Plankton

Plankton (from the Greek planktos, meaning "drifter" or "wanderer") 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. 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. Plankton are a diverse group, which traditionally were divided into two trophic (feeding) groups: phytoplankton and zooplankton. Phytoplankton (autotrophic plant-like producers such as diatoms and cyanobacteria) synthesize their own food, while zooplankton (heterotrophic consumers such as radiolarians and copepods) get their food like animals do, by predating and eating other life forms. In recent years research has shown 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 the recognition of a third group, called the mixoplankton. A fourth group are planktonic decomposers, which include microscopic fungi (mycoplankton 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. Microscopic plankton, smaller than about one millimetre in size, play crucial roles maintaining the health and balance of aquatic ecosystems. Phytoplankton (generally microscopic) are responsible for roughly half of Earth's oxygen production through photosynthesis and play a major role in carbon sequestration. Together, these largely unseen microplankton drive primary production, support local food webs and cycle nutrients. Marine microorganisms have been variously estimated to make up between 70 and 90 percent of the ocean biomass. They influence global biogeochemical processes and largely drive the biological pump (which removes carbon dioxide from the atmosphere and exports carbon to deeper waters). Altogether, plankton form the foundation of the marine food web, supporting many commercially important species from forage fish to baleen whales. 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.

Overview

Apart from aeroplankton, plankton inhabits oceans, seas, estuaries, rivers, lakes and ponds. Local abundance varies horizontally, vertically and seasonally. The primary cause of this variability is the availability of light. Nearly all plankton ecosystems are driven by the input of solar energy (but see chemosynthesis), confining nearly all primary production to surface waters, and to geographical regions and seasons having abundant light. A secondary variable is nutrient availability. The amount and distribution of plankton depends on available nutrients, the state of water and a large amount of other plankton. The local distribution of plankton can be affected by wind-driven Langmuir circulation and the biological effects of this physical process. Although large areas of the tropical and sub-tropical oceans have abundant light, they experience relatively low primary production because they offer limited nutrients such as nitrate, phosphate and silicate. This results from large-scale ocean circulation and water column stratification. In such regions, primary production usually occurs at greater depth, although at a reduced level (because of reduced light). While plankton are most abundant in surface waters, they live throughout the water column. At depths where no primary production occurs, zooplankton and bacterioplankton instead consume organic material sinking from more productive surface waters above. This flux of sinking material, so-called marine snow, can be especially high following the termination of spring blooms. Despite significant macronutrient concentrations, some ocean regions are unproductive (so-called HNLC regions). The micronutrient iron is deficient in these regions, and adding it can lead to the formation of phytoplankton algal blooms. Iron primarily reaches the ocean through the deposition of dust on the sea surface. Paradoxically, oceanic areas adjacent to unproductive, arid land thus typically have abundant phytoplankton (e.g., the eastern Atlantic Ocean, where trade winds bring dust from the Sahara Desert in north Africa). Within the plankton, holoplankton spend their entire life cycle as plankton (e.g. most algae, copepods, salps, and some jellyfish). By contrast, meroplankton are only planktic for part of their lives (usually the larval stage), and then graduate to either a nektic (swimming) or benthic (sea floor) existence. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.

Microscopic plankton

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