Answer:
sunlight
Explanation:
Marine phytoplanktons are not found at great depths in the ocean because of the lack of sunlight at these depths.
Phytoplanktons need sunlight to survive. They utilize light for food production through photosynthesis. Without light, this process would become limited and the phytoplanktons will die of starvation.
At great ocean depths, sunlight penetration becomes limited and thus, phytoplanktons also stay away from these depths.
