How does light affect the population of plankton?

That depends on the depth. plankton or phytoplankton need sunlight for the process of photosynthesis in order to create their food. The deeper into the ocean the less light is available. The area where there is the most production of food is the uppermost layer since more photosynthesis can occur. This area is called the euphotic zone and extends to about 230 feet of 70 meters below the surface. Very little photosynthesis occurs below 330ft Remember that just beneath the surface the level of sunlight is actually too intense for some plankton and for some the conditions at this level are not optimal. The best area is about 62-90 feet below the surface.