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Inoceanicbiogeochemistry, thef-ratio is the fraction of totalprimary production fuelled bynitrate (as opposed to that fuelled by othernitrogencompounds such asammonium). The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the firstpapers estimating global oceanic production.[1] This fraction was originally believed significant because it appeared to directly relate to thesinking (export) flux oforganicmarine snow from thesurface ocean by thebiological 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]
Gravitational sinking oforganisms (or the remains of organisms) transfers particulate organiccarbon from the surface waters of the ocean to itsdeep interior. This process is known as the biological pump, and quantifying it is of interest to scientists because it is an important aspect of theEarth'scarbon 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 thesolubility 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 deployingsediment traps which intercept and store material as it sinks down thewater column. However, this is a relatively difficult process, since traps can be awkward to deploy or recover, and they must be leftin 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 aproxy 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 asurea ((NH2)2CO). These forms are used byautotrophicphytoplankton to synthesise organic molecules such asamino acids (the building blocks ofproteins).Grazing of phytoplankton byzooplankton and larger organisms transfers this organic nitrogen up thefood chain and throughout the marinefood-web.
When nitrogenous organic molecules are ultimatelymetabolised by organisms, they are returned to the water column as ammonium (or more complex molecules that are then metabolised to ammonium). This is known asregeneration, 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 asregenerated production.[5]
However, ammonium can also beoxidised to nitrate (via nitrite), by the process of nitrification. This is performed by differentbacteria in two stages :
Crucially, this process is believed to only occur in the absence oflight (or as some otherfunction of depth). In the ocean, this leads to a vertical separation of nitrification fromprimary production, and confines it to theaphotic 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 asnew 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 ultimatelymixed orupwelled into the surface ocean to repeat the cycle.
Consequently, the significance of new production lies in its connection to sinking material. Atequilibrium, 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 asdiatoms) 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 asProchlorococcus) which are kept in check by microzooplankton.[7][8]
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, anecosystem model of theoligotrophicsubtropical gyre region (specifically theBATS 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 thepe-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 isdissimilatory nitrate reduction to ammonium (DNRA). In low oxygen environments, such asoxygen minimum zones andseafloor sediments,chemoorganoheterotrophic microbes use nitrate as anelectron acceptor forrespiration,[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]
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