Bioaccumulation of discharged mercury from submarine fumaroles due to copepod life history traits in northern Kagoshima Bay, Japan

Kagoshima Bay is a deep and semi-enclosed embayment with submarine fumarolic activities. High mercury contents have occasionally been reported for some fishes and this was hypothesized to be due to a bioaccumulation of discharged mercury from the submarine fumaroles. However, there is still no information about how the discharged mercury is accumulated into fishes through the planktonic food web. Here, we demonstrate a bioaccumulation of discharged mercury through copepod life history adapted to the embayment. Calanus sinicus was a minor contribution to the numerical abundance of zooplankton communities in both surface and deep layers during summer. In the deepest basin in northern Kagoshima Bay, copepodite stage 5 of this copepod species concentrated occurred at high abundances below the thermocline throughout the day in summer. Zooplankton communities residing in the deep waters exhibited much lower protein synthetase activity than those in the surface waters, indicating low physiological activity. Metabarcoding analysis of their gut contents demonstrated that copepods, dinoflagellates and diatoms composed more than half of gut content DNA. Mercury contents were higher for all taxonomic groups of zooplankton in the deep layers compared with those in the surface layers. C. sinicus residing in the deep waters exhibited high mercury contents equivalent to carnivorous groups. These findings suggest that bioaccumulation of discharged mercury from submarine fumaroles is accelerated by aspects of this copepod’s life history such as lipid accumulation for dormancy after actively feeding on copepod fecal pellets and phytoplankton and dormant stock accumulation restricted to the deepest basin in northern Kagoshima Bay.