Interactions between Mercury and Biogeochemical Features of Aquatic Ecosystems: A Critical Review

Background: Mercury (Hg) is a pollutant known to affect the nervous system. The most threatening form of Hg is methylmercury (MeHg), which can biomagnify in aquatic biota. This critique summarizes our current understanding of the major relationships between mercury speciation in aquatic environments and various environmental factors to determine the most suitable indicators of MeHg pollution. Methods: Searches were performed using Web of Science. Ultimately, 33 studies were chosen and reviewed. Priority was given to recent studies as this review focused on the current state of our knowledge. Summary: The total amount of Hg (THg) alone cannot be used as an indicator of MeHg since the relationship between THg and MeHg is poorly correlated. Microbial DNA is often used in research to give insight into the mercury cycle and fate in aquatic systems, but further studies are needed to accurately assess MeHg concentration using DNA. Based on an early study, it was thought that water colour could indicate the amount of Hg in aquatic biota. However, subsequent work has shown this to not be the case since dissolved organic matter (DOM) can help or hinder Hg bioavailability and bioaccumulation. There is a nonlinear relationship between dissolved organic carbon (DOC) and MeHg bioaccumulation where there is a threshold concentration (~8.5 mg C L-1 DOC) above which mercury bioaccumulation is hindered. A recent study found that the methylation rate of an aquatic system was correlated to the type of organic matter present in the sediments, although it could not predict the amount of MeHg present in the water since runoff brings Hg from the catchment to the downstream aquatic system. Recent advancements have been made to understand components of the mercury cycle (e.g. land-water interactions, microbial methylation, water-sediment interactions), but a model encompassing all components has yet to be constructed.