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| image1 = Importance of Antarctic krill in biogeochemical cycles.png
| image1 = Importance of Antarctic krill in biogeochemical cycles.png
| caption1 = {{center|'''Processes in the biological pump'''}}{{center|<small> Numbers given are carbon fluxes (Gt C yr−1) in white boxes<br />and carbon masses (Gt C) in dark boxes</small>}}
| caption1 = {{center|'''Processes in the biological pump'''}}{{center|<small> Numbers given are carbon fluxes (Gt C yr−1) in white boxes<br />and carbon masses (Gt C) in dark boxes</small>}}
Phytoplankton convert CO2, which has dissolved from the atmosphere into the surface oceans into particulate organic carbon (POC) during primary production. Phytoplankton are then consumed by krill and small zooplankton grazers, which in turn are preyed upon by higher trophic levels. Any unconsumed phytoplankton form aggregates, and along with zooplankton faecal pellets, sink rapidly and are exported out of the mixed layer. Krill, zooplankton and microbes intercept phytoplankton in the surface ocean and sinking detrital particles at depth, consuming and respiring this POC to CO2 (dissolved inorganic carbon, DIC), such that only a small proportion of surface-produced carbon sinks to the deep ocean (i.e., depths > 1000 m). As krill and smaller zooplankton feed, they also physically fragment particles into small, slower- or non-sinking pieces (via sloppy feeding, coprorhexy if fragmenting faeces), retarding POC export. This releases dissolved organic carbon (DOC) either directly from cells or indirectly via bacterial solubilisation (yellow circle around DOC). Bacteria can then remineralise the DOC to DIC (CO2, microbial gardening). Diel vertically migrating krill, smaller zooplankton and fish can actively transport carbon to depth by consuming POC in the surface layer at night, and metabolising it at their daytime, mesopelagic residence depths. Depending on species life history, active transport may occur on a seasonal basis as well.<ref>Cavan, E.L., Belcher, A., Atkinson, A., Hill, S.L., Kawaguchi, S., McCormack, S., Meyer, B., Nicol, S., Ratnarajah, L., Schmidt, K. and Steinberg, D.K. (2019) "The importance of Antarctic krill in biogeochemical cycles". ''Nature communications'', '''10'''(1): 1–13. {{doi|10.1038/s41467-019-12668-7}}.</ref>
Phytoplankton convert CO2, which has dissolved from the atmosphere into the surface oceans into particulate organic carbon (POC) during primary production. Phytoplankton are then consumed by krill and small zooplankton grazers, which in turn are preyed upon by higher trophic levels. Any unconsumed phytoplankton form aggregates, and along with zooplankton faecal pellets, sink rapidly and are exported out of the mixed layer. Krill, zooplankton and microbes intercept phytoplankton in the surface ocean and sinking detrital particles at depth, consuming and respiring this POC to CO2 (dissolved inorganic carbon, DIC), such that only a small proportion of surface-produced carbon sinks to the deep ocean (i.e., depths > 1000 m). As krill and smaller zooplankton feed, they also physically fragment particles into small, slower- or non-sinking pieces (via sloppy feeding, coprorhexy if fragmenting faeces), retarding POC export. This releases dissolved organic carbon (DOC) either directly from cells or indirectly via bacterial solubilisation (yellow circle around DOC). Bacteria can then remineralise the DOC to DIC (CO2, microbial gardening). Diel vertically migrating krill, smaller zooplankton and fish can actively transport carbon to depth by consuming POC in the surface layer at night, and metabolising it at their daytime, mesopelagic residence depths. Depending on species life history, active transport may occur on a seasonal basis as well.<ref>Cavan, E.L., Belcher, A., Atkinson, A., Hill, S.L., Kawaguchi, S., McCormack, S., Meyer, B., Nicol, S., Ratnarajah, L., Schmidt, K. and Steinberg, D.K. (2019) "The importance of Antarctic krill in biogeochemical cycles". ''Nature communications'', '''10'''(1): 1–13. {{doi|10.1038/s41467-019-12668-7}}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>
}}
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Revision as of 11:20, 28 March 2020

The pelagic food web, showing the central involvement of marine microorganisms in how the ocean imports nutrients from and then exports them back to the atmosphere and ocean floor.
Part of a series of overviews on
Marine life

Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers, such as mature forests, grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production.

