Plankton consist of very tiny organisms located in marine and freshwater environments, according to the Encyclopedia of the Earth. The three very broad essential types of plankton are phytoplankton, zooplankton and bacterioplankton. These organisms frequently represent the bottom of the food chain in aquatic environments and are key to maintaining ecological balance.
The presence of plankton in aquatic environments is dependent upon several factors. One factor is the quantity and quality of nutrients found in the waters in question. Other determining factors include the overall condition of the water, as well as the presence of other species of plankton nearby. Quantities of plankton populations vary given the ocean depth and season of the year, the Encyclopedia of the Earth states. While the movement of plankton horizontally in the water is largely governed by the tide, they do have some upward and downward mobility. Some plankton can travel vertically hundreds of meters in a single day. Deeper depths afford some plankton a measure of protection from predators who require light in order to hunt.
Plankton organisms can be algae, bacteria, protozoa, crustaceans, mollusks and members of most phylums, according to the Encyclopaedia Britannica. Britannica notes that phytoplankton are capable, given the proper nutrients, of transforming sunlight into energy-rich chemicals through the process of photosynthesis, the same process that plants on land use to create energy.