Influence of submarine fumaroles on the seasonal changes in mercury species in the waters of Kagoshima Bay, Japan

T-Hg, DGM, RM, and MMHg in the seawater columns (n = 132) were in the range of 0.05-3.04 ng L− 1, 0.003-1.08 ng L− 1, 0.006-1.47 ng L− 1, 0.02-0.69 ng L− 1 for an entire sample. Although the average T-Hg values at the surface and middle layers (0-100 m) for each sample showed no significant seasonal variation throughout the year, those at the deeper layer (150-200 m) were high from summer to autumn and low in winter. The seasonal changes and vertical distributions of DGM, RM, and MMHg concentrations were similar to those of T-Hg. A thermocline was observed from spring to autumn and ceased in winter. Because the thermocline prohibits vertical circulation of seawater, the fumarole impact may increase during those months.