F-ratio (oceanography)

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Empirically derived effect of temperature and Net Primary Productivity on the f-ratio, and approximate values for some large ocean regions. F-ratio.png
Empirically derived effect of temperature and Net Primary Productivity on the f-ratio, and approximate values for some large ocean regions.

In oceanic biogeochemistry, the f-ratio is the fraction of total primary production fuelled by nitrate (as opposed to that fuelled by other nitrogen compounds such as ammonium). The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production. [1] This fraction was originally believed significant because it appeared to directly relate to the sinking (export) flux of organic marine snow from the surface ocean by the biological pump. However, this interpretation relied on the assumption of a strong depth-partitioning of a parallel process, nitrification, that more recent measurements has questioned. [2]

Contents

Overview

Gravitational sinking of organisms (or the remains of organisms) transfers particulate organic carbon from the surface waters of the ocean to its deep interior. This process is known as the biological pump, and quantifying it is of interest to scientists because it is an important aspect of the Earth's carbon cycle. Essentially, this is because carbon transported to the deep ocean is isolated from the atmosphere, allowing the ocean to act as a reservoir of carbon. This biological mechanism is accompanied by a physico-chemical mechanism known as the solubility pump which also acts to transfer carbon to the ocean's deep interior.

Measuring the flux of sinking material (so-called marine snow) is usually done by deploying sediment traps which intercept and store material as it sinks down the water column. However, this is a relatively difficult process, since traps can be awkward to deploy or recover, and they must be left in situ over a long period to integrate the sinking flux. Furthermore, they are known to experience biases and to integrate horizontal as well as vertical fluxes because of water currents. [3] [4] For this reason, scientists are interested in ocean properties that can be more easily measured, and that act as a proxy for the sinking flux. The f-ratio is one such proxy.

"New" and "regenerated" production

Diagram of new and regenerated production with nitrification confined to the aphotic zone. This was used to frame the f-ratio and its link to export production. F ratio diagram.gif
Diagram of new and regenerated production with nitrification confined to the aphotic zone. This was used to frame the f-ratio and its link to export production.

Bio-available nitrogen occurs in the ocean in several forms, including simple ionic forms such as nitrate (NO3−), nitrite (NO2−) and ammonium (NH4+), and more complex organic forms such as urea ((NH2)2CO). These forms are used by autotrophic phytoplankton to synthesise organic molecules such as amino acids (the building blocks of proteins). Grazing of phytoplankton by zooplankton and larger organisms transfers this organic nitrogen up the food chain and throughout the marine food-web.

When nitrogenous organic molecules are ultimately metabolised by organisms, they are returned to the water column as ammonium (or more complex molecules that are then metabolised to ammonium). This is known as regeneration, since the ammonium can be used by phytoplankton, and again enter the food-web. Primary production fuelled by ammonium in this way is thus referred to as regenerated production. [5]

However, ammonium can also be oxidised to nitrate (via nitrite), by the process of nitrification. This is performed by different bacteria in two stages :

NH3 + O2 → NO2− + 3H+ + 2e−
NO2− + H2O → NO3− + 2H+ + 2e−

Crucially, this process is believed to only occur in the absence of light (or as some other function of depth). In the ocean, this leads to a vertical separation of nitrification from primary production, and confines it to the aphotic zone. This leads to the situation whereby any nitrate in the water column must be from the aphotic zone, and must have originated from organic material transported there by sinking. Primary production fuelled by nitrate is, therefore, making use of a "fresh" nutrient source rather than a regenerated one. Production by nitrate is thus referred to as new production. [5]

The figure at the head of this section illustrates this. Nitrate and ammonium are taken up by primary producers, processed through the food-web, and then regenerated as ammonium. Some of this return flux is released into the surface ocean (where it is available again for uptake), while some is returned at depth. The ammonium returned at depth is nitrified to nitrate, and ultimately mixed or upwelled into the surface ocean to repeat the cycle.

Consequently, the significance of new production lies in its connection to sinking material. At equilibrium, the export flux of organic material sinking into the aphotic zone is balanced by the upward flux of nitrate. By measuring how much nitrate is consumed by primary production, relative to that of regenerated ammonium, one should be able to estimate the export flux indirectly.

