Plankton

Plankton are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current.[2] The individual organisms constituting plankton are called plankters.[3] They provide a crucial source of food to many small and large aquatic organisms, such as bivalves, fish and whales.

This article is about the marine organisms. For other uses, see Plankton (disambiguation).

Plankton species diversity
Diverse assemblages consist of unicellular and multicellular organisms with different sizes, shapes, feeding strategies, ecological functions, life cycle characteristics, and environmental sensitivities.[1]
Courtesy of Christian Sardet/CNRS/Tara expeditions

Planktonic organisms include bacteria, archaea, algae, protozoa and drifting or floating animals that inhabit—for example—the pelagic zone of oceans, seas, or bodies of fresh water. Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification.

Though many planktonic species are microscopic in size, plankton includes organisms over a wide range of sizes, including large organisms such as jellyfish.[4] Technically the term does not include organisms on the surface of the water, which are called pleuston—or those that swim actively in the water, which are called nekton.

Terminology
Some marine diatoms—a key phytoplankton group

The name plankton is derived from the Greek adjective πλαγκτός (planktos), meaning errant, and by extension, wanderer or drifter,[5] and was coined by Victor Hensen in 1887.[6][7] While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by the surrounding water movement, and plankton typically flow with 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.

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.[8]

The amount and distribution of plankton depends on available nutrients, the state of water and a large amount of other plankton.[9]

The study of plankton is termed planktology and a planktonic individual is referred to as a plankter.[10] 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 (in this case, "-on" which indicates the word is neuter) is normally dropped, using only the root of the word in the derivation.[11]

Trophic groups
An amphipod (Hyperia macrocephala)

Plankton are primarily divided into broad functional (or trophic level) groups:

  • Mixotrophs. Plankton have traditionally been categorized as producer, consumer and recycler groups, but some plankton are able to benefit from more than just one trophic level. In this mixed trophic strategy—known as mixotrophy—organisms act as both producers and consumers, either at the same time or switching between modes of nutrition in response to ambient conditions. This makes it possible to use photosynthesis for growth when nutrients and light are abundant, but switching to eat phytoplankton, zooplankton or each other when growing conditions are poor. Mixotrophs are divided into two groups; constitutive mixotrophs, CMs, which are able to perform photosynthesis on their own, and non-constitutive mixotrophs, NCMs, which use phagocytosis to engulf phototrophic prey that are either kept alive inside the host cell which benefit from its photosynthesis, or they digest their prey except for the plastids which continues to perform photosynthesis (kleptoplasty).[15]

Recognition of the importance of mixotrophy as an ecological strategy is increasing,[16] as well as the wider role this may play in marine biogeochemistry.[17] Studies have shown that mixotrophs are much more important for the marine ecology than previously assumed, and comprise more than half of all microscopic plankton.[18][19] Their presence act as a buffer that prevents the collapse of ecosystems during times with little to no light.[20]

Size groups
Macroplankton: a Janthina janthina snail (with bubble float) cast up onto a beach in Maui

Plankton are also often described in terms of size.[21] Usually the following divisions are used:

Group Size range
    (ESD)		 Examples
Megaplankton> 20 cmmetazoans; e.g. jellyfish; ctenophores; salps and pyrosomes (pelagic Tunicata); Cephalopoda; Amphipoda
Macroplankton2→20 cmmetazoans; e.g. Pteropods; Chaetognaths; Euphausiacea (krill); Medusae; ctenophores; salps, doliolids and pyrosomes (pelagic Tunicata); Cephalopoda; Janthinidae (one family of gastropods); Amphipoda
Mesoplankton0.2→20 mmmetazoans; e.g. copepods; Medusae; Cladocera; Ostracoda; Chaetognaths; Pteropods; Tunicata
Microplankton20→200 µmlarge eukaryotic protists; most phytoplankton; Protozoa Foraminifera; tintinnids; other ciliates; Rotifera; juvenile metazoans - Crustacea (copepod nauplii)
Nanoplankton2→20 µmsmall eukaryotic protists; Small Diatoms; Small Flagellates; Pyrrophyta; Chrysophyta; Chlorophyta; Xanthophyta
Picoplankton0.2→2 µmsmall eukaryotic protists; bacteria; Chrysophyta
Femtoplankton< 0.2 µmmarine viruses

However, some of these terms may be used with very different boundaries, especially on the larger end. 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.

