Trophic transfer characteristics of mercury affected by food source in coral reef fish from the South China Sea.

Mercury (Hg), particularly neurotoxic methylmercury (MeHg), persists globally and bioaccumulates in marine food webs. Hg biomagnification and ecological risks remain poorly characterized for the offshore oligotrophic Nansha Islands coral reef system with low background Hg. We measured total Hg (THg) and MeHg concentrations, plus carbon and nitrogen stable isotopes, in 144 coral reef fish from 41 species. Using proportional contributions of four basal food sources, namely organic matter from predominantly autotrophic corals (PACs), predominantly heterotrophic corals (PHCs), algae and plankton, we evaluated Hg trophic transfer in this system. Trophic magnification factors were 5.88 for THg and 6.76 for MeHg, much higher than those reported for nearshore ecosystems in China. Higher Hg biomagnification was associated with larger dietary fractions of coral-derived organic matter (OMC, including PACs and PHCs), indicating basal food-source heterogeneity modulates Hg trophic transfer. MeHg target hazard quotient (THQ) in some high-trophic-level fish exceeded safety thresholds, and 47.2% of samples presented THQ > 1, with particular risks for children. Even in minimally disturbed offshore waters with low background Hg levels, intense Hg biomagnification may lead to substantial health risks. Basal food-source composition is associated with Hg biomagnification, providing a reference for sustainable fishery governance and seafood consumption recommendations.