Plankton (singular plankter) are the diverse collection of organisms that live in the water column of large bodies of water and are unable to swim against a current. They provide a crucial source of food to many large aquatic organisms, such as fish and whales.
These organisms include bacteria, archaea, algae, protozoa and drifting or floating animals that inhabit, for example, the pelagic zone of oceans, seas, or bodies of fresh water. Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification.
Though many planktonic species are microscopic in size, plankton includes organisms covering a wide range of sizes, including large organisms such as jellyfish. Technically the term does not include those organisms on the surface of the water which are called Pleuston or those which swim actively in the water which are termed Nekton.