Because of this inversion, it is the zooplankton that make up most of the marine animal biomass. As primary consumers, zooplankton are the crucial link between the primary producers (mainly phytoplankton) and the rest of the marine food web (secondary consumers).[1]

If phytoplankton dies before it is eaten, it descends through the euphotic zone as part of the marine snow and settles into the depths of sea. In this way, phytoplankton sequester about 2 billion tons of carbon dioxide into the ocean each year, causing the ocean to become a sink of carbon dioxide holding about 90% of all sequestered carbon.[2] The ocean produces about half of the world's oxygen and stores 50 times more carbon dioxide than the atmosphere.[3]

An ecosystem cannot be understood without knowledge of how its food web determines the flow of materials and energy. Phytoplankton autotrophically produced biomass by converting inorganic compounds into organic ones. In this way, phytoplankton function as the foundation of the marine food web by supporting all other life in the ocean. The second central process in the marine food web is the microbial loop. This loop degrades marine bacteria and archaea, remineralises organic and inorganic matter, and then recycles the product either within the pelagic food web or by depositing it as sediment on the seafloor.[4]

Trophic levels

Main article: trophic levels
marine food chain
(typical)
  Sun
↓
phytoplankton
↓
herbivorous zooplankton
↓
carnivorous zooplankton
↓
  filter feeder
↓
  predatory vertebrate

All life forms in the sea have the potential and perhaps the destiny to become food for something else. In the ocean, the food chain typically starts with phytoplankton and follows a course such as:

Phytoplankton → herbivorous zooplankton → carnivorous zooplankton → filter feeder → predatory vertebrate

Classic food web for grey seals in the Baltic Sea containing several typical marine food chains [5]

Phytoplankton are the main primary producers at the bottom of the marine food chain. Since they are at the first level in the food chain they are said to have a trophic level of 1 (from the Greek trophē meaning food). Phytoplankton use photosynthesis to convert inorganic carbon into protoplasm. They are then consumed by microscopic animals called zooplankton.

Zooplankton comprise the second trophic level in the food chain, and include microscopic one-celled organisms called protozoa as well as small crustaceans, such as copepods and krill, and the larva of fish, squid, lobsters and crabs.

In turn, small zooplankton are consumed by both larger predatory zooplankters, such as krill, and by forage fish, which are small, schooling, filter-feeding fish. This makes up the third trophic level in the food chain.

The fourth trophic level consists of predatory fish, marine mammals and seabirds that consume forage fish. Examples are swordfish, seals and gannets.

Apex predators, such as orcas, which can consume seals, and shortfin mako sharks, which can consume swordfish, make up a fifth trophic level. Baleen whales can consume zooplankton and krill directly, leading to a food chain with only three or four trophic levels.

In practice, trophic levels are not always simple integers because the same consumer species often feeds across more than one trophic level.[6][7] For example a large marine vertebrate may eat smaller predatory fish but may also eat filter feeders; the stingray eats crustaceans, but the hammerhead eats both crustaceans and stingrays. Animals can also eat each other; the cod eats smaller cod as well as crayfish, and crayfish eat cod larvae. The feeding habits of a juvenile animal, and, as a consequence, its trophic level, can change as it grows up.

The fisheries scientist Daniel Pauly sets the values of trophic levels to one in plants and detritus, two in herbivores and detritivores (primary consumers), three in secondary consumers, and so on. The definition of the trophic level, TL, for any consumer species is:[8]

where is the fractional trophic level of the prey j, and represents the fraction of j in the diet of i. In the case of marine ecosystems, the trophic level of most fish and other marine consumers takes value between 2.0 and 5.0. The upper value, 5.0, is unusual, even for large fish,[9] though it occurs in apex predators of marine mammals, such as polar bears and killer whales.[10]

Primary producers

At the base of the ocean food web are single-celled algae and other plant-like organisms known as phytoplankton. Like plants on land, phytoplankton use chlorophyll and other light-harvesting pigments to carry out photosynthesis, absorbing atmospheric carbon dioxide to produce sugars for fuel. Chlorophyll in the water changes the way it reflects and absorbs sunlight, allowing scientists to map the amount and location of phytoplankton. These measurements give scientists valuable insights into the health of the ocean environment, and help scientists study the ocean carbon cycle.[11]