As an aside, the f-ratio can also reveal important aspects of local ecosystem function. [6] High f-ratio values are typically associated with productive ecosystems dominated by large, eukaryotic phytoplankton (such as diatoms) that are grazed by large zooplankton (and, in turn, by larger organisms such as fish). By contrast, low f-ratio values are generally associated with low biomass, oligotrophic food webs consisting of small, prokaryotic phytoplankton (such as Prochlorococcus ) which are kept in check by microzooplankton. [7] [8]

Assumptions

Diagram of new and regenerated production but including photic zone nitrification. By providing an additional source of surface nitrate, this disrupts the original inferred link between the f-ratio and export production. F ratio diagram 2.gif
Diagram of new and regenerated production but including photic zone nitrification. By providing an additional source of surface nitrate, this disrupts the original inferred link between the f-ratio and export production.

A fundamental assumption in this interpretation of the f-ratio is the spatial separation of primary production and nitrification. Indeed, in their original paper, Eppley & Peterson noted that: "To relate new production to export requires that nitrification in the euphotic zone be negligible." [1] However, subsequent observational work on the distribution of nitrification has found that nitrification can occur at shallower depths, and even within the photic zone. [9] [10] [11]

As the adjacent diagram shows, if ammonium is indeed nitrified to nitrate in the ocean's surface waters it essentially "short circuits" the deep pathway of nitrate. In practice, this would lead to an overestimation of new production and a higher f-ratio, since some of the ostensibly new production would actually be fuelled by recently nitrified nitrate that had never left the surface ocean. After including nitrification measurements in its parameterisation, an ecosystem model of the oligotrophic subtropical gyre region (specifically the BATS site) found that, on an annual basis, around 40% of surface nitrate was recently nitrified (rising to almost 90% during summer). [12] A further study synthesising geographically diverse nitrification measurements found high variability but no relationship with depth, and applied this in a global-scale model to estimate that up to a half of surface nitrate is supplied by surface nitrification rather than upwelling. [2]

Although measurements of the rate of nitrification are still relatively rare, they do suggest that the f-ratio is not as straightforward a proxy for the biological pump as was once thought. For this reason, some workers have proposed distinguishing between the f-ratio and the ratio of particulate export to primary production, which they term the pe-ratio. [8] While quantitatively different from the f-ratio, the pe-ratio shows similar qualitative variation between high productivity/high biomass/high export regimes and low productivity/low biomass/low export regimes.

In addition, a further process that potentially complicates the use of the f-ratio to estimate "new" and "regenerated" production is dissimilatory nitrate reduction to ammonium (DNRA). In low oxygen environments, such as oxygen minimum zones and seafloor sediments, chemoorganoheterotrophic microbes use nitrate as an electron acceptor for respiration, [13] reducing it to nitrite, then to ammonium. Since, like nitrification, DNRA alters the balance in the availability of nitrate and ammonium, it has the potential to introduce inaccuracy to the calculated f-ratio. However, as DNRA's occurrence is limited to anaerobic situations, [14] its importance is less widespread than nitrification, although it can occur in association with primary producers. [15] [16]

See also

Related Research Articles

<span class="mw-page-title-main">Nitrogen cycle</span> Biogeochemical cycle by which nitrogen is converted into various chemical forms

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

<span class="mw-page-title-main">Biological pump</span> Carbon capture process in oceans

The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments. In other words, it is a biologically mediated process which results in the sequestering of carbon in the deep ocean away from the atmosphere and the land. The biological pump is the biological component of the "marine carbon pump" which contains both a physical and biological component. It is the part of the broader oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis (soft-tissue pump), as well as the cycling of calcium carbonate (CaCO3) formed into shells by certain organisms such as plankton and mollusks (carbonate pump).

<span class="mw-page-title-main">Nitrification</span> Biological oxidation of ammonia/ammonium to nitrate

Nitrification is the biological oxidation of ammonia to nitrate via the intermediary nitrite. Nitrification is an important step in the nitrogen cycle in soil. The process of complete nitrification may occur through separate organisms or entirely within one organism, as in comammox bacteria. The transformation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an aerobic process performed by small groups of autotrophic bacteria and archaea.

The mesopelagiczone, also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at the depth where only 1% of incident light reaches and ends where there is no light; the depths of this zone are between approximately 200 to 1,000 meters below the ocean surface.

