Question

Why do phytoplanktons live near the surface of water?

Answer 1

Phytoplankton tend to live near the surface of the water due to :-
the high concentrations of salt particles that are found in the air. 

Answer 2

Sunlight can vary in the ocean. Down deep, sunlight is very dim or absent, so phytoplankton can only grow near the surface. But generally, phytoplankton growth is limited by nutrients. Most of the nutrients in the ocean come from other animals and plants which have died, and decomposed.
Hope it help you.