I studied a shallow water (O-13m water depths) hydrothermal vent in Bahía Concepción, Baja California Sur, México. Investigations using side-scan sonar and SCUBA suggested that the submarine hydrothermal activity is mainly controlled by a NW-SE trending onshore-offshore fault. Compared to seawater, vent fluids are enriched in Ca, Mn, HC03, SiO2, B, As, Li, Fe, Rb, and Sr, and depleted in Na, Cl, Mg, and S04, with calculated end-member temperatures up to 220°C. The vent gas is primarily nitrogen, carbon dioxide, and methane, and the geochemistry of the gas suggests that it may be derived from the thermal alteration of algal matter. The submarine hydrothermal activity is negatively affecting abundances and diversity of infaunal assemblages. The carbon and nitrogen stable isotope ratios of some of the animals feeding around the vent, particularly the sea cucumber Holothuria inhabilis, were significantly different than those from the same species collected away from venting activity
