Plankton

Some marine diatoms (a key phytoplankton group) seen through a microscope.

Plankton consists of the tiny organisms that float in the water and drift with it.

Plankton is a plural or mass noun; the term for an individual organism within the plankton is a plankter. Plankton is to be contrasted with: the nekton, the large active swimmers like fish, who swim in the water rather than drifting with it: the neuston, the animals like water-striders and whirligig beetles that live on the surface-tension film at the surface; and the benthos, the animals like flounders or sea urchins that live on the lake or ocean floor.

The word was coined by the German marine biologist Viktor Hensen, who also developed the Hensen net, a standardized cone-shaped net, and procedures for using it to collect and measure quantities of plankton.

Plankton live in both salt and fresh water. Plankton include both plant life (phytoplankton) and animal life (zooplankton). Typical organisms of the zooplankton are about the size of a pinhead. Like dust specks in the air, they can be easily seen by snorkelers or scuba divers when they are crosslit by bright sunlight. Common elements of the plankton include tiny jellyfish and comb jellies, copepods, and dinoflagellates.

Plankton are an important element in the marine food chain. Notably, the largest animal on Earth, the blue whale, feeds entirely on tiny plankton; it filters the water through its whalebone combs, which become coated with plankton, which it then licks off the combs with its tongue.

Although zooplankton drift with the water, they do swim at low speeds and over small distances. In particular, most zooplankton undergo a diurnal vertical migration, swimming higher in the water during the night and sinking to lower depths. (Jocular zoologists once gave a paper the rhyming title "Diurnal migrations of plankton crustaceans.")

Some dinoflagellates are bioluminescent, and create the luminous trails seen behind ships at night in tropical waters. Kipling described the effect in his poem The Long Trail:

O the blazing tropic night, when the wake's a welt of light

  That holds the hot sky tame,

And the steady fore-foot snores through the planet-powder'd floors

  Where the scared whale flukes in flame!