Oxygen enters seawater:

Question

Quizplus · Accepted Answer

The main source of oxygen in seawater is through the process of photosynthesis by marine plants and algae, which release oxygen as a byproduct. This oxygen then diffuses into the surrounding seawater. Additionally, oxygen from the atmosphere can also dissolve into the surface layer of seawater through diffusion. Respiration of animals and decomposition of plant and animal remains also release some oxygen into seawater, but these sources are not as significant as photosynthesis and diffusion from the surface. The oxidation of metal ions in seawater does not release significant amounts of oxygen.

Oxygen Enters Seawater