"Plankton" refers to the trillions of little organisms that float in the world's oceans at the pelagic zone, within 200 m (656 ft) from the surface, where there is sufficient light for photosynthesis. Like every other ecosystem in the world, the basis of plankton are the photosynthetic bacteria (cyanobacteria), algae, and other autotrophs that soak up the Sun's rays and reproduce by using trace amounts of carbon, nitrogen, phosphorous, and silicon from sea water as nutrients to divide and grow. These microorganisms exist in a boom-and-bust pattern, usually twice a year, when nutrients are plentiful and the organisms quickly reproduce.

Plankton are classified into about five levels based on their size: nanoplankton (<20 microns), microplankton (<0.2 mm), mesoplankton (<2 mm), macroplankton (<2 cm), and megaplankton (2 cm+). Like in most ecosystems, organisms are more numerous when they are smaller. Unlike terrestrial ecosystems, where there is abundant and macroscopic foliage, planktonic animals have more carnivorous tendencies. Herbivorous animals are more common on the sea floor, where more plant material is available.

Organisms found in plankton, in approximate descending order of frequency, include numerous cyanobacteria, flagellate protists, diatoms, rotifers, copepods (1 mm crustaceans), arrow worms, ostracods (tiny seed-shaped crustaceans), krill, tunicates (blob-like filter feeders that are actually primitive chordates), pteropods (planktonic gastropods), jellyfish, ctenophores, water fleas (crustaceans), pyrosomes (bioluminescent tunicates), cephalopods, fish, etc. At the highest level, rare large animals such as whales occasionally pass through plankton and eat whatever fish they can find. The combined annual prey of all whales in the world exceeds 100 million tons, which is greater than the total annual human consumption of seafood.

To humans, plankton is only indirectly meaningful -- it serves as the primary food source of everything else in the ocean. Occasionally, some members of the plankton, especially bacteria, are so visibly bioluminescent that "milky seas" -- glowing water -- are created. Areas as large as 6,000 square miles have been observed displaying the milky seas effect, and scientists are very interested in learning more about the conditions that give rise to this rare phenomenon.