Impact of mercury discharged from submarine volcano on inner bay ecosystems

Organic mercury, inorganic mercury and total mercury concentrations in phytoplankton (<0.1 mm) and zooplankton (>0.1 mm) collected in Kagoshima Bay, Japan were measured from 2017 to 2019 to estimate the impact of mercury discharged from submarine volcanoes on ecosystems; submarine volcanic activity continues at a depth of 200 m in the inner part of Kagoshima Bay. The total mercury concentrations in phyto- and zooplankton collected by vertical hauling at 0-200 m at just above the submarine volcano were in the range of 0.11-2.0 mg kg^-1 (avg. 0.67 mg kg^-1) and 0.090-0.56 mg kg^-1 (avg. 0.21 mg kg^-1), respectively. These values were one order of magnitude higher than the values in plankton collected in the central part of Kagoshima Bay. Organic mercury concentrations in phyto- and zooplankton were <0.010-0.071 mg kg^-1 (avg. 0.028 mg kg^-1) and 0.012-0.25 mg kg^-1 (avg. 0.10 mg kg^-1), respectively, for the inner part, and <0.010-0.040 mg kg^-1 (avg. 0.010 mg kg^-1) and <0.010-0.025 mg kg^-1 (avg. 0.012 mg kg^-1), respectively, for the central part. The values obtained in the inner part of the bay increased in summer and decreased in winter, which was consistent with changes in seawater mercury concentrations affected by volcanic activity. The organic mercury concentration in zooplankton collected just above the submarine volcano showed a size dependency, and a higher value was observed in the larger size, which suggested that the discharged mercury from the volcano was absorbed and concentrated through the ecosystem.