Investigating the Importance of the Coastal Zone as a Source of Methylmercury to the Ocean

汞(Hg)从自然和人为来源释放到环境中，本世纪的输入大幅增加，并使海洋中的汞沉积增加了三倍。然而，海洋鱼类中汞的浓度(主要是以甲基汞(CH3汞)形式)的浓度一致上升的证据是矛盾的。事实上，在迄今为止所研究的大多数生态系统中，汞输入和CH3汞产生之间的关系很弱，这表明许多因素影响汞甲基化的速度。切萨皮克生物实验室和本尼迪克特河口研究实验室的研究人员一直在研究淡水和河口系统中的这些相互作用和关系，在这个项目中，他们将根据这一提议将调查扩展到沿海海洋。虽然以前的海洋研究集中在空气-水交换以及海洋分布和生物地球化学方面，但沿海地区的研究相对较少，尽管它在全球汞循环中发挥着不可或缺的作用。研究人员推测，它是海洋中甲基化汞的潜在来源。海洋中甲烷汞的初步预算表明，就地净生产是最重要的来源。人们普遍认为，硫酸盐还原细菌是环境中主要的汞甲基化物质，因此，生产力高的沿海水域下的沉积物应该是汞甲基化的重要场所，其中硫酸盐还原是主要的矿化机制。研究人员推测，由于这些沉积物中铁和S之间的相互作用，高浓度的硫化物不会影响汞对甲基化细菌的生物有效性，而高浓度的硫化物已被证明抑制了汞的甲基化。因此，主要的工作假设是，海岸带和陆地-海洋边缘是汞甲基化的重要地点，是开放海洋的甲烷汞净来源，并占全球海洋甲烷汞收支的重要部分。此外，研究人员提出，这种甲烷汞的大部分来自海岸和陆架沉积物中的原位细菌甲基化。因此，主要目标是测量从切萨皮克湾上游到中大西洋海湾大陆架外缘的沉积物中总汞和CH3汞的浓度和形态，以及随深度变化的辅助参数(有机碳、铁、锰、S)。此外，研究小组将在空间和时间上确定这些沉积物中甲基化和去甲基化的相对速率，并调查这些测量与汞形态、沉积物生物地球化学和微生物活动之间的关系。将使用顺序提取方法检查汞在固相中的分布(形态)。还将研究汞在溶解和固相中的形态对汞甲基化的影响。

ABSTRACTOCE-0351050Mercury (Hg) is released to the environment from natural and anthropogenic sources and inputs have increased substantially this century, and have increased Hg deposition to the ocean, the dominant Hg input, by a factor of three. However, the evidence that the concentration of Hg, mostly as methylmercury (CH3Hg), in ocean fish has increased in concert is contradictory. Indeed, in most ecosystems studied to date, the relationship between Hg input and CH3Hg production is weak, suggesting that many factors influence the rate of Hg methylation. Researchers at the Chesapeake Biological Laboratory and the Benedict Estuarine Research Laboratory have been studying these interactions and relationships in freshwater and estuarine systems, and in this project they will extend investigations to the coastal ocean under this proposal.While prior ocean studies have focused on the air-water exchange, and on ocean distributions and biogeochemistry, the coastal zone has been relatively little studied even though it plays an integral role in the global Hg cycle. The researchers hypothesize that it is a potential source of methylated Hg to the ocean. A preliminary budget for CH3Hg in the ocean indicates that net in situ production is the most important source. It is generally accepted that sulfate- reducing bacteria are the primary Hg methylators in the environment and thus sediments under highly productive coastal waters, where sulfate-reduction is the main mineralization mechanism, should be important locations for Hg methylation. The researchers surmise that because of the interaction between Fe and S in these sediments, Hg bioavailability to methylating bacteria will not be effected by high sulfide concentration, which has been shown to inhibit Hg methylation. The primary working hypothesis is, therefore, that the coastal zone and the land-ocean margin are important sites of Hg methylation, are net sources of CH3Hg to the open ocean, and contribute an important fraction of the global ocean CH3Hg budget. Furthermore, the researchers propose that most of this CH3Hg is derived from in situ bacterial methylation within coastal and shelf sediments. The primary objectives are therefore to measure the concentration and speciation of total Hg and CH3Hg, and ancillary parameters (organic carbon, Fe, Mn, S) in the dissolved and solid phase, and with depth, in sediments collected from sites ranging from the upper Chesapeake Bay to the outer reaches of the shelf of the Mid-Atlantic Bight. In addition, researcher team will determine the relative rate of methylation and demethylation in these sediments, spatially and temporally, and investigate relationships between these measures and Hg speciation, sediment biogeochemistry and microbial activity. The distribution (speciation) of Hg in the solid phase will be examined using sequential extraction approaches. The effect of Hg speciation in the dissolved and solid phase on Hg methylation will also be examined.