Obtaining both light and nutrients is problematic.
Light is available where nutrients are scarce, and vice versa.
Plants on land solve this problem with roots in the ground to take up nutrients and leaves above ground to absorb light. Phytoplankton in the sea must perform the same functions in a single cell. Light from the sun penetrates only a short distance into the ocean surface. Consequently, most phytoplankton species live in the the upper 100 meters of clear water. Nutrients, on the other hand, come from the sea bottom.