Macrobenthic community structure within and beneath the oxygen minimum zone, NW Arabian Sea

Abstract

Investigations of macrobenthos were carried out within and beneath the oxygen minimum zone (OMZ, <0.5 ml l −1) during Fall 1994 on the Oman margin, NW Arabian Sea. Six stations (400, 700, 850, 1000, 1250 and 3400 m) were characterized with respect to macrofaunal abundance, biomass, body size, taxonomic composition, diversity and lifestyles, and the relation of these parameters to environmental conditions. The OMZ (400–1000 m) was dominated by a dense (5818–19,183 ind m −2), soft-bodied assemblage consisting largely (86–99%) of surface-feeding polychaetes. Spionids and cirratulids dominated at the 400- and 700-m stations, paraonids and ampharetids at the 850- and 1000-m stations. Molluscs and most crustaceans were common only below the OMZ (⩾1250 m); a species of the amphipod Ampelisca was abundant within the OMZ, however. Both density and biomass were elevated within the OMZ relative to stations below but body size did not differ significantly among stations. The lower OMZ boundary (0.5 ml l −1) was not a zone of enhanced macrofaunal standing stock, as originally hypothesized. However, abundance maxima at 700–850 m may reflect an oxygen threshold (0.15–0.20 ml l −1) above which macrofauna take advantage of organically enriched sediments. Incidence of burrowing and subsurface-deposit feeding increased below the OMZ. Species richness ( E[ S 100]), diversity ( H′) and evenness ( J′) were lower and dominance (R1D) was higher within than beneath the OMZ. Within-station (between-boxcore) faunal heterogeneity increased markedly below the OMZ. Surface sediment pigment concentrations and oxygen together explained 96–99% of the variance in measures of E[S 100], H′ and J′ across the transect; grain size and % TOC did not yield significant regressions. Pigments, assumed to reflect food availability and possibly oxygen effects on organic matter preservation, were negatively correlated with species richness and evenness, and positively correlated with dominance. The reverse was true for water depth. Macrobenthic patterns of calcification and lifestyle within the Oman margin OMZ (0.13–0.3 ml l −1) match the dysaerobic biofacies of paleo-environmental reconstruction models.