Answer:
More sunlight is available at the surface of ocean to carry out photosynthesis.
Step-by-step explanation:
Phytoplankton are autotrophic organisms and use sunlight to carry out the process of photosynthesis. Sunlight is required for photosynthesis to occur. More sunlight is available at the surface of ocean. Availability of sunlight reduces with increasing depth of water due to presence of suspended particles in water. To obtain maximum sunlight to support photosynthesis, phytoplankton are found near the surface of ocean water.