Organisms belonging to the group of phytoplankton are autotrophic. This means that they produce energy through photosynthesis, a process where water and carbon dioxide are transformed into oxygen and sugar with the help of sunlight.

The amount of phytoplankton in the oceans and freshwater ecosystems of planet Earth is enormous. They form the basis for the food chains and food webs of water ecosystems. When phytoplankton photosynthesize, they produce oxygen that is released into the surrounding water. Therefore, phytoplankton contribute significantly to the oxygen concentration of water ecosystems.

Phytoplankton are found everywhere in the water, but photosynthesis can only happen only near the surface of the water, where there is enough sunlight. This is why phytoplankton are mostly found near the surface of the water.

Some phytoplankton produce poisonous substances. Most of the time these substances are not dangerous. However, when large concentrations of plankton occur, they can be harmful even to humans. Some mass concentrations of blue-green algae or cyanobacteria contain poisonous substances that are harmful to humans and other animals. If blue-green algae can be seen on the surface of the water, one should not swim in it, as contact can result in nausea and a rash.

Phytoplankton are studied attentively, as changes in species can tell a lot about changes in water quality. When the amount of nutrients in the water increases, the number of diatoms and cyanobacteria in the water likewise grows. This phenomenon is known as eutrophication.


Mass concentrations of cyanobacteria are easily distinguishable.