The microscopic size range of small planktonic algae varies from one tenth to one thousandth of a millimetre. Most live either as single cells or as cellular communities, i.e. colonies.
The majority of phytoplankton are autotrophic
Using sunlight, phytoplankton algae are able to photosynthesise their own food from water, carbon dioxide and inorganic compounds. As they grow, they absorb huge amounts of carbon dioxide. Thus, not only are they important for marine ecosystems but also for oxygen production in the entire atmosphere of the planet.
Phytoplankton also include heterotrophic, secondary species that require organic matter to survive.
Phytoplankton may become so seasonally abundant, they form algal blooms
The most well-known phytoplankton phenomena are associated with the excess seasonal proliferation of various types of plankton, also known as algal blooms. Diatoms and dinoflagellates proliferate in the spring while various small flagellate algae are at their most abundant in the early summer. Monocellular and simple filamentous blue-green algae, or cyanobacteria, bloom in summer proper.
The sheer abundance of plankton and blue-green algae in the water column, as well as on the surface are indicated by the colour change and turbidity of the water.
Microscopic phytoplankton drift in the sunlit layers close to the water’s surface.
Zooplankton graze on phytoplankton
Somewhat larger than phytoplankton, zooplankton is composed of organisms such as flagellate protozoa, ciliates, rotifers, copepods, and water fleas. They also include the larval and juvenile stages of growing benthic animals.
Zooplankton follow phytoplankton that passively float in the water, either by using the water currents or slowly swimming. In addition to food, their movement is regulated by light, temperature, and so-called predation pressure.