Plankton is a diverse group oforganismsthat live in thewater columnand cannot swim against a current.They provide a crucial source of food to many large aquatic organisms, such asfish andwhales.These organisms include driftinganimals,algae, or bacteriathat inhabit thepelagic zoneofoceans,seas, or bodies of fresh water. Plankton typically flows with ocean currents. While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behavior calleddiel vertical migration), their horizontal position is primarily determined by the surroundingcurrents. This is in contrast tonekton organisms that can swim against the ambient flow and control their position.Plankton abundance and distribution are strongly dependent on factors such as ambientnutrientconcentrations, the physical state of the water column, and the abundance of other plankton.Trophic groupsPlankton inhabit oceans, seas, lakes, ponds. Local abundance varies horizontally, vertically and seasonally. The primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy (but seechemosynthesis), confining primary production to surface waters, and to geographical regions and seasons having abundant light.A secondary variable is nutrient availability. Although large areas of thetropicalandsub-tropicaloceans have abundant light, they experience relatively low primary production because they offer limited nutrients such asnitrate,phosphateandsilicate. This results from large-scaleocean circulationand water columnstratification. In such regions, primary production usually occurs at greater depth, although at a reduced level (because of reduced light). The micronutrientironis deficient in these regions, and adding it can lead to the formation ofbloomsof many kinds of phytoplankton.Iron primarily reaches the ocean through the deposition of dust on the sea surface. Plankton is primarily divided into broad functional (ortrophic level) groups: Phytoplankton(from Greekphyton, or plant),autotrophic,prokaryoticor eukaryoticalgaethat live near the water surface where there is sufficientlightto supportphotosynthesis. Among the more important groups are thediatoms, cyanobacteria,dinoflagellatesandcoccolithophores. Zooplankton(from Greekzoon, or animal), smallprotozoansormetazoans (e.g.crustaceansand otheranimals) that feed on other plankton andtelonemia. Some of theeggsandlarvaeof larger animals, such as fish, crustaceans, andannelids, are included here. Bacterioplanktonbacteriaandarchaea, which play an important role inremineralisingorganic material down the water column (note that the prokaryotic phytoplankton is also bacterioplankton).This scheme divides the plankton community into broadproducer,consumerandrecyclergroups. However, determining the trophic level of some plankton is not straightforward. For example, although mostdinoflagellatesare photosyntheticproducers orheterotrophicconsumers, many species aremixotrophicdepending upon circumstances.Phytoplanktons:

Zooplanktons: