F-ratio 4184055 205019222 2008-04-11T22:41:58Z Mdd 113850 /* See also */ [[Category:Systems ecology]] {{dablink|For other senses of this term, see [[f-ratio (disambiguation)]].}} {{lowercase|title=f-ratio}} 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]] [[chemical compound|compounds]] such as [[ammonium]]). This fraction is significant because it is assumed to be directly related to the [[export production|sinking (export) flux]] of [[organic compound|organic]] [[marine snow]] from the [[photic zone|surface ocean]] by the [[biological pump]]. The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first [[Academic publishing#Academic paper|papers]] estimating global oceanic production<ref name=eppley>Eppley, R. W. and Peterson, B. J. (1979) [http://www.nature.com/nature/journal/v282/n5740/abs/282677a0.html Particulate organic matter flux and planktonic new production in the deep ocean.] ''Nature'' '''282''', 677-680.</ref>. ==Overview== [[Gravity|Gravitational]] sinking of [[organism]]s (or the remains of organisms) transfers [[carbon]] from the surface waters of the ocean to its [[aphotic zone|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 must be left ''in situ'' over a long period to integrate the sinking flux. For this reason, scientists are interested in ocean properties that can be more easily measured, and that act as a [[Proxy (statistics)|proxy]] for the sinking flux. The f-ratio is one such proxy. =="New" and "regenerated" production== [[Image:F_ratio_diagram.gif|thumb|right|Diagram of '''new''' and '''regenerated''' production]] Bioavailable nitrogen occurs in the ocean in several forms, including simple ionic forms such as nitrate (NO<sub>3</sub><sup>&minus;</sup>), [[nitrite]] (NO<sub>2</sub><sup>&minus;</sup>) and ammonium (NH<sub>4</sub><sup>+</sup>), and more complex organic forms such as [[urea]] ((NH<sub>2</sub>)<sub>2</sub>CO). These forms are utilised by [[autotroph|autotrophic]] [[phytoplankton]] to synthesise organic molecules such as [[amino acid]]s (the building blocks of [[protein]]s). [[Grazing]] of phytoplankton by [[plankton|zooplankton]] and larger organisms transfers this organic nitrogen up the [[food chain]] and throughout the marine foodweb. When nitrogenous organic molecules are ultimately [[metabolism|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 foodweb. Primary production fuelled by ammonium in this way is thus referred to as '''regenerated production'''<ref name=dugdale>Dugdale, R. C. and Goering, J. J. (1967) [http://www.aslo.org/lo/toc/vol_12/issue_2/0196.pdf Uptake of new and regenerated forms of nitrogen in primary production.] ''Limnol. Oceanogr.'' '''12''', 196-206.</ref>. However, ammonium can also be [[oxidation|oxidised]] to nitrate (via nitrite), by the process of [[nitrification]]. This is performed by different [[bacteria]] in two stages : :::<b>NH<sub>3</sub></b> + O<sub>2</sub> → NO<sub>2</sub><sup>&minus;</sup> + 3H<sup>+</sup> + 2e<sup>&minus;</sup> <br> :::NO<sub>2</sub><sup>&minus;</sup> + H<sub>2</sub>O → <b>NO<sub>3</sub><sup>&minus;</sup></b> + 2H<sup>+</sup> + 2e<sup>&minus;</sup> Crucially, this process is believed to only occur in the absence of [[light]] (or as some other [[function (mathematics)|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'''<ref name=dugdale/>. The figure at the head of this section illustrates this. Nitrate and ammonium are taken up by primary producers, processed through the foodweb, 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 [[turbulence|mixed]] or [[upwelling|upwelled]] into the surface ocean to repeat the cycle. Consequently, the significance of new production lies in its connection to sinking material. At [[dynamic equilibrium|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<ref>Allen, A. E., Howard-Jones, M. H., Booth, M. G., Frischer, M. E., Verity, P. G., Bronk, D. A. and Sanderson, M. P. (2002) [http://dx.doi.org/10.1016/s0924-7963(02)00171-9 Importance of heterotrophic bacterial assimilation of ammonium and nitrate in the Barents Sea during summer.] ''Journal of Marine Systems'' '''38''', 93-108.</ref>. 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, [[oligotrophic]] food webs consisting of small, [[prokaryotic]] phytoplankton (such as ''[[Prochlorococcus]]'') which are kept in check by microzooplankton<ref>Laws, E. A., Falkowski, P. G., Smith, W. O., Ducklow, H. and McCarthy, J. J. (2000) [http://www.agu.org/pubs/crossref/2000/1999GB001229.shtml Temperature effects on export production in the open ocean.] ''Global Biogeochemical Cycles'' '''14''', 1231-1246.</ref><ref name=dunne>Dunne, J. P., Armstrong, R. A., Gnanadesikan, A., and Sarmiento, J. L. (2005) [http://www.agu.org/pubs/crossref/2005/2004GB002390.shtml Empirical and mechanistic models for the particle export ratio.] ''Global Biogeochemical Cycles'' '''19''', GB4026, doi:10.1029/2005GB002390.</ref>. ==Assumptions== [[Image:F_ratio_diagram_2.gif|thumb|right|Is nitrification really confined to the photic zone?]] 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"<ref name=eppley/>. However, subsequent observational work on the distribution of nitrification has found that nitrification can occur at shallower depths, and even within the photic zone<ref>Dore, J. E. and Karl, D. M. (1996) [http://www.jstor.org/view/00243590/dm995043/99p0297a/0 Nitrification in the euphotic zone as a source for nitrite, nitrate, and nitrous oxide at Station ALOHA.] ''Limnol. Oceanogr.'' '''41''', 1619-1628.</ref><ref>Raimbault, P., Slawyk, G., Boudjellal, B., Coatanoan, C., Conan, P., Coste, B., Garcia, N., Moutin, T. and Pujo-Pay, M. (1999) [http://www.agu.org/pubs/crossref/1999/1998JC900004.shtml Carbon and nitrogen uptake and export in the equatorial Pacific at 150°W: Evidence of an efficient regenerated production cycle.] ''J. Geophys. Res.'' '''104''', 3341-3356.</ref><ref>Diaz, F. and Raimbault, P. (2000) [http://www.int-res.com/abstracts/meps/v197/p51-65/ 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.</ref>. 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|ecosystem model]] of the [[oligotrophic]] [[subtropical gyre]] region (specifically the [[Bermuda Atlantic Time-series Study|BATS]] site) found that, on an annual basis, around 40% of surface nitrate was recently nitrified (rising to almost 90% during summer)<ref name=martin06>Martin, A. P. and Pondaven, P. (2006) [http://www.agu.org/pubs/crossref/2006/2005GB002608.shtml New primary production and nitrification in the western subtropical North Atlantic: a modelling study.] ''Global Biogeochemical Cycles'' '''20''', doi:10.1029/2005GB002608.</ref>. 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'''<ref name=dunne/>. 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. <br style="clear:both;"> ==References== {{reflist|2}} ==See also== * [[Biological pump]] * [[Nitrification]] * [[Primary production]] [[Category:Aquatic ecology]] [[Category:Biological oceanography]] [[Category:Chemical oceanography]] [[Category:Geochemistry]] [[Category:Nitrogen]] [[Category:Oceanography]] [[Category:Systems ecology]]