Why do dissolved oxygen levels vary at different depths in the ocean?

Dissolved oxygen levels in the ocean vary significantly with depth due to a combination of biological activity and physical mixing processes.1

In near-surface waters, oxygen concentrations are primarily established through exchange with the atmosphere. Additionally, photosynthetic organisms in these shallow, sunlit depths produce oxygen, often keeping these waters well-oxygenated.1

At intermediate depths, oxygen concentrations typically reach a minimum. This occurs because oxygen is consumed by the biological decay of organic matter—a process often referred to as the remineralization of biological debris—without the replenishment provided by atmospheric exchange or photosynthesis.1

In contrast, deep ocean waters often exhibit relatively high oxygen levels. This is largely due to the large-scale physical circulation of the ocean, specifically the sinking of cold, dense, oxygen-rich surface waters from high latitudes. These deepwater currents transport oxygen from the surface to the ocean floor, ensuring that deep waters remain oxygenated despite the lack of local production. Consequently, dissolved oxygen concentrations in the deep ocean often appear as a mirror image of dissolved nutrient concentrations, which increase at depth as organic matter is broken down.123