Ocean chlorophyll concentration October 2019
Green indicates where there are a lot of phytoplankton, while blue indicates where there are few phytoplankton. – NASA Earth Observatory 2019.[11]
Prochlorococcus, an influential bacterium which produces much of the world's oxygen

Among the phytoplankton are members from a phylum of bacteria called cyanobacteria. Marine cyanobacteria include the smallest known photosynthetic organisms. The smallest of all, Prochlorococcus, is just 0.5 to 0.8 micrometres across.[12] In terms of individual numbers, Prochlorococcus is possibly the most plentiful species on Earth: a single millilitre of surface seawater can contain 100,000 cells or more. Worldwide there are estimated to be several octillion (~1027) individuals.[13] Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans.[14] The bacterium accounts for about 20% of the oxygen in the Earth's atmosphere.[15]

In oceans, most primary production is performed by algae. This is a contrast to on land, where most primary production is performed by vascular plants. Algae ranges from single floating cells to attached seaweeds, while vascular plants are represented in the ocean by groups such as the seagrasses. Larger producers, such as seagrasses and seaweeds, are mostly confined to the littoral zone and shallow waters, where they attach to the underlying substrate and are still within the photic zone. But most of the primary production by algae is performed by the phytoplankton.

  • Phytoplankton form the base of the ocean foodchain
  • Phytoplankton
    Phytoplankton
  • Dinoflagellate
    Dinoflagellate
  • Diatoms
    Diatoms

Thus, in ocean environments, the first bottom trophic level is occupied principally by phytoplankton, microscopic drifting organisms, mostly one-celled algae, that float in the sea. Most phytoplankton are too small to be seen individually with the unaided eye. They can appear as a green discoloration of the water when they are present in high enough numbers. Since they increase their biomass mostly through photosynthesis they live in the sun-lit surface layer (euphotic zone) of the sea.

The most important groups of phytoplankton include the diatoms and dinoflagellates. Diatoms are especially important in oceans, where according to some estimates they contribute up to 45% of the total ocean's primary production.[16] Diatoms are usually microscopic, although some species can reach up to 2 millimetres in length.

Primary consumers

The second trophic level (primary consumers) is occupied by zooplankton which feed off the phytoplankton. Together with the phytoplankton, they form the base of the food pyramid that supports most of the world's great fishing grounds. Zooplankton are tiny animals found with the phytoplankton in oceanic surface waters, and include tiny crustaceans, and fish larvae and fry (recently hatched fish). Most zooplankton are filter feeders, and they use appendages to strain the phytoplankton in the water. Some larger zooplankton also feed on smaller zooplankton. Some zooplankton can jump about a bit to avoid predators, but they can't really swim. Like phytoplankton, they float with the currents, tides and winds instead. Zooplanktons can reproduce rapidly, their populations can increase up to thirty percent a day under favourable conditions. Many live short and productive lives and reach maturity quickly.

  • Zooplankton form a second level in the ocean food chain
  • Segmented worm
    Segmented worm
  • Tiny shrimp-like crustaceans
    Tiny shrimp-like crustaceans
  • Juvenile planktonic squid
    Juvenile planktonic squid

Particularly important groups of zooplankton are the copepods and krill. These are not shown in the images above, but are discussed in more detail later. Copepods are a group of small crustaceans found in ocean and freshwater habitats. They are the biggest source of protein in the sea,[17] and are important prey for forage fish. Krill constitute the next biggest source of protein. Krill are particularly large predator zooplankton which feed on smaller zooplankton. This means they really belong to the third trophic level, secondary consumers, along with the forage fish.

Together, phytoplankton and zooplankton make up most of the plankton in the sea. Plankton is the term applied to any small drifting organisms that float in the sea (Greek planktos = wanderer or drifter). By definition, organisms classified as plankton are unable to swim against ocean currents; they cannot resist the ambient current and control their position. In ocean environments, the first two trophic levels are occupied mainly by plankton. Plankton are divided into producers and consumers. The producers are the phytoplankton (Greek phyton = plant) and the consumers, who eat the phytoplankton, are the zooplankton (Greek zoon = animal).

