Macrofaunal community structure and zonation of an Ecuadorian sandy beach (bay of Valdivia)

dThe sandy beach macrofauna of the Bay of Valdivia (Ecuador) was sampled in August-September 1999 along six replicate transects between the high and low water line. The sediment consisted of well-sorted, fine to medium sand. Taking into account the dimensionless fall velocity (Omega) and the relative tidal range, the beach was characterized as an exposed, low tide terrace - rip beach. The distribution of the macrofauna was mainly determined by the elevation on the beach. Thirty-one taxa were found throughout the study, varying between 10 and 22 taxa per transect. Molluscs were the most dominant taxon (overall average : 285 ind/m(2), max. 2135 ind/m(2)), followed by crustaceans. The gastropod Olivella semistriata (overall average : 243 ind/m(2), max. 2131 ind/m(2)) was the most abundant species. The crustaceans were the most diverse taxon (10 spp.); Haustorius sp., Excirolana braziliensis and Emerita rathbunae were the most abundant species. Densities of polychaetes were low in comparison with the previous two taxa mentioned (overall average : 13 ind/m(2)). The total average density over the entire bay was 370 ind/m(2). Three zones of macrofaunal distribution along the tidal gradient could be distinguished : an upper beach, a middle beach and a lower beach assemblage. The upper beach assemblage consisted mainly of Excirolana braziliensis and ghost crabs. The middle beach assemblage had highest macrofaunal densities and was dominated by Olivella semistriata and Haustorius sp. The most diverse assemblage was found on the low beach, with representatives of different taxa, but with lower densities. These results are in conformity with other studies along the Pacific coast of South America and fit into the zonation scheme of DAHL (1952). Given that many other studies from South America were done at temperate latitudes, we conclude that, although differences in species composition were found, the general pattern of macrobenthos zonation on sandy beaches is similar in tropical regions.