The gas found in high levels near the surface of the ocean due to plant photosynthesis is oxygen (O2).

Here's why:

* Photosynthesis: Plants, including phytoplankton in the ocean, use sunlight, water, and carbon dioxide (CO2) to create their own food (sugars). A byproduct of this process is oxygen.

* Release of Oxygen: The oxygen produced by photosynthesis is released into the surrounding water and eventually the atmosphere.

* Surface Concentration: Since phytoplankton are abundant near the ocean's surface where sunlight is plentiful, the oxygen concentration is higher in this area.

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