Plankton
Introduction
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current. The individual organisms constituting plankton are called plankters. They provide a crucial source of food to many large aquatic organisms, such as fish and whales.
Classification
Plankton are primarily divided into broad functional (or trophic level) groups:
1. Phytoplankton: Autotrophic, prokaryotic 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, dinoflagellates and coccolithophores.
2. Zooplankton: 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.
3. Bacterioplankton: Bacteria and archaea, which play an important role in remineralising organic material down the water column (note that prokaryotic phytoplankton are also bacterioplankton).
Size groups
Plankton are also often described in terms of size. Usually the following divisions are used:
1. Picoplankton: less than 2 µm 2. Nanoplankton: 2–20 µm 3. Microplankton: 20–200 µm 4. Mesoplankton: 200–2000 µm 5. Macroplankton: 2–20 cm
Distribution
The distribution of plankton is usually determined by environmental factors like light, temperature, and nutrients availability. The vertical distribution of plankton is called the planktonic zone, which can be further divided into the euphotic, disphotic, and aphotic zones.
Ecological significance
Plankton plays a critical role in the biogeochemical cycle, facilitating the cycling of essential nutrients in the aquatic environment. They are the base of the aquatic food web, serving as food for larger aquatic organisms. Phytoplankton, in particular, contribute significantly to global primary production, more than terrestrial forests.
Threats and conservation
Plankton populations are threatened by various factors, including climate change, ocean acidification, and human activities such as overfishing and pollution. Conservation efforts are crucial to maintain the health of plankton populations and, by extension, the health of the entire marine ecosystem.