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

Investigations of macrobenthos were carried out within and beneath the oxyg en 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 3400m) were cha racterized with respect to macrofaunal abundance, biomass, body size, taxon omic composition, diversity and lifestyles, and the relation of these param eters to environmental conditions. The OMZ (400-1000 m) was dominated by a dense (5818-19,183 ind m(-2)), soft-bodied assemblage consisting largely (8 6-99%) of surface-feeding polychaetes, Spionids and cirratulids dominated a t the 400- and 700-m stations, paraonids and ampharetids at the 850- and 10 00-m stations. Molluscs and most crustaceans were common only below the OMZ ( greater than or equal to 1250 m); a species of the amphipod Ampelisca wa s abundant within the OMZ, however. Both density and biomass were elevated within the OMZ relative to stations below but body size did not differ sign ificantly among stations. The lower OMZ boundary (0.5 ml l(-1)) was not a z one of enhanced macrofaunal standing stock, as originally hypothesized. How ever, abundance maxima at 700-850m 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. Wi thin-station (between-boxcore) faunal heterogeneity increased markedly belo w the OMZ. Surface sediment pigment concentrations and oxygen together expl ained 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. Pigme nts, assumed to reflect food availability and possibly oxygen effects on or ganic matter preservation, were negatively correlated with species richness and evenness, and positively correlated with dominance. The reverse was tr ue for water depth. Macrobenthic patterns of calcification and lifestyle wi thin the Oman margin OMZ (0.13-0.3 mi l(-1)) match the dysaerobic biofacies of paleo-environmental reconstruction models. (C) 1999 Elsevier Science Lt d. All rights reserved.