercury (MeHg) levels in sediment and peatland porewaters following additions of sulfate. In the current ecosystem-scale study, sulfate was added to half of an experimental wetland at the Marcell Experimental Forest located in northeastern Minnesota, increasing annual sulfate load by approximately four times relative to the control half of the wetland. Sulfate was added on four separate occasions during 2002 and delivered via a sprinkler system constructed on the southeast half (1.0 ha) of the S6 experimental wetland. MeHg levels were monitored in porewater and in outflow from the wetland. Prior to the first sulfate addition, MeHg concentrations (filtered, 0.7 microm) were not statistically different between the control (0.47 +/- 0.10 ng L(-1), n = 12; mean +/- one standard error) and experimental 0.52 +/- 0.05 ng L(-1), n = 18) halves. Following the first addition in May 2002, MeHg porewater concentrations increased to 1.63 +/- 0.27 ng L(-1) two weeks after the addition, a 3-fold increase. Subsequent additions in July and September 2002 did not raise porewater MeHg, but the applied sulfate was not observed in porewaters 24 h after addition. MeHg concentrations in outflow from the wetland also increased leading to an estimated 2.4x increase of MeHg flux from the wetland. Our results demonstrate enhanced methylation and increased MeHg concentrations within the wetland and in outflow from the wetland suggesting that decreasing sulfate deposition rates would lower MeHg export from wetlands.
A dynamic model that couples air−water exchange and
phytoplankton uptake of persistent organic pollutants has
been developed and then applied to PCB data from a
small experimental lake. A sensitivity analysis of the model,
taking into account the influence of physical environmental
conditions such as temperature, wind speed, and mixing
depth as well as plankton-related parameters such as biomass
and growth rate was carried out for a number of PCBs
with different physical−chemical properties. The results
indicate that air−water exchange dynamics are influenced
not only by physical parameters but also by phytoplankton
biomass and growth rate. New phytoplankton production
results in substantially longer times to reach equilibrium.
Phytoplankton uptake-induced depletion of the dissolved
phase concentration maintains air and water phases out
of equilibrium. Furthermore, PCBs in phytoplankton also take
longer times to reach equilibrium with the dissolved
water phase when the latter is supported by diffusive air−water exchange. However, both model analysis and
model application to the Experimental Lakes Area of
northwestern Ontario (Canada) suggest that the gas phase
supports the concentrations of persistent organic
pollutants, such as PCBs, in atmospherically driven
aquatic environments.
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