Jellyfish are easy to capture and digest and may be more important as food sources than was previously thought.[18]

Traditionally jellyfish have been viewed as trophic dead ends, minor players in the marine food web, gelatinous organisms with a body plan largely based on water that offers little nutritional value for other organisms apart from a few specialised predators such as the ocean sunfish and the leatherback sea turtle.[19][18] That view has recently been challenged. Jellyfish, and more generally "gelatinous zooplankton" which include salps and ctenophores, are very diverse, fragile with no hard parts, difficult to see and monitor, subject to rapid population swings and often live inconveniently far from shore or deep in the ocean. It is difficult for scientists to detect and analyse jellyfish in the guts of predators, since they turn to mush when eaten and are rapidly digested.[19] But jellyfish bloom in vast numbers, and it has been shown they form major components in the diets of tuna, spearfish and swordfish as well as various birds and invertebrates such as octopus, sea cucumbers, crabs and amphipods.[20][18] "Despite their low energy density, the contribution of jellyfish to the energy budgets of predators may be much greater than assumed because of rapid digestion, low capture costs, availability, and selective feeding on the more energy-rich components. Feeding on jellyfish may make marine predators susceptible to ingestion of plastics."[18]

Higher order consumers

Marine invertebrates
Fish
Predator fish sizing up schooling forage fish

Forage fish occupy central positions in the ocean food webs. The organisms it eats are at a lower trophic level, and the organisms that eat it are at a higher trophic level. Forage fish occupy middle levels in the food web, serving as a dominant prey to higher level fish, seabirds and mammals.

Other marine vertebrates

In 2010 researchers found whales carry nutrients from the depths of the ocean back to the surface using a process they called the whale pump.[22] Whales feed at deeper levels in the ocean where krill is found, but return regularly to the surface to breathe. There whales defecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface where phytoplankton consume it. In the Gulf of Maine the whale pump provides more nitrogen than the rivers.[23]

Microorganisms

On average there are "more than one million microbial cells in every drop of seawater, and their collective metabolisms not only recycle nutrients that can then be used by larger organisms but also catalyze key chemical transformations that maintain Earth’s habitability".[24]
See also: Marine microorganism

There has been increasing recognition in recent years that marine microorganisms play much bigger roles in marine ecosystems than was previously thought. Developments in metagenomics gives researchers an ability to reveal previously hidden diversities of microscopic life, offering a powerful lens for viewing the microbial world and the potential to revolutionise understanding of the living world.[25] Metabarcoding dietary analysis techniques are being used to reconstruct food webs at higher levels of taxonomic resolution and are revealing deeper complexities in the web of interactions.[26]

Microorganisms play key roles in marine food webs. The viral shunt pathway is a mechanism that prevents marine microbial particulate organic matter (POM) from migrating up trophic levels by recycling them into dissolved organic matter (DOM), which can be readily taken up by microorganisms.[27] Viral shunting helps maintain diversity within the microbial ecosystem by preventing a single species of marine microbe from dominating the micro-environment.[28] The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM.[29]

Ocean particulate organic matter (POM)
as imaged by a satellite in 2011
Cycling of marine phytoplankton. Phytoplankton live in the photic zone of the ocean, where photosynthesis is possible. During photosynthesis, they assimilate carbon dioxide and release oxygen. If solar radiation is too high, phytoplankton may fall victim to photodegradation. For growth, phytoplankton cells depend on nutrients, which enter the ocean by rivers, continental weathering, and glacial ice meltwater on the poles. Phytoplankton release dissolved organic carbon (DOC) into the ocean. Since phytoplankton are the basis of marine food webs, they serve as prey for zooplankton, fish larvae and other heterotrophic organisms. They can also be degraded by bacteria or by viral lysis. Although some phytoplankton cells, such as dinoflagellates, are able to migrate vertically, they are still incapable of actively moving against currents, so they slowly sink and ultimately fertilize the seafloor with dead cells and detritus.[30]
DOM, POM and the viral shunt
Connections between the different compartments of the living (bacteria/viruses and phyto−/zooplankton) and the nonliving (DOM/POM and inorganic matter) environment [31]
The viral shunt pathway facilitates the flow of dissolved organic matter (DOM) and particulate organic matter (POM) through the marine food web

