PATTERNS IN MACROFAUNAL ABUNDANCE AND DIVERSITY AT A SHALLOW-WATER HYDROTHERMAL VENT NEAR PUNTA SANTA BARBARA, BAHÍA CONCEPCIÓN, B.C.S., MEXICO

Geothermal fluids are being released through rock and soft sediments near Punta Santa Barbara, Bahía Concepción. Macrofauna and geochemical parameters were sampled in the soft sediment at 13m water depth using push-cores. In areas affected by venting, macrofaunal abundance and diversity was significantly reduced, compared to regions where no venting was evident. This pattern could be most likely attributed to an intolerance of pore water temperatures as high as 92°C. However, precipitates of iron oxides appeared to be colonized by morphologically diverse thermophilic bacteria that cover the sediment in regions of venting, which may support nutrition for some epifaunal animals. Crustaceans make up the largest infaunal biomass inside venting regions, whereas molluscs and polychaetes make up the largest biomass outside venting. Analyses of pore water fluid showed its chemistry to be complex; containing noxious elements such as arsenic. The abundance and diversity of macrofauna was related to the geochemical parameters, and were similar to ecological patterns demonstrated for other shallow water hydrothermal vents.