Plankton can be divided into broad functional (or trophic level) groups:
- Phytoplankton are tiny (usually unicellular) algae that live near the water surface where there is sufficient light to support photosynthesis. Among the more important groups are the diatoms, coccolithophores, cyanobacteria and dinoflagellates.
- Zooplankton are small protists or metazoans (e.g. crustaceans and other animals) that feed on the phytoplankton. Larval stages of larger animals, such as fish, crustaceans, and annelids are included here. Zooplankton are in turn consumed by small fishes.
- Bacterioplankton are bacteria and archaea which play an important role in nutrient cycles in the water column.
This classification divides the plankton into broad producer, consumer and recycler groups. In reality, the trophic level of some plankton is not straightforward. For example, although most dinoflagellates are either photosynthetic producers or heterotrophic consumers, some species can do both, depending upon the circumstances.
Within the plankton, holoplankton are those that spend their entire life cycle in the plankton, while meroplankton are those organisms that are only planktonic for part of their lives (usually the larval stage), and then move into the nekton or a benthic habitat. Examples of meroplankton include larvae of sea urchins, starfish, clams, crustaceans, worms and most fish. Plankton can also be categorized by their size range (e.g., macroplankton, microplankton, nannoplankton, and picoplankton).
Some zooplankton can swim up to several hundreds of meters vertically in a single day (called diel vertical migration), but their horizontal position is primarily determined by currents in the water. Plankton are unable to swim against ocean currents, while larger nekton such as fish and squid can swim against the flow of the water. Diel vertical migrations by zooplankton consist of moving up in the water at night and down in the day time. Being in deeper water during the daylight provides protection from predators that use vision to capture prey. This behavior is seen in animals from all different phyla that are in the plankton, so it must have enormous survival value.
Plankton abundance and distribution are strongly dependent on factors such as nutrient concentrations, the state of the water, and the abundance of other plankton. Their abundance varies horizontally, vertically and seasonally. The primary sources of this variability are the availability of light and nutrients. In temperate climates, springtime brings increased light and higher temperatures, resulting in a spring bloom of phytoplankton, followed by zooplankton. During the summer, dead organisms sink to the bottom where bacteria and fungi break down the tissues in the process of decay. This decomposition restores the nutrients, which concentrate on the bottom, although the phytoplankton that need them are on the top of the water column. Consequently the rate of photosynthesis declines. During the summer, the water column is stratified with warmer water staying on top and cooler water staying on the bottom. Only when water movements can bring the regenerated nutrients up closer to the surface can the phytoplankton bloom again. This happens in the fall, when temperatures fall and the now cooler surface water sinks down and bottom water comes up (upwelling) causing the stratification to break down and another small bloom to occur. Although tropical oceans have abundant light, they have relatively low primary production because of low levels of nutrients such as nitrate and phosphate, due to perpetual stratification of the water column. While plankton are found in the greatest abundance in surface waters, in areas that are too deep for primary production to occur, zooplankton and bacterioplankton can make use of organic material that sinks down from the surface waters above.