Fungi

Main article: Marine fungi
Roles of fungi in the marine carbon cycle [32]

Coastal webs

Continental shelves
Typical food web on a continental shelf


  • Byrnes, J.E., Reynolds, P.L. and Stachowicz, J.J. (2007) "Invasions and extinctions reshape coastal marine food webs". PloS one, 2(3): e295. doi:10.1371/journal.pone.0000295




Seagrass meadows
Cumulative visualization of a number of seagrass food webs from different regions and with different eutrophication levels Different coloured dots represent trophic groups from different trophic levels with black  =  primary producers, dark to light grey  =  secondary producers, and the lightest grey being top predators. The grey links represent feeding links.[33]




Estuaries
Food web diagram of the Venice Lagoon
with 27 nodes or functional groups. Colors of flows depict different fishing target (artisanal fisheries in blue, and clam fishery in red) and non-target species (for clam harvesting, in green). [34][35]






Coral reefs
Food web reconstruction by DNA barcodes at the coral reef of Moorea, French Polynesia. Dietary partitioning among three predatory fish species as detected using metabarcoding dietary analysis. The taxonomic resolution provided by the metabarcoding approach highlights a complex interaction web and demonstrates that levels of trophic partitioning among coral reef fishes have likely been underestimated.[26][36]

DNA barcoding can be used to construct food web structures with better taxonomic resolution at the web nodes. This provides more specific species identification and greater clarity about exactly who eats whom. "DNA barcodes and DNA information may allow new approaches to the construction of larger interaction webs, and overcome some hurdles to achieving adequate sample size".[26]


Deep ocean webs

Mesopelagic fishes
  • Mesopelagic food web
  • Impact of mesopelagic species on the global carbon budget [37] DVM = diel vertical migration           NM = non-migration
    Impact of mesopelagic species on the global carbon budget [37]
    DVM = diel vertical migration           NM = non-migration

Scientists are starting to explore in more detail the largely unknown twilight zone of the mesopelagic, 200 to 1,000 metres deep. This layer is responsible for removing about 4 billion tonnes of carbon dioxide from the atmosphere each year. The mesopelagic layer is inhabited by most of the marine fish biomass.[38]

Mesopelagic bristlemouths may be the most abundant vertebrates on the planet, though little is known about them.[38]
Oceanic pelagic food web showing energy flow from micronekton to top predators. Line thickness is scaled to the proportion in the diet.[41]

According to a 2017 study, narcomedusae consume the greatest diversity of mesopelagic prey, followed by physonect siphonophores, ctenophores and cephalopods. The importance of the so called "jelly web" is only beginning to be understood, but it seems medusae, ctenophores and siphonophores can be key predators in deep pelagic food webs with ecological impacts similar to predator fish and squid. Traditionally gelatinous predators were thought ineffectual providers of marine trophic pathways, but they appear to have substantial and integral roles in deep pelagic food webs.[42] Diel vertical migration, an important active transport mechanism, allows mesozooplankton to sequester carbon dioxide from the atmosphere as well as supply carbon needs for other mesopelagic organisms.[43]

Seeps and vents
Seep and vent interactions with surrounding deep-sea ecosystems
The y axis is meters above bottom on a log scale. DOC: Dissolved Organic Carbon, POC: Particulate Organic Carbon, SMS: Seafloor Massive Sulfide.[44]
Conceptual diagram of faunal community structure and food-web patterns along fluid-flux gradients within Guaymas seep and vent ecosystems.[45][46][47]

Polar webs

Polar topographies
The Antarctica is a frozen landmass
surrounded by oceans
The Arctic is a frozen ocean
surrounded by landmasses
The annual pulse of ice and snow at the poles

Arctic and Antarctic marine systems have very different topographical structures and as a consequence have very different food web structures.[48] Both Arctic and Antarctic pelagic food webs have characteristic energy flows controlled largely by a few key species. But there is no single generic web for either. Alternative pathways are important for resilience and maintaining energy flows. However, these more complicated alternatives provide less energy flow to upper trophic-level species. "Food-web structure may be similar in different regions, but the individual species that dominate mid-trophic levels vary across polar regions".[49]