High-nutrient, low-chlorophyll (HNLC) regions are regions of the ocean where the abundance of phytoplankton is low and fairly constant despite the availability of macronutrients. Phytoplankton rely on a suite of nutrients for cellular function. Macronutrients are generally available in higher quantities in surface ocean waters, and are the typical components of common garden fertilizers. Micronutrients are generally available in lower quantities and include trace metals. Macronutrients are typically available in millimolar concentrations, while micronutrients are generally available in micro- to nanomolar concentrations. In general, nitrogen tends to be a limiting ocean nutrient, but in HNLC regions it is never significantly depleted. Instead, these regions tend to be limited by low concentrations of metabolizable iron. Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport.

<span class="mw-page-title-main">Redfield ratio</span>

The Redfield ratio or Redfield stoichiometry is the consistent atomic ratio of carbon, nitrogen and phosphorus found in marine phytoplankton and throughout the deep oceans.

<i>Nitrobacter</i> Genus of bacteria

Nitrobacter is a genus comprising rod-shaped, gram-negative, and chemoautotrophic bacteria. The name Nitrobacter derives from the Latin neuter gender noun nitrum, nitri, alkalis; the Ancient Greek noun βακτηρία, βακτηρίᾱς, rod. They are non-motile and reproduce via budding or binary fission. Nitrobacter cells are obligate aerobes and have a doubling time of about 13 hours.

<span class="mw-page-title-main">Ocean fertilization</span> Type of climate engineering

Ocean fertilization or ocean nourishment is a type of technology for carbon dioxide removal from the ocean based on the purposeful introduction of plant nutrients to the upper ocean to increase marine food production and to remove carbon dioxide from the atmosphere. Ocean nutrient fertilization, for example iron fertilization, could stimulate photosynthesis in phytoplankton. The phytoplankton would convert the ocean's dissolved carbon dioxide into carbohydrate, some of which would sink into the deeper ocean before oxidizing. More than a dozen open-sea experiments confirmed that adding iron to the ocean increases photosynthesis in phytoplankton by up to 30 times.

<i>Nitrosopumilus</i> Genus of archaea

Nitrosopumilus maritimus is an extremely common archaeon living in seawater. It is the first member of the Group 1a Nitrososphaerota to be isolated in pure culture. Gene sequences suggest that the Group 1a Nitrososphaerota are ubiquitous with the oligotrophic surface ocean and can be found in most non-coastal marine waters around the planet. It is one of the smallest living organisms at 0.2 micrometers in diameter. Cells in the species N. maritimus are shaped like peanuts and can be found both as individuals and in loose aggregates. They oxidize ammonia to nitrite and members of N. maritimus can oxidize ammonia at levels as low as 10 nanomolar, near the limit to sustain its life. Archaea in the species N. maritimus live in oxygen-depleted habitats. Oxygen needed for ammonia oxidation might be produced by novel pathway which generates oxygen and dinitrogen. N. maritimus is thus among organisms which are able to produce oxygen in dark.

<span class="mw-page-title-main">Marine snow</span> Shower of organic detritus in the ocean

In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below, which is referred to as the biological pump. Export production is the amount of organic matter produced in the ocean by primary production that is not recycled (remineralised) before it sinks into the aphotic zone. Because of the role of export production in the ocean's biological pump, it is typically measured in units of carbon. The term was coined by explorer William Beebe as observed from his bathysphere. As the origin of marine snow lies in activities within the productive photic zone, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic activity and ocean currents. Marine snow can be an important food source for organisms living in the aphotic zone, particularly for organisms that live very deep in the water column.

<span class="mw-page-title-main">Bacterioplankton</span> Bacterial component of the plankton that drifts in the water column

Bacterioplankton refers to the bacterial component of the plankton that drifts in the water column. The name comes from the Ancient Greek word πλανκτος, meaning "wanderer" or "drifter", and bacterium, a Latin term coined in the 19th century by Christian Gottfried Ehrenberg. They are found in both seawater and freshwater.

In biological oceanography, new production is supported by nutrient inputs from outside the euphotic zone, especially upwelling of nutrients from deep water, but also from terrestrial and atmosphere sources. New production depends on mixing and vertical advective processes associated with the circulation.

<span class="mw-page-title-main">Oceanic carbon cycle</span> Ocean/atmosphere carbon exchange process

The oceanic carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon and organic carbon. Part of the marine carbon cycle transforms carbon between non-living and living matter.

<span class="mw-page-title-main">Particulate organic matter</span>

Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 millimeters (53 μm) to 2 millimeters.

