ABSTRACT
Abstract Mid-water oxygen minima (0.5ml l1 dissolved O2) intercept the continental margins along much of the eastern Pacific Ocean, off west Africa and in the Arabian Sea and Bay of Bengal, creating extensive stretches of sea floor exposed to permanent, severe oxygen depletion. These seafloor oxygen minimum zones (OMZs) typically occur at bathyal depths between 200m and 1000m, and are major sites of carbon burial along the continental margins. Despite extreme oxygen depletion, protozoan and metazoan assemblages thrive in these environments. Metazoan adaptations include small, thin bodies, enhanced respiratory surface area, blood pigments such as haemoglobin, biogenic structure formation for stability in soupy sediments, an increased number of pyruvate oxidoreductases, and the presence of sulphide-oxidising symbionts. The organic-rich sediments of these regions often support mats of large sulphideoxidising bacteria (Thioploca, Beggiatoa, Thiomargarita), and high-density, low-diversity metazoan assemblages. Densities of protistan and metazoan meiofauna are typically elevated in OMZs, probably due to high tolerance of hypoxia, an abundant food supply, and release from predation. Macrofauna and megafauna often exhibit dense aggregations at OMZ edges, but depressed densities and low diversity in the OMZ core, where oxygen concentration is lowest. Taxa most tolerant of severe oxygen depletion (0.2ml l1) in seafloor OMZs include calcareous foraminiferans, nematodes, and annelids. Agglutinated protozoans, harpacticoid copepods, and calcified invertebrates are typically less tolerant. High dominance and relatively low species richness are exhibited by foraminiferans, metazoan meiofauna, and macrofauna within OMZs. At dissolved oxygen concentrations below 0.15ml l1, bioturbation is reduced, the mixed layer is shallow, and chemosynthesis-based nutrition (via heterotrophy and symbiosis) becomes important.
