Why are phytoplankton essential to aquatic life zones? filter pollution from water provide basis of food chains moderate water temperature produce chemicals that can be used against predators

The answer is provide basis of food chains.

Plankton includes a diverse group of organisms that live in the water, but are not capable of active swimming against the current. They are usually present in the upper part of the ocean, where is enough sunlight.
They are the basis of food chains in water ecosystems. They provide food for different animals, such as jellyfish, shrimps, whales, etc. 

Covalent bond is the correct answer