Answer:
Sunlight and carbon dioxide.
Step-by-step explanation:
Phytoplankton are found in the photic zone mainly because of sunlight and carbon dioxide.
Why are phytoplankton restricted to photic zone?
The top layer, that is, nearest to the ocean surface is known as the photic zone, it is also known as the sunlight layer. Phytoplankton are the planktonic aquatic species, they are the shallow water creatures, which are restricted to the photic zone in ocean. This is mainly due to the fact that they are totally dependent upon photosynthesis for their survival.
In order to perform the process of photosynthesis, the autotrophs like phytoplankton needs three
major things, that is,
Carbon dioxide
Water
Sunlight
Therefore, mainly to get carbon dioxide and sunlight, these species are confined to photic zone, as when goes deeper in the ocean, the availability of sunlight gets limited and photosynthesis would fail to take place.