Tracing monomethylmercury origins in planktivorous migratory fish in the coastal East China Sea: Evidences from mercury and carbon stable isotopes.

Under strong impacts of land-sea interactions, estuarine and coastal ecosystems exhibit large spatial variations in geochemical properties and monomethylmercury (MMHg) distribution, which may induce habitat-dependent MMHg bioaccumulation to migratory fish that remains unclear. This study investigates MMHg origins in two planktivorous migratory fish with distinct habitats (anadromous Coilia nasus and oceanodromous Scomber japonicus) in the East China Sea (ECS), based on the seasonal and inter-tissue variations of stable isotopes of MMHg (especially Δ199HgMMHg) and carbon (δ13C) in fish and site-specific isotopes conserved in zooplankton. The Δ199HgTHg (-0.12-0.39 ‰) and δ13C (-28.6 to -18.8 ‰) in zooplankton generally increased seawards, providing regional fingerprints for source tracking. For both fish species, the significantly higher Δ199HgMMHg and δ13C values in muscle (reflecting long-term exposure) than liver (reflecting short-term exposure) indicate seaward habitats as the long-term dietary sources. Compared with the isotopic signatures in zooplankton in the ECS inner shelf, the comparable δ13C and Δ199HgMMHg in C. nasus indicate the ECS inner shelf as its major MMHg source, while the significantly higher Δ199HgMMHg in S. japonicus indicates the ECS outer shelf as the major MMHg source. This study demonstrates Δ199HgMMHg can provide robust information for MMHg origin in migratory fish in coastal ecosystems.