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Most plants and animals live in the Sunlit Zone.
they live in cold water.
The pelagic ecosystem is largely dependent on the phytoplankton inhabiting the upper, sunlit regions, where most ocean organisms live
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.
The sunlit zone is the top layer of the ocean, it is 0-600 feet down.
open-ocean surface zone
Plankton
The sunlit portion
Phytoplankton
The answer to your question is phytoplankton
Phytoplankton