System efficiency of methylmercury production, an important indicator for evaluating the long-term risk of mercury: From a case study in the Jiaozhou Bay to global coastal seas

Abstract

With the implementation of the Minamata Convention, total Hg (THg) in coastal seas are expected to be reduced. However, methylmercury (MeHg) levels in aquatic environments depend not only on THg, but also the system efficiency of MeHg production (represented by MeHg/THg ratio in seawater) whose variations with time remain unclear. By choosing the Jiaozhou Bay (JZB) as a typical coastal system, combined with the published data from the global coastal systems, system efficiency of MeHg production in coastal seas were investigated. The mass budget of MeHg showed that the in situ production and degradation are the major source and sink of MeHg in the JZB. The relationships of MeHg in the seawater and fish with MeHg/THg in seawater indicate that system efficiency of MeHg production may control the risk of MeHg in coastal systems. The sulfate and nitrate in seawater, organic matter in sediment, biotic methylation, and transport of inorganic Hg from the seawater to sediment were identified to be critical parameters and processes for the MeHg/THg in global coastal seas. The findings of this study highlight the importance of monitoring the system efficiency of MeHg production and its controlling processes and parameters for evaluating the long-term risk of Hg.