Phytoplankton is the base of several aquatic food webs. It’s a group of microorganisms consisting of about 5,000 known species. Most of them are buoyant and float near the surface of the water. The major types of phytoplankton are diatoms, golden-brown algae, blue-green algae, green algae and dinoflagellates. They reside in the euphotic zone of the ocean, which ranges 200 to 300 meters below the surface. These microorganisms are abundant in the ocean, but their presence went unnoticed for a long time. After improvements in technology, however, they were finally made visible in the 1970s.