Ecomare

Oxygen in seawater has two sources: from the atmosphere and plants, which produce hydrocarbons and oxygen (originating from CO₂) from water and carbon dioxide when there is sufficient sunlight. Therefore, there is enough oxygen found on or near the sea surface. It is too dark for green plants to grow at greater depths, so oxygen is barely produced there. Too much oxygen is often produced on the sea surface. The least amount of oxygen is found in water 200 to 500 meters deep.