Climate-forced variability of ocean hypoxia

Oxygen (O₂) is a critical constraint on marine ecosystems. As oceanic O₂ falls to hypoxic concentrations, habitability for aerobic organisms decreases rapidly. We show that the spatial extent of hypoxia is highly sensitive to small changes in the ocean's O₂ content, with maximum responses at suboxic concentrations where anaerobic metabolisms predominate. In model-based reconstructions of historical oxygen changes, the world's largest suboxic zone, in the Pacific Ocean, varies in size by a factor of 2. This is attributable to climate-driven changes in the depth of the tropical and subtropical thermocline that have multiplicative effects on respiration rates in low-O₂ water. The same mechanism yields even larger fluctuations in the rate of nitrogen removal by denitrification, creating a link between decadal climate oscillations and the nutrient limitation of marine photosynthesis.