Torrential rain and extreme flooding caused by Atlantic hurricanes mobilize a large pool of organic matter (OM) from coastal forested watersheds in the southeastern United States. However, the mobilization of toxic metals such as mercury (Hg) that are associated with this vast pool of OM are rarely measured. This study aims to assess the variations of total Hg (THg) and methylmercury (MeHg) levels and the isotopic compositions of Hg in a blackwater river (Waccamaw River, SC, U.S.A.) during two recent extreme flooding events induced by Hurricane Joaquin (October 2015) and Hurricane Matthew (October 2016). We show that extreme flooding considerably increased filtered THg and MeHg concentrations associated with aromatic dissolved organic matter. During a 2‐month sampling window each year (October–November), we estimate that about 27% (2015) and 78% (2016) of the average amount of Hg deposited atmospherically during these 2 months was exported via the river. The isotopic composition of Hg in the river waters was changed only slightly by the substantial inputs of runoff from surrounding landscapes, in which mass‐dependent fractionation (as δ²⁰²Hg) decreased from −1.47 to −1.67‰ and mass‐independent fractionation (as ∆¹⁹⁹Hg) decreased from −0.15 to −0.37‰. The slight variations in Hg isotopic composition during such extreme flooding events imply that sources of Hg in the river are nearly unchanged even under the very high wet deposition of Hg derived from the intensive rainfall. The majority of Hg exported by the river (74–85%) is estimated to have been derived from dry deposition to the watersheds. An increase in frequency and intensity of Atlantic hurricanes is expected in the next few decades due to further warming of ocean surface waters. We predict that increased hurricanes will mobilize more dry‐deposited Hg and in situ produced MeHg from these coastal watersheds where MeHg can be extensively bioaccumulated and biomagnified in the downstream aquatic food webs.

中文翻译：

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受极端洪水事件影响的黑水河中汞的浓度和同位素组成

大西洋飓风造成的暴雨和极端洪水从美国东南部沿海森林流域中调动了大量有机物（OM）。但是，很少测量与这种庞大的OM库相关的有毒金属如汞（Hg）的迁移。这项研究旨在评估黑水河（Waccamaw河，美国南卡罗来纳州）在飓风“华金”引起的最近两次极端洪灾事件中（2015年10月）的总汞（THg）和甲基汞（MeHg）水平的变化以及汞的同位素组成）和马修飓风（2016年10月）。我们表明，极端洪水大大增加了与芳香族溶解有机物有关的过滤后THg和MeHg浓度。每年（10月至11月）的2个月采样期内，我们估计，在这两个月中，大气中平均汞沉积量的27％（2015年）和78％（2016年）是通过河流出口的。河流水体中汞的同位素组成仅因周围景观的径流的大量输入而略有变化，在这种情况下，质量分数取决于质量分数（δ²⁰²汞柱）从-1.47降至-1.67‰和大规模独立分馏（如Δ ¹⁹⁹Hg）从-0.15降低到-0.37‰。在这种极端洪水事件中，汞同位素组成的细微变化表明，即使在强降雨引起的非常高的汞湿沉降下，河流中的汞来源也几乎没有变化。据估计，河水出口的大部分汞（74-85％）来自干沉积物到集水区。由于海洋表层水的进一步变暖，预计未来几十年大西洋飓风的频率和强度将增加。我们预测，飓风的增加将动员来自这些沿海集水区的更多干法沉积的汞和原位产生的甲基汞，这些甲基汞可在下游水生食物网中被大量生物富集和生物放大。