Nearshore benthic communities and bioengineers from the macrotidal San Jorge Gulf: Patagonia, Argentina

Abstract

San Jorge Gulf, located in northern Patagonia, is characterized by extensive soft bottoms and large tidal ranges. A marine bot-tom section of 3 x 2 km, located off Caleta Olivia (Santa Cruz Province, Argentina) was sampled intensively during August 2008, covering a total of 35 oceanographic stations, with depths ranging from 25 to 48 m. Despite the small size of the surveyed area, and the short distance between stations, the study showed important bottom diversity, with patchiness distribution and associated with different biological assemblages. Through the geological and biological samples analysis, and subsequent statistical treat-ment, four main types of substratum were defined in the area, from those with very fine dominant sedimentary fraction to others with gravel dominance and also rocky platforms. The study area, of high hydrological energy, impacts on sediment dynamics, thus clogging phenomena are observed on rocky bottoms benthic communities. This clogging effect is mitigated in part by the bioengineering action of the polychaete Eunice fraunfeldi, whose long leathery tubes allow the settlement of various epibenthic species in areas dominated by soft bottoms, and thus reflecting the importance of bioengineering species creating special sub-strate for epibenthic communities.