Plankton, it is defined by Webster’s Dictionary as “the usually microscopic animal and plant life found floating or drifting in the ocean or in bodies of fresh water, used as food by nearly all aquatic animals”. Why does it say “usually microscopic”? Because plankton is technically any living organism that drifts and is susceptible to the currents in the water, this is mainly algae, diatoms and dinoflagelates, but also includes jellyfish and even Sargasso weed, which in one area is so big that a sea was named for it! Plankton can also be separated into two subcategories, zooplankton and phytoplankton.
Zooplankton is defined by the website Answers. com as “Small floating or weakly swimming animals that drift with water currents and, with plankton). Included are many animals, from single-celled radiolarians to the eggs or larvae of herrings, crabs, and lobsters. Permanent plankton (holoplankton), such as copepods, spend their lives as plankton. Temporary plankton (meroplankton), such as young starfish, clams, worms, and other bottom-dwelling animals, live and feed as plankton until they become adults.” This simplified means zooplankton is the animals in the ocean that primarily ride the currents and can move very little on their own.
Phytoplanktion on the other had is described again on About.com as “Flora of freely floating, often minute organisms that drift with water currents. Like land vegetation, phytoplankton uses carbon dioxide, releases oxygen, and converts minerals to a form animals can use. In fresh water, large numbers of green cyanobacteria may affect the taste of drinking water. Oceanic phytoplankton is the primary food source, directly or indirectly, of all sea organisms.” Phytoplanton is plants that float or drift in the water, yet are still alive, this is to differentiate them from plants that have been uprooted and are no longer viable!.
Zooplankton, just like zoology is dealing with the animals that are plankton and phytoplankton is the plant life. They are similar in that they both live in water, drift around on the currents, and are primarily microscopic. If looking at them under a microscope, though, one can be easily separated from the other in that phytoplankton has chlorophyll and zooplankton does not! Phytoplankton adsorb and remove carbon from the seawater and are the basis for almost all marine life.
It is interesting to note that in the oceans the smallest living organisms, the plankton feed the largest creatures in the world, the Great Blue Whales!