PLANKTON
Plankton are organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). Marine plankton include drifting organisms that inhabit the saltwater of oceans and the brackish waters of estuaries. Freshwater plankton are similar to marine plankton, but are found in lakes and rivers. An individual plankton organism in the plankton is called a plankter. In the ocean plankton provide a crucial source of food, particularly for larger filter-feeding animals, such as bivalves, sponges, forage fish and baleen whales.
Plankton includes organisms from species across all the major biological kingdoms, ranging in size from the microscopic (such as bacteria, archaea, protozoa and microscopic algae and fungi) to larger organisms (such as jellyfish and ctenophores). This is because plankton are defined by their ecological niche and level of motility rather than by any phylogenetic or taxonomic classification. The plankton category differentiates organisms from those that can swim against a current, called nekton, and those that live on the deep sea floor, called benthos. Organisms that float on or near the water's surface are called neuston. Neuston that drift as water currents or wind take them, and lack the swimming ability to counter this, form a special subgroup of plankton. Mostly plankton just drift where currents take them, though some, like jellyfish, swim slowly but not fast enough to generally overcome the influence of currents.
Microscopic plankton, smaller than about one millimetre in size, play crucial roles in marine ecosystems. They are a diverse group, which traditionally were divided into two trophic (feeding) groups: phytoplankton and zooplankton. Phytoplankton, such as diatoms and some dinoflagellates, synthesis their own food like plants on land, while zooplankton, such as radiolarians, foraminifera and copepods, get their food like animals do, by predating and eating other life forms. In recent years, there has been a recognition that unicellular plankton often combine photosynthesis and ingestion within their single cell, such as Mesodinium and many dinoflagellates, which means they can act in both the above feeding modes. This has resulted in a third group being recognized, the mixoplankton. A fourth group are the planktonic decomposers. These include mycoplankton (microscopic fungi) and mobile zoospores, bacterioplankton and aquatic viruses. These decomposers recycle organic nutrients so they can be used again as food by other plankton through processes such as the mycoloop, microbial loop and viral shunt. Together, these largely unseen microscopic plankton drive primary production, support local food webs, cycle nutrients, largely drive the biological pump, and influence global biogeochemical processes. They are the foundation of the marine food web, essential for maintaining the health and balance of marine ecosystems.
Although plankton are usually thought of as inhabiting water, there are also airborne versions that live part of their lives drifting in the atmosphere. These aeroplankton can include plant spores, pollen and wind-scattered seeds. They can also include microorganisms swept into the air from terrestrial dust storms and oceanic plankton swept into the air by sea spray.
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