title: Isotopic study of mercury sources and transfer between a freshwater lake and adjacent forest food web.
abstract: Studies of monomethylmercury (MMHg) sources and biogeochemical pathways have been extensive in aquatic ecosystems, but limited in forest ecosystems. Increasing evidence suggests that there is significant mercury (Hg) exchange between aquatic and forest ecosystems. We use Hg stable isotope ratios (δ(202)Hg and Δ(199)Hg) to investigate the relative importance of MMHg sources and assess Hg transfer pathways between Douglas Lake and adjacent forests located at the University of Michigan Biological Station, USA. We characterize Hg isotopic compositions of basal resources and use linear regression of % MMHg versus δ(202)Hg and Δ(199)Hg to estimate Hg isotope values for inorganic mercury (IHg) and MMHg in the aquatic and adjacent forest food webs. In the aquatic ecosystem, we found that lake sediment represents a mixture of IHg pools deposited via watershed runoff and precipitation. The δ(202)Hg and Δ(199)Hg values estimated for IHg are consistent with other studies that measured forest floor in temperate forests. The Δ(199)Hg value estimated for MMHg in the aquatic food web indicates that MMHg is subjected to ~20% photochemical degradation prior to bioaccumulation. In the forest ecosystem, we found a significant negative relationship between total Hg and δ(202)Hg and Δ(199)Hg of soil collected at multiple distances from the lakeshore and lake sediment. This suggests that IHg input from watershed runoff provides an important Hg transfer pathway between the forest and aquatic ecosystems. We measured Δ(199)Hg values for high trophic level insects and compared these insects at multiple distances perpendicular to the lake shoreline. The Δ(199)Hg values correspond to the % canopy cover suggesting that forest MMHg is subjected to varying extents of photochemical degradation and the extent may be controlled by sunlight. Our study demonstrates that the use of Hg isotopes adds important new insight into the relative importance of MMHg sources and complex Hg transfer pathways across ecosystem boundaries.
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave, Ann Arbor, MI 48109, United States. Electronic address: saeyunk@umich.edu.
authors.forename: Sae Yun
authors.initials: SY
authors.lastname: Kwon
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave, Ann Arbor, MI 48109, United States; Department of Ecology and Evolutionary Biology, University of Michigan, 830 N. University Ave, Ann Arbor, MI 48109, United States.
authors.forename: Joel D
authors.initials: JD
authors.lastname: Blum
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Michigan, 830 N. University Ave, Ann Arbor, MI 48109, United States.
authors.forename: Knute J
authors.initials: KJ
authors.lastname: Nadelhoffer
authors.affiliation: Department of Environmental Health Sciences, University of Michigan, 1415 Washington Heights, Ann Arbor, MI 48109, United States.
authors.forename: J
authors.initials: J
authors.lastname: Timothy Dvonch
authors.affiliation: Department of Biology, University of North Carolina at Greensboro, 321 McIver Street, Greensboro, NC 27402, United States.
authors.forename: Martin Tsz-Ki
authors.initials: MT
authors.lastname: Tsui
mesh_terms: D004784:Environmental Monitoring / Q000379:methods*; D020387:Food Chain; D060106:Lakes / Q000737:chemistry*; D008628:Mercury / Q000032:analysis*; D008629:Mercury Isotopes / Q000032:analysis; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D008629:Mercury Isotopes; D014874:Water Pollutants, Chemical; D008628:Mercury
keywords: Emergent insects; Food webs; Forests; Lakes; Mercury stable isotope; Methylmercury