Humpback whale straining krill
Penguins and polar bears never meet
The Antarctic has penguins but no polar bears
The Arctic has polar bears but no penguins
Arctic
Polar bear food webs
Traditional arctic marine food web with a focus on macroorganisms
Contemporary arctic marine food web with a greater focus on the role of microorganisms

The Arctic food web is complex. The loss of sea ice can ultimately affect the entire food web, from algae and plankton to fish to mammals. The impact of climate change on a particular species can ripple through a food web and affect a wide range of other organisms... Not only is the decline of sea ice impairing polar bear populations by reducing the extent of their primary habitat, it is also negatively impacting them via food web effects. Declines in the duration and extent of sea ice in the Arctic leads to declines in the abundance of ice algae, which thrive in nutrient-rich pockets in the ice. These algae are eaten by zooplankton, which are in turn eaten by Arctic cod, an important food source for many marine mammals, including seals. Seals are eaten by polar bears. Hence, declines in ice algae can contribute to declines in polar bear populations.[50]

Pteropod (sea angel)
Pteropods: Swimming snails of the sea
The bacterium Marinomonas arctica grows inside Arctic sea ice at subzero temperatures
Walrus are keystone species in the Arctic but are not found in the Antarctic.
Arctic food web with mixotrophy
Yellow arrows: flow of energy from the sun to photosynthetic organisms (autotrophs and mixotrophs)
Gray arrows: flow of carbon to heterotrophs
Green arrows: major pathways of carbon flow to or from mixotrophs
HCIL: heterotrophic ciliates; MCIL: mixotrophic ciliates; HNF: heterotrophic nanoflagellates; DOC: dissolved organic carbon; HDIN: heterotrophic dinoflagellates [51]
Antarctic
Importance of Antarctic krill in biogeochemical cycles
Processes in the biological pump
Numbers given are carbon fluxes (Gt C yr−1) in white boxes
and carbon masses (Gt C) in dark boxes
Phytoplankton convert CO2, which has dissolved from the atmosphere into the surface oceans into particulate organic carbon (POC) during primary production. Phytoplankton are then consumed by krill and small zooplankton grazers, which in turn are preyed upon by higher trophic levels. Any unconsumed phytoplankton form aggregates, and along with zooplankton faecal pellets, sink rapidly and are exported out of the mixed layer. Krill, zooplankton and microbes intercept phytoplankton in the surface ocean and sinking detrital particles at depth, consuming and respiring this POC to CO2 (dissolved inorganic carbon, DIC), such that only a small proportion of surface-produced carbon sinks to the deep ocean (i.e., depths > 1000 m). As krill and smaller zooplankton feed, they also physically fragment particles into small, slower- or non-sinking pieces (via sloppy feeding, coprorhexy if fragmenting faeces), retarding POC export. This releases dissolved organic carbon (DOC) either directly from cells or indirectly via bacterial solubilisation (yellow circle around DOC). Bacteria can then remineralise the DOC to DIC (CO2, microbial gardening). Diel vertically migrating krill, smaller zooplankton and fish can actively transport carbon to depth by consuming POC in the surface layer at night, and metabolising it at their daytime, mesopelagic residence depths. Depending on species life history, active transport may occur on a seasonal basis as well.[54]
Antarctic marine food web
Potter Cove 2018. Vertical position indicates trophic level and node widths are proportional to total degree (in and out). Node colors represent functional groups.[55][56]
Common-enemy graph of Antarctic food web
Potter Cove 2018. Nodes represent basal species and links indirect interactions (shared predators). Node and link widths are proportional to number of shared predators. Node colors represent functional groups.[55]

Terrestrial comparisons

Biomass pyramids
Compared to terrestrial biomass pyramids, aquatic pyramids are generally inverted at the base
Marine producers use less biomass than terrestrial producers
The minute but ubiquitous and highly active bacterium Prochlorococcus runs through its life cycle in one day, collectively generating 20% of all global oxygen.
By contrast, a single bristlecone pine can tie up a lot of relatively inert biomass for thousands of years with little photosynthetic activity.[57]

Marine environments can have inversions in their biomass pyramids. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is generally larger than the biomass of primary producers. This happens because the ocean's primary producers are mostly tiny phytoplankton which have r-strategist traits of growing and reproducing rapidly, so a small mass can have a fast rate of primary production. In contrast, many terrestrial primary producers, such as mature forests, have K-strategist traits of growing and reproducing slowly, so a much larger mass is needed to achieve the same rate of primary production.

