Department of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave, Ann Arbor, MI 48109, United States.
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.
Sci Total Environ. 2015 Nov 1;532:220-9. doi: 10.1016/j.scitotenv.2015.06.012. Epub 2015 Jun 11.
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.
对水生生态系统中单甲基汞 (MMHg) 来源和生物地球化学途径的研究已经非常广泛，但在森林生态系统中却有限。越来越多的证据表明，水生和森林生态系统之间存在着大量的汞 (Hg) 交换。我们使用 Hg 稳定同位素比值（δ(202)Hg 和 Δ(199)Hg）来研究 MMHg 来源的相对重要性，并评估美国密歇根大学生物站的道格拉斯湖及其相邻森林之间的 Hg 转移途径。我们描述了基础资源的 Hg 同位素组成，并使用 % MMHg 与 δ(202)Hg 和 Δ(199)Hg 的线性回归来估计水生和相邻森林食物网中无机汞 (IHg) 和 MMHg 的 Hg 同位素值。在水生生态系统中，我们发现湖底沉积物代表了通过流域径流水和降水沉积的 IHg 池的混合物。为 IHg 估计的 δ(202)Hg 和 Δ(199)Hg 值与其他在温带森林中测量林地表层的研究一致。水生食物网中 MMHg 的 Δ(199)Hg 值表明，在生物积累之前，MMHg 经历了约 20%的光化学降解。在森林生态系统中，我们发现从多个距离湖岸和湖底沉积物采集的土壤中的总 Hg 与 δ(202)Hg 和 Δ(199)Hg 之间存在显著的负相关关系。这表明来自流域径流水的 IHg 输入是森林和水生生态系统之间的重要 Hg 转移途径。我们测量了多个垂直于湖岸线的距离处的高营养级昆虫的 Δ(199)Hg 值，并对这些昆虫进行了比较。Δ(199)Hg 值与树冠覆盖率相对应，表明森林 MMHg 经历了不同程度的光化学降解，程度可能受阳光控制。我们的研究表明，使用 Hg 同位素可以为 MMHg 来源的相对重要性和跨生态系统边界的复杂 Hg 转移途径提供重要的新见解。
