ECOLOGICAL CONSEQUENCES OF SHALLOW-WATER HYDROTHERMAL VENTING ALONG THE EL REQUESÓN FAULT ZONE BAHÍA CONCEPCIÓN, B.C.S, MÉXICO

Bahía Concepción is a prime example of the extensional basins and accommodation zones that formed along the Baja California peninsula associated with the opening of the proto-Gulf. Its narrow (5-10 Km), elongate (40 Km) shape results from a half-graben controlled by northwest-southeast trending faults. The most prominent of these faults make up the Concepción fault zone on the peninsula forming the eastern margin of the bay. The steep escarpment along much of the western shore of Bahía Concepción suggests that this side of the bay is also bound by faults, referred to here as the El Requesón fault zone. This fault zone is currently acting as a conduit for geothermal fluids and gases. Intertidal hot springs have been documented in a mangrove stand near Playa Santispac, and in several other locations. We have extensively studied one area of active hydrothermal venting along an NW-SE trending onshore-offshore fault near Punta Santa Barbara, where geothermal fluids and gas bubbles are being released in the intertidal and shallow subtidal (to 13m water depths) through rocks and soft sediment in a roughly linear trend extending over 750m of coastline. This hydrothermal venting is affecting the ecology of the area. Infaunal densities and diversities are significantly lower in soft sediments affected by venting, likely due to intolerance of temperatures that reach as high as 92ºC. Conversely, abundances and diversity of organisms appear to be enhanced in rocky habitats, particularly in fish and epifaunal filter-feeding invertebrate assemblages that may benefit from enhanced water motion and higher temperatures. Precipitates of iron oxides heavily colonized by a community of morphologically diverse thermophilic bacteria, cyanobacteria, and diatoms form thin mats on rocks and sediment affected by venting, and some animals appear to be consuming these mats. Stable carbon and nitrogen isotope analyses indicate that these mats are providing a substantial amount of nutrition to Holothuria inhabilis, a deposit feeding sea cucumber. These shallow-water vents along the El Requesón fault zone may be providing additional nutritional input to background fauna typically dependent solely on photosynthetically derived carbon sources.