The subtidal macrobenthic assemblages of Bahía San Sebastián (Tierra del Fuego, Argentina)

Abstract

San Sebastián Bay (SSB, 53º 00’ S - 53º 19’ S, 68º 10’ W - 68º 40’ W), a shallow and protected environment on the eastern coast of Tierra del Fuego (Argentina), is a prime habitat for several species of nearctic migratory birds that visit the area during summer to feed in its huge intertidal mudflats. The area also has great economic value, due to the extraction and transport of hydrocarbons. Twenty-three dredge stations were analysed, 18 of which were located within SSB, and the other 5 on the shallow continental shelf off Tierra del Fuego. Presence/absence data were used to classify stations and species by cluster analysis. Two algae and 113 taxa of macrobenthic invertebrates belonging to 12 animal phyla were identified. Species richness was one order of magnitude higher (Mean: 29.0 sp.st-1) in stations located outside or in the southern unprotected section of SSB than in most muddy or sandy stations of the bay (Mean: 3.7 sp.st-1). Hard-bottom stations on the shallow continental shelf outside SSB were dominated by a highly diversified assemblage of sessile filter-feeders, mainly bryozoans, hydrozoans, ascidians, sponges, and the bivalve Aulacomya atra. Soft-sediment stations within SSB were mainly populated by relatively few species of deposit-feeders, such as the orbiniid polychaete Phylo felix, which was particularly frequent and widespread. Living specimens of the bivalve Mulinia edulis were found just in one station, but shells of this clam are common, representing one of the few hard substrata available for epifaunal species on muddy and sandy bottoms. Densely ramified colonies of the hydrozoan Amphisbetia operculata serve as substrate and refuge for many bryozoans, amphipods, picnogonids and juveniles of the mussel, Mytilus chilensis.