Examples: The bristlecone pine can live for thousands of years, and has a very low production/biomass ratio. The cyanobacterium Prochlorococcus lives for about 24 hours, and has a very high production/biomass ratio.

Mature forests have a lot of biomass invested in secondary growth which has low productivity
Ocean or marine biomass, in a reversal of terrestrial biomass, can increase at higher trophic levels.[58]

In oceans, most primary production is performed by algae. This is a contrast to on land, where most primary production is performed by vascular plants.

Aquatic producers, such as planktonic algae or aquatic plants, lack the large accumulation of secondary growth that exists in the woody trees of terrestrial ecosystems. However, they are able to reproduce quickly enough to support a larger biomass of grazers. This inverts the pyramid. Primary consumers have longer lifespans and slower growth rates that accumulates more biomass than the producers they consume. Phytoplankton live just a few days, whereas the zooplankton eating the phytoplankton live for several weeks and the fish eating the zooplankton live for several consecutive years.[59] Aquatic predators also tend to have a lower death rate than the smaller consumers, which contributes to the inverted pyramidal pattern. Population structure, migration rates, and environmental refuge for prey are other possible causes for pyramids with biomass inverted. Energy pyramids, however, will always have an upright pyramid shape if all sources of food energy are included, since this is dictated by the second law of thermodynamics."[60][61]

Comparison of productivity in marine and terrestrial ecosystems [62]
Ecosystem Net primary productivity
billion tonnes per year 		Total plant biomass
billion tonnes 		Turnover time
years
Marine
45–55
1–2
0.02–0.06
Terrestrial
55–70
600–1000
9–20

Anthropogenic effects

Fishing down the food web [63]
Overfishing
Acidification

Pteropods and brittle stars together form the base of the Arctic food webs and both are seriously damaged by acidification. Pteropods shells dissolve with increasing acidification and brittle stars lose muscle mass when re-growing appendages.[64] Additionally the brittle star's eggs die within a few days when exposed to expected conditions resulting from Arctic acidification.[65] Acidification threatens to destroy Arctic food webs from the base up. Arctic waters are changing rapidly and are advanced in the process of becoming undersaturated with aragonite.[66] Arctic food webs are considered simple, meaning there are few steps in the food chain from small organisms to larger predators. For example, pteropods are "a key prey item of a number of higher predators – larger plankton, fish, seabirds, whales".[67]

Effects of ocean acidification
Unhealthy pteropod showing effects of ocean acidification
Ocean acidification causes brittle stars to lose muscle mass
      Pteropods and brittle stars form the base of Arctic food webs
Climate change

"Our results show how future climate change can potentially weaken marine food webs through reduced energy flow to higher trophic levels and a shift towards a more detritus-based system, leading to food web simplification and altered producer–consumer dynamics, both of which have important implications for the structuring of benthic communities."[68][69]

"...increased temperatures reduce the vital flow of energy from the primary food producers at the bottom (e.g. algae), to intermediate consumers (herbivores), to predators at the top of marine food webs. Such disturbances in energy transfer can potentially lead to a decrease in food availability for top predators, which in turn, can lead to negative impacts for many marine species within these food webs... "Whilst climate change increased the productivity of plants, this was mainly due to an expansion of cyanobacteria (small blue-green algae)," said Mr Ullah. "This increased primary productivity does not support food webs, however, because these cyanobacteria are largely unpalatable and they are not consumed by herbivores. Understanding how ecosystems function under the effects of global warming is a challenge in ecological research. Most research on ocean warming involves simplified, short-term experiments based on only one or a few species."[69]

The distribution of anthropogenic stressors faced by marine species threatened with extinction in various marine regions of the world. Numbers in the pie charts indicate the percentage contribution of an anthropogenic stressors’ impact in a specific marine region.[70][71]
Anthropogenic stressors to marine species threatened with extinction [72][73]

References

  1. ^ U S Department of Energy (2008) Carbon Cycling and Biosequestration page 81, Workshop report DOE/SC-108, U.S. Department of Energy Office of Science.
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source: https://en.wikipedia.org/w/index.php?title=Marine_food_web&diff=947772612&oldid=947752417

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