Dissolved oxygen levels in the surface ocean can be altered due to several factors. Oxygen solubility is controlled by temperature (warming decreases solubility) and salinity (freshening increases solubility). Stratification controls mixing of oxygen rich surface water with the subsurface ocean. Changes in oxygen levels can also be associated with episodic evens such as algal blooms or typhoons.
Oxygen minimum zones (OMZs) exist in some ocean basins and result from the combined influences of circulation and microbial processes.