Macrobenthic zonation caused by the oxygen minimum zone on the shelf and slope off central Chile

The relationship between macrobenthic (⩾300 μm) zonation and the oxygen minimum zone (OMZ: O2<0.5 ml L−1) was studied in shelf and slope sediments (122–840 m depth) off Concepción Bay, central Chile. Four study sites were sampled during March–April 1999 for abiotic factors, macrofaunal density, biomass, mean individual size, and diversity. Within the OMZ (122–206 m), the macrofaunal density was high (16,478–21,381 individuals m−2) and 69–89% of the organisms were soft-bodied. Density was highest (21,381 individuals m−2), biomass lowest (16.95 g wet weight m−2), and individual size smallest (0.07 mg C individuals–1) at the shelf break site (206 m). Polychaete worms made up 71% of the total abundance, crustaceans 16%, and mollusks only 2%. Total abundance beneath the OMZ (mid-slope site, ∼840 m) was 49% crustaceans and 43% polychaetes. Although existing literature originally led to the hypothesis that both diversity and biomass within the OMZ would be lower than beneath the OMZ, in the present study this was only true for diversity. Biomass distribution, on the other hand, was concave along the depth gradient; the highest values were near the upper edge of (122 m) and beneath (840 m) the OMZ. Indices of the macrofaunal community structure varied in relation to bottom-water oxygen concentration, chlorophyll-a, phaeopigments, and sulfide concentration, but not in relation to grain size, C, N, mud, porosity, redox potential, a bottom-water temperature.