Jeffrey D. Jeremiason

Assessment of mercury bioaccumulation within the pelagic food web of lakes in the western Great Lakes region

While mercury is a health hazard to humans and wildlife, the biogeochemical processes responsible for its bioaccumulation in pelagic food webs are still being examined. Previous studies have indicated both “bottom-up” control of piscivorous fish Hg content through methylmercury.(MeHg) supply, as well as site-specific trophic factors. We evaluated ten studies from the western Great Lakes region to examine the similarity of MeHg trophic transfer efficiency within the pelagic food web, and assessed regional-scale spatial variability. Analyses of bioaccumulation and biomagnification factors between water, seston, zooplankton, and preyfish indicated that the largest increases in MeHg occurred at the base of the food web, and that the relative extent of trophic transfer was similar between sites. Positive correlations were observed between aqueous unfiltered MeHg, total Hg, and dissolved organic carbon, and measures of the efficiency of MeHg trophic transfer were consistent across widely disparate systems (both natural and experimentally manipulated) throughout North America. Such similarity suggests that the aqueous supply of MeHg is largely controlling bioaccumulation in pelagic food webs, while local, lake-specific variability can result from an array of trophic (biological) factors.

The effects of hydrologic fluctuation and sulfate regeneration on mercury cycling in an experimental peatland

A series of severe droughts during the course of a long-term, atmospheric sulfate-deposition experiment in a boreal peatland in northern Minnesota created a unique opportunity to study how methylmercury (MeHg) production responds to drying and rewetting events in peatlands under variable levels of sulfate loading. Peat oxidation during extended dry periods mobilized sulfate, MeHg, and total mercury (HgT) to peatland pore-waters during rewetting events. Pore-water sulfate concentrations were inversely related to antecedent moisture conditions and proportional to past and current levels of atmospheric sulfate deposition. Severe drying events caused oxidative release of MeHg to pore-waters and also resulted in increased net MeHg production likely because available sulfate stimulated the activity of sulfate-reducing bacteria, an important group of Hg-methylating bacteria in peatlands. Rewetting events led to increased MeHg concentrations across the peatland, but concentrations were highest in peat receiving elevated atmospheric sulfate deposition. Dissolved HgT concentrations also increased in peatland pore-waters following drought, but were not affected by sulfate loading and did not appear to be directly controlled by dissolved organic carbon mobilization to peatland pore-waters. Peatlands are often considered to be sinks for sulfate and HgT in the landscape and sources of MeHg. Hydrologic fluctuations not only serve to release previously sequestered sulfate and HgT from peatlands, but may also increase the strength of peatlands as sources of MeHg to downstream aquatic systems, particularly in regions that have experienced elevated levels of atmospheric sulfate deposition.A series of severe droughts during the course of a long-term, atmospheric sulfate-deposition experiment in a boreal peatland in northern Minnesota created a unique opportunity to study how methylmercury (MeHg) production responds to drying and rewetting events in peatlands under variable levels of sulfate loading. Peat oxidation during extended dry periods mobilized sulfate, MeHg, and total mercury (HgT) to peatland pore waters during rewetting events. Pore water sulfate concentrations were inversely related to antecedent moisture conditions and proportional to past and current levels of atmospheric sulfate deposition. Severe drying events caused oxidative release of MeHg to pore waters and resulted in increased net MeHg production likely because available sulfate stimulated the activity of sulfate-reducing bacteria, an important group of Hg-methylating bacteria in peatlands. Rewetting events led to increased MeHg concentrations across the peatland, but concentrations were highest in peat receiving elevated atmospheric sulfate deposition. Dissolved HgT concentrations also increased in peatland pore waters following drought but were not affected by sulfate loading and did not appear to be directly controlled by dissolved organic carbon mobilization to peatland pore waters. Peatlands are often considered to be sinks for sulfate and HgT in the landscape and sources of MeHg. Hydrologic fluctuations not only serve to release previously sequestered sulfate and HgT from peatlands but may also increase the strength of peatlands as sources of MeHg to downstream aquatic systems, particularly in regions that have experienced elevated levels of atmospheric sulfate deposition.

A Comparison of Mercury Cycling in Lakes Michigan and Superior

ABSTRACT Mercury cycling in Lake Superior and Lake Michigan was evaluated based on measurements of mercury levels, modeling of evasional fluxes, and development of first-order mass balance models. Total mercury, methylmercury, and dissolved gaseous mercury were measured on sampling cruises in Lake Michigan (2005) and Lake Superior (2006). Average total mercury concentrations in unfiltered surface water were higher in Lake Michigan (420 ± 40 pg/L) compared to Lake Superior (210 ± 20 pg/L). Methylmercury levels were below the detection limit in Lake Michigan. Larger sample volumes were collected to lower detection limits in Lake Superior in 2006 and methylmercury levels averaged 7 ± 6 pg/L. Dissolved gaseous mercury concentrations were also higher in Lake Michigan (27 ± 7 pg/L) compared to Lake Superior (14 ± 8 pg/L). Evasional fluxes were estimated using a two-film model for air-water exchange. The annual evasional flux in Lake Michigan was determined to be ∼380 kg/yr from Lake Michigan and ∼160 kg/yr from Lake Superior. Total mercury burdens in each lake were estimated to be ∼2500 kg in Superior and ∼2100 kg in Lake Michigan demonstrating that evasional fluxes play an important role in the mass balance of each lake, particularly Lake Michigan. A simple first-order mass balance model demonstrates the importance of air-water exchange and sedimentation as primary removal processes for Hg in each lake. Uncertainties in the mass balance model are highlighted due to lack of key data, particularly in Lake Superior.