Why does phytoplankton stay near the surface of the ocean?

Phytoplankton stay near the surface of the ocean primarily to access sunlight, which is essential for photosynthesis. The upper layers of the ocean receive abundant light, enabling phytoplankton to produce energy and nutrients. Additionally, being in the photic zone allows them to take advantage of nutrient upwellings and currents that can support their growth. These factors together ensure their survival and proliferation in this vital ecological niche.