Megafaunal responses to strong oxygen gradients on the Pakistan margin of the Arabian Sea

The Arabian Sea oxygen minimum zone (OMZ), which intersects the continental margin between approximately 100 and 1200 m, is one of the world's largest deep-water oxygen-deficient water masses. We analysed megafaunal organisms seen in images obtained using a wide-angle survey photographic (WASP) system at nine sites (140–1850 m water depth) across the OMZ on the Pakistan Margin during the late-monsoon period (August–September 2003). The visible megafauna comprised: (1) the megabenthos sensu strictu (s.s.), (2) large polychaetes and (3) the benthopelagic megafauna (fish, natant decapods and octopods). Large protozoans, mainly the foraminiferan Pelosina sp., were counted but not included in the megafauna. The megabenthos s.s. were rare at the seasonally hypoxic 140-m site (O2=0.11 ml l−1), entirely absent in the OMZ core and most of the lower transition zone (300–900 m; O2=0.12–0.15 ml l−1), but peaked in abundance (27.94 indiv. m−2) at 1000 m (O2=0.16 ml l−1). Densities were much lower at 1100 and 1200 m (0.52–0.69 indiv. m−2; O2=0.25–0.38 ml l−1), and declined to minimal values (0.01 indiv. m−2) at 1850 m (O2=1.68 ml l−1). There was no correlation with depth, dissolved-oxygen concentration or sediment organic chemistry variables (%Corg, %Total N, C:N, δ13C, δ15N). Pelosina sp. was the only strictly benthic organism visible at 400 and 700 m. Fish and natant decapods were fairly common at 300 m, and fish were the only metazoans seen in photographs from 700 m. Large polychaetes, almost certainly Linopherus sp., were very abundant in photographs from 900 m, where megabenthos s.s. were absent, and somewhat less abundant at 1000 m. Suspension-feeding cnidarians and tunicates were abundant at 1100 and 1200 m, respectively. The number of megabenthos s.s. species visible at each site ranged from six (1000 and 1850 m) to 11 (1100 and 1200 m). Diversity (H′(loge)) was the lowest at 1000 and 1850 m and the highest at 1100 m, with intermediate values at 140 and 1200 m. Dominance was the highest (>99%) at 1000 m, high (87%) at 1200 m and lower (32–50%) at 140, 1100 and 1850 m. Benthopelagic megafauna appeared more tolerant of dysoxia than the megabenthos s.s., although densities peaked at 1100 m, slightly deeper than for the megabenthos s.s.. The presence of uneaten carrion (dead fish and natant decapods) between 300 and 900 m suggests that scavengers were either not active or not present within the OMZ. The megabenthos s.s. appeared to respond to oxygen concentrations above a threshold value (∼0.15–0.16 ml l−1). The ophiuroid-dominated abundance peak at 1000 m, and the abrupt changes in megabenthic assemblage composition between 1000 and 1200 m, were probably expressions of an ‘edge effect’, known from other OMZs and believed to reflect a threshold release from physiological oxygen limitation accompanied by an abundant food supply.