Plankton

Plankton are important because small fish, whales, and zooplankton eat it as regular food. And if phytoplankton didn't exist, those larger fish that eat the animals who eat the phytoplankton, would die out. They have adapted to make food and nutrients from the sun and the way they move in the water is adaptations. 

Phytoplankton: need sunlight for food and nutrients. 

Zooplankton: need the phytoplankton for food and nutrients.