There are up to 2,000 species of plankton in the Baltic Sea

The most abundant plant group in the Baltic Sea is phytoplankton. It is estimated that there are approximately 2,000 species, of which about 100 are common. The term plankton describes a diverse collection of organisms that live in large bodies of open water and are unable to swim against a current. They are often so small that they cannot be seen with the naked eye.

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.

 Cloudy water due to microscopic plankton blooms.
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. The one thing common to this diverse group of living planktonic groups is that they all live in open water and feed on phytoplankton.

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.

 White amphipods are shown to be only a few millimetres long under the microscope. Copepod under the microscope with black background.