Photosynthesis introduces into the ocean.

The process of photosynthesis helps to produce oxygen in ocean by the help of blue green algae, phytoplankton. Like other plants, they also do photosynthesis , that uses sunlight and carbon dioxide to make food. ... This oxygen gets mixed into the ocean and helps the aquatic animals to survive.

Hope this helps.

btw this isn't my answer, it's a previous answer from another question but unfortunately i can't post the link. :/