Oxygen Minimum Zone Benthos: Adaptation and Community Response to Hypoxia

Mid-water oxygen minima ( 0.5ml l 1 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 l ) 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 l , bioturbation is reduced, the mixed layer is shallow, and chemosynthesis-based nutrition (via heterotrophy and symbiosis) becomes important. OMZs represent a major oceanographic boundary for many species. As they expand and contract over geological time, OMZs may influence genetic diversity and play a key role in the evolution of species at bathyal depths. These ecosystems may preview the types of adaptations, species, and processes that will prevail with increasing hypoxia over ecological and evolutionary time. However, many questions remain unanswered concerning controls on faunal standing stocks in OMZs, and the physiological, enzymatic, metabolic, reproductive and molecular adaptations that permit benthic animals to live in OMZs. As global warming and eutrophication reduce oxygenation of the world ocean, there is a pressing need to understand the functional consequences of oxygen depletion in marine ecosystems.