Dissimilatory nitrate reduction to ammonium (DNRA), also known as nitrate/nitrite ammonification, is the result of anaerobic respiration by chemoorganoheterotrophic microbes using nitrate (NO3−) as an electron acceptor for respiration. In anaerobic conditions microbes which undertake DNRA oxidise organic matter and use nitrate (rather than oxygen) as an electron acceptor, reducing it to nitrite, then ammonium (NO3−→NO2−→NH4+).

<span class="mw-page-title-main">Marine biogeochemical cycles</span>

Marine biogeochemical cycles are biogeochemical cycles that occur within marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. These biogeochemical cycles are the pathways chemical substances and elements move through within the marine environment. In addition, substances and elements can be imported into or exported from the marine environment. These imports and exports can occur as exchanges with the atmosphere above, the ocean floor below, or as runoff from the land.

An oxygen minimum zone (OMZ) is characterized as an oxygen-deficient layer in the world's oceans. Typically found between 200m to 1500m deep below regions of high productivity, such as the western coasts of continents. OMZs can be seasonal following the spring-summer upwelling season. Upwelling of nutrient-rich water leads to high productivity and labile organic matter, that is respired by heterotrophs as it sinks down the water column. High respiration rates deplete the oxygen in the water column to concentrations of 2 mg/L or less forming the OMZ. OMZs are expanding, with increasing ocean deoxygenation. Under these oxygen-starved conditions, energy is diverted from higher trophic levels to microbial communities that have evolved to use other biogeochemical species instead of oxygen, these species include Nitrate, Nitrite, Sulphate etc. Several Bacteria and Archea have adapted to live in these environments by using these alternate chemical species and thrive. The most abundant phyla in OMZs are Pseudomonadota, Bacteroidota, Actinomycetota, and Planctomycetota.

<span class="mw-page-title-main">Viral shunt</span>

The viral shunt 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. The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM.

<span class="mw-page-title-main">Martin curve</span> Mathematical representation of particulate organic carbon export to ocean floor

The Martin curve is a power law used by oceanographers to describe the export to the ocean floor of particulate organic carbon (POC). The curve is controlled with two parameters: the reference depth in the water column, and a remineralisation parameter which is a measure of the rate at which the vertical flux of POC attenuates. It is named after the American oceanographer John Martin.

Low-nutrient, low-chlorophyll (LNLC)regions are aquatic zones that are low in nutrients and consequently have low rate of primary production, as indicated by low chlorophyll concentrations. These regions can be described as oligotrophic, and about 75% of the world's oceans encompass LNLC regions. A majority of LNLC regions are associated with subtropical gyres but are also present in areas of the Mediterranean Sea, and some inland lakes. Physical processes limit nutrient availability in LNLC regions, which favors nutrient recycling in the photic zone and selects for smaller phytoplankton species. LNLC regions are generally not found near coasts, owing to the fact that coastal areas receive more nutrients from terrestrial sources and upwelling. In marine systems, seasonal and decadal variability of primary productivity in LNLC regions is driven in part by large-scale climatic regimes leading to important effects on the global carbon cycle and the oceanic carbon cycle.

