
In
ocean
The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...
ic
biogeochemistry
Biogeochemistry is the Branches of science, scientific discipline that involves the study of the chemistry, chemical, physics, physical, geology, geological, and biology, biological processes and reactions that govern the composition of the natu ...
, the f-ratio is the fraction of total
primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
fuelled by
nitrate
Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
(as opposed to that fuelled by other
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
compounds such as
ammonium
Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...
). The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first
papers estimating global oceanic production.
This fraction was originally believed significant because it appeared to directly relate to the
sinking (export) flux of
organic marine snow
In the deep ocean, marine snow (also known as "ocean dandruff") 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 ...
from the
surface ocean by the
biological pump
The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven Carbon sequestration, sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sedim ...
. However, this interpretation relied on the assumption of a strong depth-partitioning of a parallel process,
nitrification
''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 ent ...
, that more recent measurements has questioned.
Overview
Gravitational sinking of
organism
An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s (or the remains of organisms) transfers particulate organic
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
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
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's
carbon cycle
The carbon cycle is a part of the biogeochemical cycle where carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth. Other major biogeochemical cycles include the nitrogen cycle and the water cycl ...
. 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. 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

Bio-available nitrogen occurs in the ocean in several forms, including simple ionic forms such as nitrate (NO
3−),
nitrite
The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...
(NO
2−) and ammonium (NH
4+), and more complex organic forms such as
urea
Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...
((NH
2)
2CO). These forms are used by
autotroph
An autotroph is an organism that can convert Abiotic component, abiotic sources of energy into energy stored in organic compounds, which can be used by Heterotroph, other organisms. Autotrophs produce complex organic compounds (such as carbohy ...
ic
phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
to synthesise organic molecules such as
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s (the building blocks of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s).
Grazing
In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to free range (roam around) and consume wild vegetations in order to feed conversion ratio, convert the otherwise indigestible (by human diges ...
of phytoplankton by
zooplankton
Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...
and larger organisms transfers this organic nitrogen up the
food chain
A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...
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''.
However, ammonium can also be
oxidised to nitrate (via nitrite), by the process of nitrification. This is performed by different
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
in two stages :
:::NH
3 + O
2 → NO
2− + 3H
+ + 2e
−
:::NO
2− + H
2O → NO
3− + 2H
+ + 2e
−
Crucially, this process is believed to only occur in the absence of
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
(or as some other
function of depth). In the ocean, this leads to a vertical separation of nitrification from
primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
, 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''.
[
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. High f-ratio values are typically associated with productive ecosystems dominated by large, ]eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
phytoplankton (such as diatom
A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
s) 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
A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...
phytoplankton (such as ''Prochlorococcus
''Prochlorococcus'' is a genus of very small (0.6 μm) marine cyanobacteria with an unusual pigmentation ( chlorophyll ''a2'' and ''b2''). These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosyn ...
'') which are kept in check by microzooplankton.
Assumptions
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."[ However, subsequent observational work on the distribution of nitrification has found that nitrification can occur at shallower depths, and even within the photic zone.]
As the adjacent diagram shows, if ammonium is indeed nitrified to nitrate in the ocean's surface waters it essentially "short circuit
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...
s" 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
Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...
site) found that, on an annual basis, around 40% of surface nitrate was recently nitrified (rising to almost 90% during summer). 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.
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''.[ 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
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. Electron acceptors are oxidizing agents.
The electron accepting power of an electron acceptor is measured by its redox potential.
In the ...
for respiration, 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, its importance is less widespread than nitrification, although it can occur in association with primary producers.
See also
* Marine snow
In the deep ocean, marine snow (also known as "ocean dandruff") 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 ...
* Biological pump
The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven Carbon sequestration, sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sedim ...
References
{{lowercase title, title=f-ratio
Aquatic ecology
Biological oceanography
Chemical oceanography
Nitrates
Systems ecology
Biogeochemistry