Dissolved Oxygen (DO)

Dissolved oxygen is the concentration of O2 in seawater, supplied by photosynthesis and air-sea exchange and consumed by respiration and remineralization. Solubility falls with rising temperature and salinity, so warm and salty water holds less. The interior pattern, high in newly ventilated water and low in old, sluggish water, makes DO a tracer of circulation and a hard limit on marine life, with mid-depth oxygen-minimum zones and coastal dead zones at the extremes.