Submarine fumaroles are located in the 200-m deep basin of the head area of Kagoshima Bay.To estimate the influence of fumarolic activity on distribution of mercury the seasonal changes of total mercury(T-Hg),Hg2+,Hg0 and methylmercury(MeHg)in water were studied.Samples had been collected at 5 locations in the bay head area and at 3 locations in the central area; the bay head area is connected to the central area only through the West-Sakurajima Passage,which is approximately 3 km wide and 40 m deep.Sea water samples were collected with a CTD-Rosette water sampler from May,2011 to December,2012; samples were taken at depth intervals of 50 m starting from the surface.For T-Hg measurement,the water samples were filtered into Teflon bottles and a solution of BrCl was added into the water.After UV irradiation for one night,NH2OH-HCl solution was added and mercury concentration in the final solution was measured by CVAFS using SnCl2 as a reducing agent.Hg2+ was measured by CVAFS in the same manner as T-Hg,without the BrCl oxidation step.Unfiltered water samples were bubbled with nitrogen gas and Hg0 swept from the water samples was measured by CVAFS.MeHg in water samples was extracted into a dithizone toluene solution and after several steps of purification,MeHg dithizonate was measured by GC-ECD.A thermocline was observed from spring to autumn,and ceased in winter.The water body near the fumaroles showed low pH values and the area showing the low pH values gradually became large with the formation of the seasonal thermocline.Since vertical circulation of sea water is prohibited by the thermocline,the water in the deeper layer may be strongly affected by hydrothermal fumarolic activity.In the bay head area,while the mercury concentrations in surface and middle layer(0 – 100m)showed no significant variation through the year,those in deeper layer(150 – 200 m)were higher in summer and lower in winter.In the central area,Hg concentrations and pH values were stable through a year.The seasonal distribution of mercury corresponded to pH.These phenomena strongly suggest that the source of Hg in Kagoshima Bay is the hydrothermal fumarolic activity.Although the effect of the geothermal activity may be within a limited area of bay head,MeHg corresponds to 23.7±17.1%of T-Hg.The impact of MeHg on biological systems should be investigated further.