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

Abstract

Submarine fumaroles are located in the 200-m deep basin of the northern part of Kagoshima Bay. To estimate the influence of mercury emitted from these fumaroles, the seasonal changes in total mercury (T-Hg), reactive mercury (RM), dissolved gaseous mercury (DGM), and monomethylmercury (MMHg) in water were studied.Seawater samples were collected from five separate locations in the northern part from May, 2011 to May, 2012 using a CTD-Rosette water sampler; the samples were taken every 50m, starting from the surface.T-Hg, DGM, RM, and MMHg in the seawater columns (n=132) were in the range of 0.05–3.04ngL−1, 0.003–1.08ngL−1, 0.006–1.47ngL−1, 0.02–0.69ngL−1 for an entire sample. Although the average T-Hg values at the surface and middle layers (0–100m) for each sample showed no significant seasonal variation throughout the year, those at the deeper layer (150–200m) 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.