Plankton (singular 'plankter') are any organisms that live in the water column and are incapable of swimming against a current. They provide a crucial source of food to many large aquatic organisms, such as fish and whales.
These organisms include drifting animals, protists, archaea, algae, or bacteria that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic ortaxonomic classification.
Though many planktic (or planktonic—see the next section below) species are microscopic in size, plankton includes organisms covering a wide range of sizes, including large organisms such as jellyfish.
The name plankton is derived from the Greek adjective πλαγκτός - planktos, meaning "errant", and by extension "wanderer" or "drifter". Plankton typically flow with ocean currents. 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 currents. This is in contrast to nekton organisms that can swim against the ambient flow and control their position (e.g. squid, fish, and marine mammals).
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 or benthic (sea floor) existence. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.
Plankton abundance and distribution are strongly dependent on factors such as ambient nutrient concentrations, the physical state of the water column, and the abundance of other plankton.
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 formal 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.
Plankton are primarily divided into broad functional (or trophic level) groups:
• Phytoplankton (from Greek phyton, or plant), 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.
• Zooplankton (from Greek zoon, or animal), small protozoans or metazoans (e.g. crustaceans and other animals) that feed on other plankton and telonemia. Some of the eggs and larvae of larger animals, such as fish, crustaceans, and annelids, are included here.
• Bacterioplankton, bacteria and archaea, which play an important role in remineralising organic material down the water column (note that the prokaryotic phytoplankton are also bacterioplankton).
This scheme divides the plankton community into broad producer, consumer and recycler groups. However, determining the trophic level of some plankton is not straightforward. For example, although most dinoflagellates are either photosynthetic producers or heterotrophic consumers, many species are mixotrophic depending upon circumstances.
Plankton are also often described in terms of size. Usually the following divisions are used:
Group Size range
Mega-plankton > 20 mm metazoans; e.g. jellyfish; ctenophores; salps and pyrosomes (pelagic Tunicata); Cephalopoda
Macro plankton 2→20 mm metazoans; e.g. Pteropods; Chaetognaths; Euphausiacea (krill); Medusae; ctenophores; salps, doliolids and pyrosomes (pelagic Tunicata); Cephalopoda
Mesoplankton 0.2→2 mm metazoans; e.g. copepods; Medusae; Cladocera; Ostracoda; Chaetognaths; Pteropods; Tunicata; Heteropoda
Micro-plankton 20→200 µm
large eukaryotic protists; most phytoplankton; Protozoa (Foraminifera); ciliates; Rotifera; juvenile metazoans - Crustacea (copepodnauplii)
Nanoplankton 2→20 µm small eukaryotic protists; Small Diatoms; Small Flagellates; Pyrrophyta; Chrysophyta; Chlorophyta; Xanthophyta
0.2→2 µm small eukaryotic protists; bacteria; Chrysophyta
Femtoplankton < 0.2 µm marine viruses
However, some of these terms may be used with very different boundaries, especially on the larger end of the scale. 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 micro-plankton 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.
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 macro-nutrient concentrations, some ocean regions are unproductive (so-called HNLC regions). The micro-nutrientcient in these regions, and adding it can lead to the formation of blooms of many kinds of phytoplankton. 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.
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.
Primarily by grazing on phytoplankton, zooplankton provide carbon to the planktic food-web, either respiring it to provide metabolic energy, or upon death as biomass or detritus. Organic Material tends to be more dense than seawater and as a result it sinks into open ocean ecosystems away from the coastlines transporting carbon along with it. This process is known as the biological pump, and it is one reason that oceans constitute the largest carbon sink on Earth.
It might be possible to increase the ocean's uptake of carbon dioxide generated through human activities by increasing plankton production through "seeding", primarily with the micro nutrient 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.
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 inter-annual scales phytoplankton levels temporarily plummet during El Nino 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. 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 they cannot swim against currents.
Grazing zooplankton could be key to marine chlorophyll decline
A much discussed 2010 study reported that chlorophyll concentrations in the world's oceans – an indicator of phytoplankton growth – declined over the 20th century at a rate of around 1% a year. Physical factors associated with climate change, such as higher sea-surface temperatures and reduced mixing depths that decrease nutrient concentrations in the upper layers of the ocean, go some way to explaining the decline. But biological processes may also be coming into play, for example a difference in the change in metabolic rates as temperatures rise of phytoplankton and the zooplankton that feed on them.
Now, a team from Germany, Switzerland, Canada and the US has modelled the chlorophyll changes using the POTSMOM-C ocean-atmosphere-sea-ice biogeochemistry model under the A1FI scenario for 1800–2100, and various increases in metabolic rate for zooplankton.
The team's simulations could reproduce about one quarter of the observed 20th century chlorophyll(a) decline when using a temperature coefficient (Q10) of zooplankton metabolism between 2 and 4, and a Q10 for phytoplankton growth of roughly 1.9.
Ocean warming could result in a relative boost in zooplankton compared to phytoplankton as respiration may be more temperature-dependent than photosynthesis.
The researchers recalibrated the standard ecosystem model, which had assumed a Q10 for zooplankton grazing of roughly 1.1. This led to a projection of a decline in global chlorophyll(a) of more than half by the end of the 21st century.
The model projected a rise of 6.9 °C in mean sea-surface temperature by 2100. For a zooplankton Q10 of 1.1, the team projected a 22.5% drop in chlorophyll by 2100. A zooplankton Q10 of 2 indicated a 48.1% decrease in chlorophyll, a Q10 of 3 gave a 54.4% reduction and a Q10 of 4 resulted in a 51% drop.
"A realistically parameterized model of zooplankton metabolism and phytoplankton growth can explain a large part of [the] observed global change in phytoplankton chlorophyll, indicating significant current and future declines," Dirk Olonscheck of the Potsdam Institute for Climate Impact Research, Germany, and ETH Zurich in Switzerland told environmentalresearchweb. "Furthermore [our results] indicate that both the marine net primary production and zooplankton biomass are much less sensitive to warming than the standing stock of phytoplankton."
Olonscheck believes this is likely to be due to a faster microbial turnover with higher temperatures. "This means that lower plankton biomass might not impact fisheries and biogeochemical cycling as drastically as one might think," he said.
Phytoplankton net primary productivity was projected to decrease 15–20% by 2100. Zooplankton, meanwhile, were projected to decline by about 6%, probably because of the reduction in the phytoplankton they feed on.
"Although the projected strong declines in phytoplankton Chla are projected to be partly compensated by an increased turnover under higher temperatures, the decline in plankton biomass constitutes a profound change in the future structure of marine ecosystems," said Olonscheck.
The team says that more realistic values of the temperature sensitivity of zooplankton metabolism should be used in future ocean-model simulations. "As our results have shown only slight deviations within the broad and likely range of Q10 = 2.0 – 4.0 for zooplankton metabolism, a value within this range seems to be appropriate," said Olonscheck. "Nevertheless, to finally confirm being within this range, comprehensive experimental research is needed to determine a reliable globally and species averaged Q10 parameter."
The scientists, who reported the results in Environmental Research Letters (ERL) , are also interested in tracing the wider ecological effects of changes in phytoplankton on fish stock productivity.