Carbon, Nitrogen, and Mercury Isotope Evidence for the Biogeochemical History of Mercury in Hawaiian Marine Bottomfish

The complex biogeochemical cycle of Hg makes identifying primary sources of fish tissue Hg problematic. To identify sources and provide insight into this cycle, we combined carbon (δ13C), nitrogen amino acid (δ15NPhe), and Hg isotope (Δ199Hg, Δ201Hg, δ202Hg) data for six species of Hawaiian marine bottomfish. Results from these isotopic systems identified individuals within species that likely fed from separate food webs. Terrestrial freshwater inputs to coastal sediments were identified as the primary source of tissue Hg in the jack species, Caranx ignobilis, which inhabit shallow marine ecosystems. Thus, coastal C. ignobilis were a biological vector transporting Hg from freshwater environments into marine ecosystems. Depth profiles of Hg isotopic compositions for bottomfish (excludung C. ignobilis) were similar, but not identical, to profiles for open-ocean pelagic fishes, suggesting that in both settings inorganic Hg, which was ultimately transformed to monomethylmercury (MeHg) and bioaccumulated, was dominantly from a single source. However, differences between pelagic fish and bottomfish profiles were attributable to mass-dependent fractionation in the benthos prior to incorporation into the food web. Results also confirmed that bottomfish relied, at least in part, on a benthic food web and identified the incorporation of deeper water oceanic MeHg sources into deeper water sediments prior to food web uptake and transfer.汞（Hg）复杂的生物地球化学循环使得识别鱼类组织汞的主要来源颇具挑战性。为了溯源汞来源并深入解析该循环，我们整合了夏威夷海域6种海水底栖鱼类的碳同位素（δ¹³C）、氨基酸态氮同位素（δ¹⁵N_苯丙氨酸，δ15NPhe）以及汞同位素（Δ¹⁹⁹Hg、Δ²⁰¹Hg、δ²⁰²Hg）数据。通过上述同位素体系的分析结果，我们甄别出同物种内可能依赖不同食物网觅食的个体。研究发现，输入近岸沉积物的陆地淡水物质，是栖息于浅海生态系统的鲹属鱼类珍鲹（Caranx ignobilis）体内组织汞的主要来源。由此可见，近岸栖息的珍鲹是将汞从淡水环境转移至海洋生态系统的生物载体。排除珍鲹之外的底栖鱼类，其体内汞同位素组成的深度分布剖面与远洋洄游鱼类的特征相似但不完全一致，表明在这两类生境中，最终被转化为一甲基汞（monomethylmercury, MeHg）并发生生物富集的无机汞，主要源自单一来源。然而，远洋鱼类与底栖鱼类的同位素剖面之间的差异，可归因于食物网摄入前底栖环境中发生的质量依赖性分馏效应。研究结果同时证实，底栖鱼类至少部分依赖底栖食物网，并明确了深水大洋的一甲基汞源在被食物网摄取和转移前，已融入深水沉积物中。