References

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  2. 1 2 Yool, A.; Martin, A.P.; Fernández, C.; Clark, D.R. (2007). "The significance of nitrification for oceanic new production". Nature . 447 (7147): 999–1002. Bibcode:2007Natur.447..999Y. doi:10.1038/nature05885. PMID   17581584. S2CID   4416535.
  3. ↑ Thomas, S.; Ridd, P.V. (2004). "Review of methods to measure short time scale sediment accumulation". Marine Geology. 207 (1–4): 95–114. Bibcode:2004MGeol.207...95T. doi:10.1016/j.margeo.2004.03.011.
  4. ↑ Buesseler, K.O.; et al. (2007). "An assessment of the use of sediment traps for estimating upper ocean particle fluxes" (PDF). J. Mar. Res. 65 (3): 345–416. doi:10.1357/002224007781567621. hdl:1912/1803. ISSN   0022-2402.
  5. 1 2 Dugdale, R.C.; Goering, J.J. (1967). "Uptake of new and regenerated forms of nitrogen in primary productivity". Limnol. Oceanogr. 12 (2): 196–206. Bibcode:1967LimOc..12..196D. doi: 10.4319/lo.1967.12.2.0196 .
  6. ↑ Allen, A.E.; Howard-Jones, M.H.; Booth, M.G.; Frischer, M.E.; Verity, P.G.; Bronk, D.A.; Sanderson, M.P. (2002). "Importance of heterotrophic bacterial assimilation of ammonium and nitrate in the Barents Sea during summer". Journal of Marine Systems. 38 (1–2): 93–108. Bibcode:2002JMS....38...93A. doi:10.1016/s0924-7963(02)00171-9.
  7. ↑ Laws, E.A.; Falkowski, P.G.; Smith, W.O.; Ducklow, H.; McCarthy, J.J. (2000). "Temperature effects on export production in the open ocean". Global Biogeochemical Cycles. 14 (4): 1231–1246. Bibcode:2000GBioC..14.1231L. doi: 10.1029/1999GB001229 .
  8. 1 2 Dunne, J.P.; Armstrong, R.A.; Gnanadesikan, A.; Sarmiento, J.L. (2005). "Empirical and mechanistic models for the particle export ratio". Global Biogeochemical Cycles. 19 (4): GB4026. Bibcode:2005GBioC..19.4026D. doi: 10.1029/2004GB002390 . S2CID   9019774.
  9. ↑ Dore, J.E.; Karl, D.M. (1996). "Nitrification in the euphotic zone as a source for nitrite, nitrate, and nitrous oxide at Station ALOHA". Limnol. Oceanogr. 41 (8): 1619–1628. Bibcode:1996LimOc..41.1619D. doi: 10.4319/lo.1996.41.8.1619 . JSTOR   00243590.
  10. ↑ Raimbault, P.; Slawyk, G.; Boudjellal, B.; Coatanoan, C.; Conan, P.; Coste, B.; Garcia, N.; Moutin, T.; Pujo-Pay, M. (1999). "Carbon and nitrogen uptake and export in the equatorial Pacific at 150°W: Evidence of an efficient regenerated production cycle". J. Geophys. Res. 104 (C2): 3341–3356. Bibcode:1999JGR...104.3341R. doi: 10.1029/1998JC900004 . S2CID   55579865.
  11. ↑ Diaz, F.; Raimbault, P. (2000). "Nitrogen regeneration and dissolved organic nitrogen release during spring in a NW Mediterranean coastal zone (Gulf of Lions): implications for the estimation of new production". Mar. Ecol. Prog. Ser. 197: 51–65. Bibcode:2000MEPS..197...51D. doi: 10.3354/meps197051 .
  12. ↑ Martin, A.P.; Pondaven, P. (2006). "New primary production and nitrification in the western subtropical North Atlantic: a modelling study". Global Biogeochemical Cycles. 20 (4): n/a. Bibcode:2006GBioC..20.4014M. doi: 10.1029/2005GB002608 .
  13. ↑ Kraft, B. Strous, M. and Tegetmeyer, H. E. (2011). "Microbial nitrate respiration – Genes, enzymes and environmental distribution". Journal of Biotechnology. 155 (1): 104–117. doi:10.1016/j.jbiotec.2010.12.025. PMID   21219945.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ↑ Lam, Phyllis and Kuypers, Marcel M. M. (2011). "Microbial Nitrogen Processes in Oxygen Minimum Zones". Annual Review of Marine Science. 3: 317–345. Bibcode:2011ARMS....3..317L. doi:10.1146/annurev-marine-120709-142814. hdl: 21.11116/0000-0001-CA25-2 . PMID   21329208.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. ↑ Kamp, Anja; Beer, Dirk de; Nitsch, Jana L.; Lavik, Gaute; Stief, Peter (2011-04-05). "Diatoms respire nitrate to survive dark and anoxic conditions". Proceedings of the National Academy of Sciences. 108 (14): 5649–5654. Bibcode:2011PNAS..108.5649K. doi: 10.1073/pnas.1015744108 . ISSN   0027-8424. PMC   3078364 . PMID   21402908.
  16. ↑ Kamp, Anja; Stief, Peter; Knappe, Jan; Beer, Dirk de (2013-12-02). "Response of the Ubiquitous Pelagic Diatom Thalassiosira weissflogii to Darkness and Anoxia". PLOS ONE. 8 (12): e82605. Bibcode:2013PLoSO...882605K. doi: 10.1371/journal.pone.0082605 . ISSN   1932-6203. PMC   3846789 . PMID   24312664.
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