F-ratio
Encyclopedia
In ocean
ic 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
). This fraction is significant because it is assumed to be directly related to the sinking (export) flux of organic
marine snow
from the surface ocean
by the biological pump
. The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production.
s (or the remains of organisms) transfers 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 trap
s 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.
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 utilised by autotroph
ic phytoplankton
to synthesise organic molecules such as amino acid
s (the building blocks of protein
s). 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.
However, ammonium can also be oxidised to nitrate (via nitrite), by the process of nitrification
. This is performed by different bacteria
in two stages :
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.
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 phytoplankton (such as diatom
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, oligotroph
ic food webs consisting of small, prokaryotic phytoplankton (such as Prochlorococcus
) which are kept in check by microzooplankton.
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 diagram to the right shows, if ammonium is indeed nitrified to nitrate in the ocean's surface waters it essentially "short 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 oligotroph
ic 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). 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 than 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.
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
ic biogeochemistry
Biogeochemistry
Biogeochemistry is the scientific discipline that involves the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment...
, the f-ratio is the fraction of total primary production
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
fuelled by nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
(as opposed to that fuelled by other nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
such as ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
). This fraction is significant because it is assumed to be directly related to the sinking (export) flux of organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
marine snow
Marine snow
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. The term was first coined by the explorer William Beebe as he...
from the surface ocean
Photic zone
The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...
by the biological pump
Biological pump
In oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.-Overview:...
. The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production.
Overview
Gravitational sinking of organismOrganism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s (or the remains of organisms) transfers carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
from the surface waters of the ocean to its deep interior
Aphotic zone
The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone...
. 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
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
. 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
Solubility pump
In oceanic biogeochemistry, the solubility pump is a physico-chemical process that transports carbon from the ocean's surface to its interior.-Overview:...
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 trap
Sediment trap
Sediment traps are instruments used in oceanography to measure the quantity of sinking particulate organic material in aquatic systems, usually oceans...
s 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
Proxy (statistics)
In statistics, a proxy variable is something that is probably not in itself of any great interest, but from which a variable of interest can be obtained...
for the sinking flux. The f-ratio is one such proxy.
"New" and "regenerated" production
Nitrite
The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...
(NO2−) and ammonium (NH4+), and more complex organic forms such as urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
((NH2)2CO). These forms are utilised by autotroph
Autotroph
An autotroph, or producer, is an organism that produces complex organic compounds from simple inorganic molecules using energy from light or inorganic chemical reactions . They are the producers in a food chain, such as plants on land or algae in water...
ic phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
to synthesise organic molecules such as amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s (the building blocks of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s). Grazing
Grazing
Grazing generally describes a type of feeding, in which a herbivore feeds on plants , and also on other multicellular autotrophs...
of phytoplankton by zooplankton
Plankton
Plankton are any drifting organisms 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 or taxonomic classification...
and larger organisms transfers this organic nitrogen up the food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
and throughout the marine food-web.
When nitrogenous organic molecules are ultimately metabolised
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
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
Nitrification
Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. Degradation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil...
. This is performed by different bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
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
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
(or as some other function
Function (mathematics)
In mathematics, a function associates one quantity, the argument of the function, also known as the input, with another quantity, the value of the function, also known as the output. A function assigns exactly one output to each input. The argument and the value may be real numbers, but they can...
of depth). In the ocean, this leads to a vertical separation of nitrification from primary production
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
, and confines it to the aphotic zone
Aphotic zone
The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this 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
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
or upwelled
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
into the surface ocean to repeat the cycle.
Consequently, the significance of new production lies in its connection to sinking material. At equilibrium
Dynamic equilibrium
A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state...
, 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 phytoplankton (such as diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
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, oligotroph
Oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...
ic food webs consisting of small, prokaryotic phytoplankton (such as Prochlorococcus
Prochlorococcus
Prochlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....
) which are kept in check by microzooplankton.
Assumptions
As the diagram to the right shows, if ammonium is indeed nitrified to nitrate in the ocean's surface waters it essentially "short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
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
Ecosystem model
An ecosystem model is an abstract, usually mathematical, representation of an ecological system , which is studied to gain a deeper understanding of the real system.Ecosystem models are formed by combining known ecological relations An ecosystem model is an abstract, usually mathematical,...
of the oligotroph
Oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...
ic subtropical gyre region (specifically the BATS
Bermuda Atlantic Time-series Study
The Bermuda Atlantic Time-series Study is a long-term oceanographic study by the Bermuda Institute of Ocean Sciences . Based on regular research cruises, it samples an area of the western Atlantic Ocean nominally at the coordinates...
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 than 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.