The reason phytoplankton have to live in shallow ocean water is because they cannot withstand the colder temperatures of deep water. they are an important food source to surface-dwelling marine organisms. they require sunlight to carry out photosynthesis. they feed on tiny organisms living at the surface.

The answer is they require sunlight to carry out photosynthesis.
Phytoplankton is basically a plant with only one cell. To obtain food, phytoplankton need to conduct a photosynthesis process just like normal plants do.
As the water got deeper, it would be harder for sunlight to pass through the water. So, most of the phytoplankton usually went to the surface in order to obtain the main source of their photosynthesis.

4) repetitive change