What will most likely happen if the plankton population decreases in this ocean system?

Planktons are a microscopic organism, that grows in oceans and seas plankton is made up of tiny plants known as phytoplankton. They require water, nutrients for their growth. They are green in color because of chlorophyll present in them which helps them to perform photosynthesis and they creating their food carbon dioxide and sunlight.

Small fishes, shrimps and some large species of fish such as whales consume them as their main food source. These fishes are prey for marine mammals in food chain. Dead phytoplanktons nourish bottom dwellers & shellfish. If the plankton population decreases in this ocean system it collapse entire marine system.