Plankton
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Plankton are the weakly swimming but mostly drifting small organisms that inhabit the water column of the ocean, seas, and bodies of freshwater. While some forms of plankton can move several hundreds of meters vertically in a single day (a behavior called vertical migration), their horizontal position is mostly determined by movement (currents) of the body of water they inhabit. Larger organisms, such as squid, fish, and marine mammals that can control their horizontal movement and swim against the average flow of the water environment, are called nekton. The study of plankton is termed planktology.
The term holoplankton refers to organisms that spend their entire life cycle as part of the plankton, such as krill, copepods, salps, and jellyfish. Meroplankton, in contrast, are only planktonic for part of their lives (usually the larval stage). Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.
Plankton concentration and distribution are sensitive to chemical and physical changes in the water.
Size groups
Plankton are often described in terms of size. Usually the following divisions are used:
- Megaplankton, 20-200 cm
- Macroplankton, 2-20 cm
- Mesoplankton, 0.2 mm-2 cm
- Microplankton, 20-200 μm
- Nanoplankton, 2-20 μm
- Picoplankton, 0.2-2 μm, mostly bacteria
- Femtoplankton, smaller than 0.2 μm, consisting of marine viruses
However, some of these terms may be used with very different boundaries, especially on the larger end of the scale. The existence and importance of nano- and even smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity.
Functional groups
Plankton are also divided into broad functional groups:
- Phytoplankton (from Greek phyton or plant), algae that live near the water surface where there is sufficient light to support photosynthesis
- Zooplankton (from Greek zoon or animal), small protozoa, crustaceans, and various other animals that feed on other plankton. Some of the eggs and larvae of larger animals, such as fish, crustaceans, and annelids, are included here.
- Bacterioplankton, bacteria and archaea, which play an important role in absorbing nutrients dissolved in the water.