Abstract：
The production of the highly toxic monomethylmercury (MeHg) is heterogenous throughout the water column. Multiple factors have been identified to significantly affect this process, such as an extended anoxic water layer and a deep-water phytoplankton maximum. However, the role of water column heterogeneity on mercury (Hg) cycling is still poorly known, especially concerning the role of zooplankton grazers. Here, four boreal lakes with contrasting characteristics were sampled (i.e., transparency and the presence/absence of fish) at both day and night in order to maximize the heterogeneity in zooplankton abundance both among and within lakes, and to investigate their potential links with Hg vertical heterogeneity. Diel variation of the concentrations of both dissolved total Hg (DTHg) and total Hg (THg) were observed, with night samples significantly higher than day samples. Although this pattern was not related to diel changes in the vertical distribution of zooplankton, results showed that the presence of large copepods (>1.2 mm) and medium-sized (0.6 to 1.2 mm) cladocerans was significantly associated with lower concentrations of DTHg in the water at a given depth, whereas the presence of medium-sized copepods was significantly associated with the concentration of THg. The presence of cladocerans was significantly associated with the ratio between the dissolved MeHg and DTHg (conventionally used as a proxy of methylation potential). Phytoplankton biomass was directly correlated with the concentration of both dissolved and total MeHg and the methylation potential. At the same time, phytoplankton biomass was inversely related to the fraction of DTHg. These results suggest a potential key role of the heterogeneity of biotic factors in the water column, especially of phytoplankton and zooplankton, in the cycling of total Hg and MeHg in boreal lakes.
摘要：
在整个水柱中，高毒性单甲基汞（MeHg）的产生是不均匀的。已经确定了多种因素来显着影响这一过程，例如延伸的缺氧水层和深水浮游植物最大值。然而,水柱异质性对汞（Hg）循环的作用仍然知之甚少，特别是关于浮游动物放牧者的作用。这里,对四个具有对比特征的北方湖泊进行了采样(即，透明度和鱼类的存在/不存在）在白天和黑夜，以最大程度地提高湖泊之间和湖泊内浮游动物丰度的异质性，并研究它们与汞垂直异质性的潜在联系。观察到溶解的总Hg（DTHg）和总Hg（THg）浓度的Diel变化，夜间样本明显高于白天样本。尽管这种模式与浮游动物垂直分布的diel变化无关，结果表明，大的co足类（>1.2毫米）和中等大小（0.6至1.2毫米）的锁骨的存在与在给定深度的水中较低的DTHg浓度显着相关，而中型co足类的存在与THg的浓度显着相关。Cladocerans的存在与溶解的MeHg和DTHg之间的比率显着相关（通常用作甲基化潜力的代表）。浮游植物的生物量与溶解和总MeHg的浓度以及甲基化潜力直接相关。同时,浮游植物生物量与DTHg的含量成反比。这些结果表明，水柱中生物因素异质性的潜在关键作用，尤其是浮游植物和浮游动物，在北方湖泊中总汞和甲基汞的循环中。
