Graduate Program in Water Resources Science, Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota 55108, USA.
Environ Sci Technol. 2011 Jul 15;45(14):5981-7. doi: 10.1021/es200332f. Epub 2011 Jun 22.
Stream ecosystems are widely contaminated by mercury (Hg) via atmospheric transport and deposition in watersheds. Dissolved organic carbon (DOC) is well-known to be the dominant ligand for aqueous methylmercury (MeHg), the bioaccumulative form of Hg in aquatic food webs. However, it is less clear if and how the concentration and character (e.g., aromaticity) of DOC influences the availability of dissolved MeHg to stream food webs. In this work, we analyzed total-Hg and/or MeHg concentrations in water, seston, and macroinvertebrates (filter-feeding hydropsychid caddisflies), and other physiochemical properties in 30 streams along a south-north geographic gradient in eastern Minnesota that corresponds to substantial changes in dominant land cover (i.e., agriculture, urban, wetland, and forest). In general, MeHg concentrations in seston and hydropsychids were higher in watersheds with more forest and wetland coverage, and increased with dissolved MeHg concentration. However, we found that the efficiency of MeHg incorporation into the stream food webs (i.e., bioconcentration factors of MeHg in both seston and hydropsychids, BCF(MeHg) = solid MeHg ÷ dissolved MeHg) decreased significantly with DOC concentration and aromaticity, suggesting that MeHg bioavailability to the base of food webs was attenuated at higher levels of terrestrial DOC. Therefore, our findings suggest that there is a dual role of DOC on MeHg cycling in streams: terrestrial DOC acts as the primary carrier ligand of dissolved MeHg for transport into surface waters, yet this aromatic DOC also attenuates dissolved MeHg uptake by aquatic food webs. Thus, consideration of MeHg bioavailability and its environmental regulation could help improve predictive models of MeHg bioaccumulation in stream ecosystems.
河流生态系统通过大气传输和在流域中的沉积而广泛受到汞（Hg）的污染。众所周知，溶解有机碳（DOC）是水中甲基汞（MeHg）的主要配体，MeHg 是水生食物网中 Hg 的生物累积形式。然而，DOC 的浓度和性质（例如，芳香度）是否以及如何影响溶解的 MeHg 对河流食物网的可利用性尚不清楚。在这项工作中，我们分析了明尼苏达州东部沿南北地理梯度的 30 条溪流中的总 Hg 和/或 MeHg 浓度以及水、悬浮物和大型无脊椎动物（滤食性石蛾）中的其他理化性质，这些溪流的主要土地覆盖（即农业、城市、湿地和森林）发生了很大变化。一般来说，悬浮物和石蛾中的 MeHg 浓度在森林和湿地覆盖度较高的流域中较高，并且随溶解 MeHg 浓度的增加而增加。然而，我们发现 MeHg 纳入河流食物网的效率（即悬浮物和石蛾中的 MeHg 生物浓缩系数，BCF(MeHg)=固体 MeHg÷溶解 MeHg）随着 DOC 浓度和芳香度的增加而显著降低，这表明陆地 DOC 水平较高时，MeHg 对食物网基础的生物可利用性减弱。因此，我们的研究结果表明，DOC 在溪流中 MeHg 循环中具有双重作用：陆地 DOC 作为溶解 MeHg 进入地表水的主要载体配体，但这种芳香性 DOC 也会减弱水生食物网对溶解 MeHg 的吸收。因此，考虑 MeHg 的生物利用度及其环境调节作用可以帮助改进预测模型。
