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A microscopic image of plankton. Plankton (singular plankter) are a diverse group of organisms that live in the water column of large bodies of water and can't swim against a current. They provide a crucial source of food to many large aquatic organisms, such as fish and whales. These organisms include drifting or floating bacteria, archaea, algae, protozoa and 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 include organisms covering a wide range of sizes, including large organisms such as jellyfish.Image credit: NSF Collection
