The term plankton describes a diverse collection of tiny organisms living in open water. They are often so small that they cannot be seen with the naked eye. Plankton consists of tiny algae particles, phytoplankton, and microscopic animals that feed on them, known as zooplankton.
Plankton, also known as drifters, live in open water. The most abundant plant group in the Baltic Sea is phytoplankton. It is estimated that there are approximately 2,000 species of phytoplankton, of which about 100 are common. 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.
Zooplankton, which feed on phytoplankton, are slightly larger than phytoplankton.
Phytoplankton algae are primary producers, meaning they generate the energy they need by photosynthesizing water, carbon dioxide, and inorganic compounds using sunlight. In the process, they produce oxygen and absorb huge amounts of carbon dioxide. Photosynthesis forms the foundation of the entire food web. Thus, not only are they important for marine ecosystems but also for oxygen production in the entire atmosphere of the planet.
Phytoplankton are found throughout the ocean, but the light needed for photosynthesis is most abundant in the surface layer of the water. Therefore, the amount of phytoplankton is highest in the surface layer of the ocean.
Phytoplankton also include heterotrophic, secondary species that require organic matter to survive.
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.
In winter, there is hardly any phytoplankton in the water – there is so little light in the sea that algae cannot photosynthesize and grow. During this time, algae also do not consume the water’s nutrient reserves, so the nutrient concentration in the water is at its highest in winter.
When spring arrives, the amount of light increases and the sea water begins to warm up. Then, the dormant forms of phytoplankton awaken, and the planktonic algae start to grow and reproduce. In spring, there is more phytoplankton in the sea than at any other time of the year.
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. The one thing common to this diverse group of living planktonic groups is that they all live in open water. Many species of zooplankton feed on phytoplankton. However, some are predators, meaning they hunt other zooplankton for food.
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.
Phytoplankton are among the world’s most important oxygen producers. Up to half of the oxygen you breathe comes from marine phytoplankton.
