Influence of organic matter inputs and grain size on soft-bottom macrobenthic biodiversity in the upwelling ecosystem of central Chile

Soft-bottom macrobenthic communities on the continental shelf of central Chile were characterised, in number, by polychaetes, followed by amphipods, bivalves and cumaceans, while biomass was dominated by Amphioplus magellanicus (Ophiuroidea). The standing stocks followed a parabolic pattern relative to depth (highest at mid-shelf) but the species were divided into inshore (22 to 50 m) and offshore (80 to 140 m) assemblages, based on the similarity in species composition. Diversity (Es) was highest in the shallow assemblage and lowest at the deeper locations, suggesting that the decline in oxygen offshore was impacting the numbers of species. Dominance followed the opposite pattern, with greatest dominance at the deepest two stations. The distribution of sizes at each site, measured as normalised biomass size spectra (NBSS), was not different among the sites, although total biomass was concentrated in the single large ophiuroid at the deepest location. Significant correlations (p < 0.05) indicated that diversity and number of species were mainly related to the organic content (i.e. total organic carbon) and sediment composition (grain size). Therefore, food supply from primary production and the physical properties of the seafloor are thought to be the main forces structuring the macrobenthos biodiversity and distribution.