Scientifically, plankton are either referred to as phytoplankton (plant plankton), or zooplankton (animal plankton). Phytoplankton are smaller than zooplankton, and a basic food source for many animals in the ocean, including zooplankton.
Phytoplankton are divided into main groups such as golden-brown algae, green algae and blue-green algae. Zooplankton range from tiny microscopic animals to macroscopic jellyfish. Phytoplankton and zooplankton live near the surface of the ocean and are susceptible to pollutants. Their health and abundance are often observed by biologists to gauge environmental conditions. Both types of plankton are vital to the ecosystems of the world's oceans, and are also producers of oxygen that humans need to breathe.