Plankton are drifting organisms that inhabit the water column of oceans, seas, and bodies of fresh water.
Plankton abundance and distribution are strongly dependent on factors such as ambient nutrients concentrations, the physical state of the water column, and the abundance of other plankton.
Plankton can be divided into three functional groups: phytoplankton, zooplankton and bacterioplankton.
Phytoplankton are autotrophic prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis.
Among the more important groups are the diatoms, cyanobacteria and dinoflagellates.
Zooplankton are small protozoans or metazoans (e.g. crustaceans and other animals) that feed on other plankton.
Some of the eggs and larvae of larger animals, such as fish, crustaceans, and annelids, are included here.
Bacterioplankton are bacteria and archaea, which play an important role in remineralising organic material down the water column (note that many phytoplankton are also bacterioplankton).
Plankton are found throughout the oceans, seas and lakes of Earth.
However, the local abundance of plankton varies horizontally, vertically and seasonally.
The primary source of this variability is the availability of light.
All plankton ecosystems are driven by the input of solar energy (but see chemosynthesis), and this confines primary production to surface waters, and to geographical regions and seasons when light is abundant.
A secondary source of variability is that of nutrient availability.
Although large areas of the tropical and sub-tropical oceans have abundant light, they experience relatively low primary production because of the poor availability of nutrients such as nitrate, phosphate and silicate.
This is a product of large-scale ocean circulation and stratification of the water column.
In such regions, primary production, still usually occurs at greater depth, although at a reduced level (because of reduced light).