How much oxygen does the ocean produce?

The answer

The ocean generates approximately 50 to 80% of Earth's oxygen, primarily through photosynthesis by phytoplankton, cyanobacteria, and algae. The single most prolific oxygen producer is Prochlorococcus, a cyanobacterium discovered only in 1986, which is responsible for roughly 20% of all oxygen in the biosphere. Ocean oxygen production varies seasonally and regionally, with blooms in spring and summer producing enormous pulses. However, rising ocean temperatures are reducing dissolved oxygen levels in a process called ocean deoxygenation, which has expanded low-oxygen dead zones by an estimated 4.5 million square kilometers since the 1960s.