Bivalves as a Mercury Bioindicator: A National Isotopic Survey along the Coast of South Korea

Mercury (Hg) is a contaminant that poses health risks for human populations relying on seafood consumption. To mitigate its impact, identifying and monitoring Hg sources have become priorities, notably under the Minamata Convention. Bivalves are commonly used as sentinels in contaminant biomonitoring but can accumulate Hg from diverse environmental media. To investigate their Hg sources, bivalves (blue mussel, Mytilus edulis, and Pacific oyster, Crassostrea gigas) and their associated sediment were sampled along the coast of South Korea and analyzed for Hg concentration and isotopic values. Oysters displayed low Δ199Hg (0.19 ± 0.19‰) and δ202Hg (−0.35 ± 0.55‰), highlighting a sedimentary source, whereas mussels exhibited higher values (0.72 ± 0.87 and 0.09 ± 0.72‰, respectively) indicating bioaccumulation from the water column. Sulfur, carbon, and nitrogen stable isotopes suggested that such a difference was not due to feeding niches. However, Hg isotopic trajectory analysis showed that environmental conditions controlling the desorption and remobilization of sediment Hg, notably via tidal flows, were likely to drive the observed source(s). While sediment Hg is not systematically reflected in biota, bivalves, which typically display Hg pools accumulated by benthic or pelagic food webs, appear valuable for Minamata Convention biomonitoring, though their fluctuating Hg sources require careful interpretation.