Plankton
From Palaeos
| LIFESTYLE |
| Benthos | Nekton | Plankton | Semi-aquatic | Sessile | Terrestrial | Vagile |
Plankton are drifting organisms that inhabit the water column of oceans, seas, and bodies of fresh water.
Contents |
Definitions
The name plankton is derived from the Greek word πλανκτος ("planktos"), meaning "wanderer" or "drifter" (Thurman, 1997). While some forms of plankton are capable of independent movement and can swim up to several hundreds of metres vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by currents in the body of water they inhabit. By definition, organisms classified as "plankton" are unable to resist ocean currents. This is in contrast to nekton, which are organisms that can swim against the ambient flow of the water environment and control their position (e.g. squid, fish, krill and marine mammals).
Within the plankton itself, holoplankton are those organisms that spend their entire life cycle as part of the plankton (e.g. most algae, copepods, salps, and jellyfish). By contrast, meroplankton are those organisms that are only planktonic for part of their lives (usually the larval stage), and then graduate to either the nekton or a benthic existence. Examples of meroplankton include the larvae of sea urchins, sea stars, crustaceans, marine worms, and most fish.
Plankton abundance and distribution are strongly dependent on factors such as ambient nutrients concentrations, the physical state of the water column, and the abundance of other plankton.
The study of plankton is termed planktology. Individual plankton are referred to as plankters.
Functional groups
Plankton are primarily divided into broad functional (or trophic level) groups:
- Phytoplankton (from Greek phyton, or plant), autotrophic pro- or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis. Among the more important groups are the diatoms, cyanobacteria and dinoflagellates.
- Zooplankton (from Greek zoon, or animal), small protozoans or metazoans (e.g. crustaceans and 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 remineralising organic material down the water column (note that many phytoplankton are also bacterioplankton).
This scheme divides the plankton community into broad producer, consumer and recycler groups. In reality, even the trophic level of some plankton is not straightforward. For example, although most dinoflagellates are either photosynthetic producers or heterotrophic consumers, many species are mixotrophic depending upon their circumstances.
Size groups
Plankton are also often described in terms of size. Usually the following divisions are used:
- Megaplankton, 2×10-1→2×100 m (20-200 cm)
- Macroplankton, 2×10-2→2×10-1 m (2-20 cm)
- Mesoplankton, 2×10-4→2×10-2 m (0.2mm-2 cm)
- Microplankton, 2×10-5→2×10-4 m (20-200 µm)
- Nanoplankton, 2×10-6→2×10-5 m (2-20 µm)
- Picoplankton, 2×10-7→2×10-6 m (0.2-2 µm), mostly bacteria
- Femtoplankton, < 2×10-7 m, (< 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.
Distribution
Plankton are found throughout the oceans, seas and lakes of Earth. However, the local abundance of plankton varies horizontally, vertically and seasonally. The primary source of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy (but see chemosynthesis), and this confines primary production to surface waters, and to geographical regions and seasons when light is abundant.
A secondary source of variability is that of nutrient availability. Although large areas of the tropical and sub-tropical oceans have abundant light, they experience relatively low primary production because of the poor availability of nutrients such as nitrate, phosphate and silicate. This is a product of large-scale ocean circulation and stratification of the water column. In such regions, primary production, still usually occurs at greater depth, although at a reduced level (because of reduced light).
While plankton are found in the greatest abundance in surface waters, they occur throughout the water column. At depths where no primary production occurs, zooplankton and bacterioplankton instead make use of organic material sinking from the more productive surface waters above. This flux of sinking material can be especially high following the termination of spring blooms.
Biogeochemical significance
Plankton ecosystems play a role in the biogeochemical cycles of many important elements. Of particular contemporary significance is their role in the ocean's carbon cycle.
As stated, phytoplankton fix carbon in sunlit surface waters via photosynthesis. Through (primarily) zooplankton grazing, this carbon enters the planktonic foodweb, where it is either respired to provide metabolic energy, or accumulates as biomass or detritus. As living or dead organic material is typically more dense than seawater it tends to sink, and in open ocean ecosystems away from the coasts this leads to the transport of carbon from surface waters to the deep. This process is known as the biological pump, and is one of the reasons that the oceans constitute the largest (active) pool of carbon on Earth.
References
- Omori, M., T. Ikeda (1992). Methods in Marine Zooplankton Ecology. Malabar, USA: Krieger Publishing Company.
- Thurman, H. V. (1997). Introductory Oceanography. New Jersey, USA: Prentice Hall College.
