Most plants and animals live in the Sunlit Zone.
The sunlit zone is the uppermost layer of the ocean where sunlight penetrates, supporting photosynthesis and providing energy for many marine organisms. This zone extends from the surface down to around 200 meters in depth, depending on water clarity.
The pelagic ecosystem is largely dependent on the phytoplankton inhabiting the upper, sunlit regions, where most ocean organisms live
they live in cold water.
The sunlit zone, also known as the euphotic zone, is the uppermost layer of the ocean where there is enough sunlight for photosynthesis to occur. It typically extends from the surface down to about 200 meters deep, varying depending on water clarity and the angle of the sun.
That is the photic zone, or euphotice zone.
The SUNLIT ZONE is this warm and sunny zone supports most of the life in the ocean.
Diatoms are primarily found in the sunlit zone of the ocean, also known as the euphotic zone. This zone receives enough sunlight for photosynthesis to occur, which is essential for diatoms as they are photosynthetic organisms. They are most abundant in surface waters where sunlight penetration is optimal.
open-ocean surface zone
Plankton
The answer to your question is phytoplankton
Phytoplankton