Infaunal macrobenthos of the oxygen minimum zone on the Indian western continental shelf

The Arabian Sea is characterized by a mid-depth layer of reduced dissolved oxygen (DO) concentration or oxygen minimum zone (OMZ -DO concentration <0.5 ml·l^-1) at ~150-1000 m depth. This OMZ results from the flux of labile organic matter coupled with limited intermediate depth water ventilation. Generally, benthic animals in the OMZ have morphological and physiological adaptations that maximize oxygen uptake in the limited oxygen availability. Characteristics of OMZ benthos have been described from only a few localities in the Arabian Sea. We measured the bottom water DO and studied the characteristics of infaunal macrobenthos of the Indian western continental shelf by collecting samples at 50, 100 and 200 m in depth from 7° to 22° N. The DO values observed at 200 m (0.0005-0.24 ml·l^-1) indicated that this area is lying within an OMZ. Five major taxa, namely Platyhelminthes, Sipunculoidea, Echiuroidea, Echinodermata and Cephalochordata were absent from the samples collected from this OMZ. In general, declines in total macrobenthic density and biomass and polychaete species richness and diversity were observed in this OMZ compared with the shallower depths above it. Community analyses of polychaetes revealed the dominance of species belonging to families Spionidae, Cirratulidae and Paraonidae in this OMZ. Low oxygen condition was more pronounced in the northern continental shelf edge (≤0.03 ml·l^-1), where the majority of spionids including Prionospio pinnata and cirratulids were absent; whereas amphipod, isopod and bivalve communities were not impacted.