Plankton
Plankton are organisms that drift in water but are unable to actively propel themselves against currents. 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 to larger organisms. 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, including phytoplankton and zooplankton, and serve as a foundational component of the marine food web. These largely unseen microscopic plankton drive primary production, support local food webs, cycle nutrients, and influence global biogeochemical processes. Their role is foundational 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.
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, 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.
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. 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.
Within the plankton, holoplankton spend their entire life cycle as plankton. By contrast, meroplankton are only planktic for part of their lives, and then graduate to either a nektic or benthic existence. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.
Microscopic plankton
Plankton is mostly made up of planktonic microorganisms less than one millimetre across, most visible only through a microscope. Microorganisms have been variously estimated to make up about 70%, or about 90%, of the total ocean biomass. Taken together they form the marine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in the global cycling of almost all chemical elements.Microplankton are ecological linchpins in the marine food web. They are crucial to nutrient recycling in the way they act as decomposers. They are also responsible for nearly all photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus and other nutrients and trace elements. Microplankton sequesters large amounts of carbon and produce much of the world's oxygen.
It is estimated marine viruses kill 20% of ocean microplankton biomass every day. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms which often kill other marine life. The number of viruses in the plankton decreases further offshore and deeper into the water, where there are fewer host organisms.
Terminology
The name plankton was coined by German marine biologist Victor Hensen in 1887 from shortening the word halyplankton from Greek ἅλς háls "sea" and πλανάομαι planáomai " drift" or " wander". Some forms of plankton are capable of independent vertically movement, and can swim hundreds of meters vertically in a single day. However their horizontal position is primarily determined by the surrounding water movement, so plankton typically flow with the ocean currents. This is in contrast to nekton organisms, such as fish, squid and marine mammals, which can swim against the ambient flow and control their position in the environment.The study of plankton is termed planktology and a planktonic individual is referred to as a plankter. The adjective planktonic is widely used in both the scientific and popular literature, and is a generally accepted term. However, from the standpoint of prescriptive grammar, the less-commonly used planktic is more strictly the correct adjective. When deriving English words from their Greek or Latin roots, the gender-specific ending is normally dropped, using only the root of the word in the derivation.
By habitat
Aeroplankton
are tiny lifeforms that float and drift in the air, carried by the current of the wind; they are the atmospheric analogue to oceanic plankton. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of their tiny size. Scientists can collect them for study in traps and sweep nets from aircraft, kites or balloons. Aeroplankton is made up of numerous microbes, including viruses, about 1000 different species of bacteria, around 40,000 varieties of fungi, and hundreds of species of protists, algae, mosses and liverworts that live some part of their life cycle as aeroplankton, often as spores, pollen, and wind-scattered seeds. Additionally, peripatetic microorganisms are swept into the air from terrestrial dust storms, and an even larger amount of airborne marine microorganisms are propelled high into the atmosphere in sea spray. Aeroplankton deposits hundreds of millions of airborne viruses and tens of millions of bacteria every day on every square meter around the planet. This means similar mixes of microscopic plankton taxon can be found in open bodies of water around the world.The sea surface microlayer, compared to the sub-surface waters, contains elevated concentration of bacteria and viruses. These materials can be transferred from the sea-surface to the atmosphere in the form of wind-generated aqueous aerosols due to their high vapour tension and a process known as volatilisation. When airborne, these microbes can be transported long distances to coastal regions. If they hit land they can have an effect on animal, vegetation and human health. Marine aerosols that contain viruses can travel hundreds of kilometers from their source and remain in liquid form as long as the humidity is high enough. These aerosols are able to remain suspended in the atmosphere for about 31 days. Evidence suggests that bacteria can remain viable after being transported inland through aerosols. Some reached as far as 200 meters at 30 meters above sea level. The process which transfers this material to the atmosphere causes further enrichment in both bacteria and viruses in comparison to either the SML or sub-surface waters.