A shallow submarine hydrothermal system at 22 m depth in Tachibana Bay, Nagasaki, Japan was studied. Submarine hot springwaters and sediments of the vent sites were collected for analyses of
chemical composition. Temperature of the submarine hot springwater at the vents was measured at 70～80ºC, and clear decreases in pH and salinity were observed in relation to surrounding seawater.
Si(OH)4, NH4+ and Mn were enriched in the submarine hot springwater, but Fe was relatively low compared with the onshore hot springwater near the coast. Black-color sediments with high
concentrations of Fe, S, Sb and As indicate the preferential removal of Fe relative to Mn during transportation of the hot springwater under the seafloor. Supply of inorganic nutrients such as NH4+ from the vents may stimulate phytoplankton production near the shallow submarine hydrothermal system during the stratified summer.