The microplankton and smaller groups are microorganisms and operate at low Reynolds numbers, where the viscosity of water is much more important than its mass or inertia. [22]

Distribution
World concentrations of surface ocean chlorophyll as viewed by satellite during the northern spring, averaged from 1998 to 2004. Chlorophyll is a marker for the distribution and abundance of phytoplankton.

Plankton inhabit oceans, seas, lakes, ponds. Local abundance varies horizontally, vertically and seasonally. The primary cause of this variability is the availability of light. All plankton ecosystems are driven by the input of solar energy (but see chemosynthesis), confining primary production to surface waters, and to geographical regions and seasons having abundant light.

A secondary variable is nutrient availability. 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).

Despite significant macronutrient concentrations, some ocean regions are unproductive (so-called HNLC regions).[23] The micronutrient iron is deficient in these regions, and adding it can lead to the formation of phytoplankton blooms.[24] 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).

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.

The local distribution of plankton can be affected by wind-driven Langmuir circulation and the biological effects of this physical process.

Ecological significance

Food chain

Aside from representing the bottom few levels of a food chain that supports commercially important fisheries, plankton ecosystems play a role in the biogeochemical cycles of many important chemical elements, including the ocean's carbon cycle.[25]

Carbon cycle

Primarily by grazing on phytoplankton, zooplankton provide carbon to the planktic foodweb, either respiring it to provide metabolic energy, or upon death as biomass or detritus. Organic material tends to be denser than seawater, so it sinks into open ocean ecosystems away from the coastlines, transporting carbon along with it. This process, called the biological pump, is one reason that oceans constitute the largest carbon sink on Earth. However, it has been shown to be influenced by increments of temperature.[26][27][28][29] In 2019, a study indicated that at current rates of seawater acidification, we could see Antarctic phytoplanktons smaller and less effective at storing carbon before the end of the century.[30]

It might be possible to increase the ocean's uptake of carbon dioxide (CO
2) generated through human activities by increasing plankton production through seeding, primarily with the micronutrient iron. However, this technique may not be practical at a large scale. Ocean oxygen depletion and resultant methane production (caused by the excess production remineralising at depth) is one potential drawback.[31][32]

Oxygen production

Phytoplankton absorb energy from the Sun and nutrients from the water to produce their own nourishment or energy. In the process of photosynthesis, phytoplankton release molecular oxygen (O
2) into the water as a waste byproduct. It is estimated that about 50% of the world's oxygen is produced via phytoplankton photosynthesis.[33] The rest is produced via photosynthesis on land by plants.[33] Furthermore, phytoplankton photosynthesis has controlled the atmospheric CO
2/O
2 balance since the early Precambrian Eon.[34]

Biomass variability
Amphipod with curved exoskeleton and two long and two short antennae

The growth of phytoplankton populations is dependent on light levels and nutrient availability. The chief factor limiting growth varies from region to region in the world's oceans. On a broad scale, growth of phytoplankton in the oligotrophic tropical and subtropical gyres is generally limited by nutrient supply, while light often limits phytoplankton growth in subarctic gyres. Environmental variability at multiple scales influences the nutrient and light available for phytoplankton, and as these organisms form the base of the marine food web, this variability in phytoplankton growth influences higher trophic levels. For example, at interannual scales phytoplankton levels temporarily plummet during El Niño periods, influencing populations of zooplankton, fishes, sea birds, and marine mammals.

The effects of anthropogenic warming on the global population of phytoplankton is an area of active research. Changes in the vertical stratification of the water column, the rate of temperature-dependent biological reactions, and the atmospheric supply of nutrients are expected to have important impacts on future phytoplankton productivity.[35] Additionally, changes in the mortality of phytoplankton due to rates of zooplankton grazing may be significant.

