Answer:
Due to non availability of sunlight.
Step-by-step explanation:
The region of ocean till 200 meter from the surface of ocean is known as the "sunlight zone". It is so because the sunlight can reach up to this extent only in an ocean. Therefore, this sunlight zone has the maximum number of marine plant and animal species.
Beyond 200 m , the sunlight cannot reach the ocean and hence there is a significant drop in the ocean community after the depth of 200 Meters.