Redfield ratioFalkowski & Davis, 2004 |
|
| All organisms are composed of six major elements: hydrogen,
carbon, nitrogen, oxygen, phosophorus and sulphur. The proportions among
various organisms vary. In the ocean most of the biomass is contained
in small drifting organisms (plankton) that are rich in nitrogen. (Plankton
are primarily euks). Plankton are essentially functionally similar ensembles
of metabolites, encased in shells formed from the most readily available
ingredients. Much plankton is consumed by other plankton with similar
chemical compositions. The result is that the average nitrogen:phosphorus
(N:P) ratios of plankton in the oceans are remarkably similar through
the world, averaging 16:1 by atoms. When these organisms or their body
parts sink into the ocean, their energy-rich bodies are consumed by bacteria
which, in aerobic conditions, oxidize the organic matter to form dissolved
nutrients, especially CO2, NO3^- and PO4^3-. IN 1934, Alfred Redfield
wrote a now classic paper proposing that the N:P ratio causes the ocean
to have a similar ratio of dissolved nitrate and phosphte ions. If devoid
of life, the chemical composition of the oceans would be markedly differnt.
This ratio is based on empirical evidence of these ions and the composition
of the plankton and represents a global average of interior ocean chemistry.
THe reason this ratio is remarkably similar across ocean interiors is
due to residence time of the two elements in the ocean (roughly 10^4 years)
relative to the ocean's circulation (roughly 10^3 years). As the residence
times exceeds the mixing time by an order of magnitutde, it is not surprising
that the No3/PO4 ratios are relatively constant. Thus the Redfield ratio
is an emergent property reflecting the interaction of multiple processes.
This ratio can change if any of these emergent properties changes if the
timescale are comparable to the residence times of the nutrients. Phosphorus
in the basin is highly variable spatially but not temporally. |
|
Phosophorus at Cuatro Cienegaspersonal communication J.Watts |
|
SRP (inorganic dissolved phosphorus) varies between 0.05 - 0.3 uM, with most poza's in the .1 - .15 uM range. Total phosphorus ranges between 0.05 - .4 uM, with most of the poza's in the .1 - .3 uM range. For reference, total nitrogen is in the 50 - 100 uM range, TN:TP ratio varies from 100 - 500, and the DIN:DIP varies between 20 - 50. These phosphorus numbers are at the low end, not extreme for oligotrophic P-limited sites. In general, much of inland waters are P-limited, although that isn't as common in arid regions. However, these numbers are considered low relative to lakes that have development around them and therefore anthropogenic P input. |
|
Archean phosphoruspersonal communicaiton, J. Elser C:N:P stoichiometry in the Archean - perhaps early food webs faced severe stoichiometric constraints due to high C:P ratios in primary producer biomass. (See Elser J.J. (2003) Biological stoichiometry: a theoretical framework connecting ecosystem ecology, evolution, and biochemistry for application in astrobiology. International Journal of Astrobiology, 3, 185-193). This pertains to benthic primary production not pelagic. However, it is conceivable that early oceans generated organic matter at ratios divergent from the canonical Redfield values (106:16:1), as hydrographic and geochemical conditions may have been such that severe P-limited production was operating (much P was still in the crust; atmospheric CO2 levels were high; hydrographic conditions / mixing perhaps was relatively weak; all of this might have generated organic matter with high C:P and N:P ratios, emphasis on (MIGHT.)
|
|
 |
|
References |
|
| Sterner R.W. & Elser J.J. (2002) Ecological Watanabe Y., Martini J. & Ohmoto H. (2000) Geochemical evidence for terrestrial ecosystems 2.6 billion years ago. Nature, 408, 574-578. Quigg A., Finkel Z.V., Irwin A.J., Rosenthal Y., Ho T.Y., Reinfelder
J.R., Schofield O., Morel F.M.M. & Falkowski P.G. (2003) The evolutionary
inheritance of elemental stoichiometry in marine phytoplankton. Nature,
425, 291-294 |
|