Freshly hatched fish larvae are also plankton for a few days, as long as it takes before they can swim against currents.

Plankton diversity
  • Some of the diversity found in plankton
  • Pelagibacter ubique, the most common bacteria in the ocean, plays a major role in global carbon cycles
  • The tiny cyanobacterium Prochlorococcus is a major contributor to atmospheric oxygen
  • Diatoms have glass shells (frustules) and also produce much of the worlds oxygen
  • Coccolithophores have chalk plates called coccoliths, and produced the Cliffs of Dover
  • Planktonic algae bloom of coccolithophores off the southern coast of England
  • Foraminiferans have calcious shells and produced the limestone in the Great Pyramids
  • The elaborate silicon shells of microscopic marine radiolarians can eventually produce opal
  • The sea sparkle dinoflagellate glows in the night to produce the milky seas effect
  • Copepod from Antarctica, a translucent ovoid animal with two long antennae
  • Herring larva imaged with the remains of the yolk and the long gut visible in the transparent animal
  • Icefish larvae from Antarctica have no haemoglobin
  • The sea walnut ctenophore has a transient anus which forms only when it needs to defecate[36]
  • Eel larva drifting with the gulf stream
  • Antarctic krill, probably the largest biomass of a single species on the planet
  • Tomopteris is a genus of marine planktonic polychaete
  • Microzooplankton are major grazers of the plankton: two dinoflagellates and a tintinnid ciliate).
  • Sargassum seaweed drifts with currents using air bladders to stay afloat
  • Planktonic sea foam bubbles with image of photographer

Importance to fish

Zooplankton are the initial prey item for almost all fish larvae as they switch from their yolk sacs to external feeding. Fish rely on the density and distribution of zooplankton to match that of new larvae, which can otherwise starve. Natural factors (e.g., current variations) and man-made factors (e.g. river dams, ocean acidification, rising temperatures) can strongly affect zooplankton, which can in turn strongly affect larval survival, and therefore breeding success.

The importance of both phytoplankton and zooplankton is also well-recognized in extensive and semi-intensive pond fish farming. Plankton population based pond management strategies for fish rearing have been practised by traditional fish farmers for decades, illustrating the importance of plankton even in man-made environments.

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See also
  • Aeroplankton
  • Algal bloom – Rapid increase or accumulation in the population of planktonic algae
  • Biological pump – The ocean's biologically driven sequestration of carbon from the atmosphere to the ocean interior and seafloor
  • Gelatinous zooplankton – Fragile and often translucent animals that live in the water column
  • Holoplankton
  • Ichthyoplankton – The eggs and larvae of fish that drift in the water column
  • Iron fertilization
  • Meroplankton
  • Nekton – The aggregate of actively swimming aquatic organisms in a body of water
  • Ocean acidification – Ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide
  • Paradox of the plankton – The ecological observation of high plankton diversity despite competition for few resources
  • Phytoplankton – Autotrophic members of the plankton ecosystem
  • Primary production – synthesis of organic compounds from carbon dioxide by biological organisms
  • Seston – The organisms and non-living matter swimming or floating in a water body
  • Veliger
  • Zooplankton – Heterotrophic protistan or metazoan members of the plankton ecosystem

References
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Further reading
  • Kirby, Richard R. (2010). Ocean Drifters: A Secret World Beneath the Waves. Studio Cactus Ltd, UK. ISBN 978-1-904239-10-9.
  • Dusenbery, David B. (2009). Living at Micro Scale: The Unexpected Physics of Being Small. Harvard University Press, Cambridge, Massachusetts ISBN 978-0-674-03116-6.
  • Kiørboe, Thomas (2008). A Mechanistic Approach to Plankton Ecology. Princeton University Press, Princeton, N.J. ISBN 978-0-691-13422-2.
  • Dolan, J.R., Agatha, S., Coats, D.W., Montagnes, D.J.S., Stocker, D.K., eds. (2013).Biology and Ecology of Tintinnid Ciliates: Models for Marine Plankton. Wiley-Blackwell, Oxford, UK ISBN 978-0-470-67151-1.
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