The above Redfield ratios are obviously mean values and
they have been shown to largely vary. For instance, in the
deep ocean*, the C:N:P ratios were in fact of the order of
14:7:1, respectively. However, the concept of Redfield ratios
continues to be very used in biological oceanography
because it remains a powerful concept to understand the
processes that limit primary production.
Among the six major constitutive elements (C, H, N, O,
P, and S) that are essential to phytoplankton activity and
biomass synthesis, nitrogen and phosphorus (Box 14.2) are
often limiting because of their uptake by microorganisms. In
contrast, they are abundant in deep waters where most of the
organic matter exported from the euphotic layer is
mineralized. With the exception of coastal areas where
nutrient can be supplied by continents, vertical mixing,
atmospheric transport, and upwellings are the main
mechanisms that supply nutrients to the euphotic zone,
thereby canceling out the nutrient limitation of primary
production. These physical mechanisms do not take place
in summer when the high surface temperature combined
with moderate winds, generates a strong thermal stratification of the water column that prevents vertical mixing.
Under these conditions primary production is strongly limited by nutrients despite high levels of incident light.
Other minerals may also limit primary production. This is
the case of silicon (Box 14.1), a frustule constituent of
diatoms whose contribution to primary production and carbon export toward the deep ocean is very important
(Box 14.1). Iron is weakly abundant in oceans where its
importance was only recognized in the 1980s. It is present
as a co-factor of enzymes responsible for the reduction of
nitrates and fixation of atmospheric dinitrogen (cf.
Sect. 14.3). Iron sources are numerous (atmospheric dust,
deep waters, and continental rocks) but large oceanic zones
away from continents (equatorial zones, sub-arctic Pacific,
and Southern Ocean) exhibit iron concentrations which are
limiting for primary production. This explains the paradox
of the so-called high nutrients but low chlorophyll (HNLC)
areas where the phytoplankton biomass is low despite high
nitrate, phosphate, and silicon concentrations.
Fate of the Net Primary Production
Regarding the fate of net primary production (NPP) it is
necessary to distinguish three scenarios (Fig. 14.12):
1. All NPP is consumed by heterotrophs in the euphotic
zone. Primary production and respiration balance each
other and then there is no organic matter available for
export to the deep ocean. This situation, that is largely
under the control of the microbial network is the most
common in the open ocean;
2. The total respiration exceeds NPP. In contrast, there is
necessarily an input of allochthonous material to sustain
heterotrophic respiration. The system is therefore open.
The microbial assemblage will control the export of
organic matter to the deep ocean;
3. A portion of photo-autotrophs and/or of heterotrophs
escapes remineralization and is exported. The vertical
flux of the sinking particulate and dissolved organic matter corresponds to the exported primary production*,
most of which is remineralized in the water column
before reaching the sediments. Except in upwelling
areas, the products of this remineralization cannot return
back to the surface when the water column depth
becomes greater than the ventilation depth (i.e., permanent pycnocline*). CO 2 and nutrients accumulate in
the deep ocean, creating a strong vertical gradient. This
is one of the mechanisms of the biological pump,
the other being the long-term storage of carbon in
sediments.
Ammonium and other reduced forms of nitrogen (urea
and amino acids) are excreted when the NPP is
remineralized in the euphotic layer.
However, remineralization that takes place in the deep
ocean produces nitrate that can be brought back to the euphotic zone by vertical mixing and upwelling. There are therefore
two dinitrogen sources for phytoplankton: one autochthonous
source (i.e., ammonium) which leads to regenerated primary production*, and one allochthonous source (nitrate
and atmospheric dinitrogen fixed by diazotrophy, cf.
Sect. 14.3.2), which leads to new primary production*.
The f ratio* represents the fraction of total primary
production due to the regenerated production. It should be
noted that a complication was not sufficiently evaluated at
the definition of these terms: nitrification (cf. Sect. 14.3.4)
can occur in the photic layer, particularly in coastal waters,
and this can lead to the presence of autochthonous nitrate.
Another term often used is the e ratio* that is the fraction of
total primary production that is exported.
Under steady-state conditions, new primary production,
exported primary production and the net community
production* should be in balance. However, in situ
measurements only rarely express a steady state of the productive oceanic system.
Global Production
Remote sensing is the only tool that gives access to global
primary production, i.e., on a planetary scale. Satellites
provide ocean color images from which are derived
parameters used by numerical models to estimate net primary production:
1. The amount of phytoplanktonic chlorophyll;
2. The photosynthetically active radiation (wavelength
range of 400–700 nm) reaching the ocean surface;
3. Temperature.
14 Biogeochemical Cycles
529
they have been shown to largely vary. For instance, in the
deep ocean*, the C:N:P ratios were in fact of the order of
14:7:1, respectively. However, the concept of Redfield ratios
continues to be very used in biological oceanography
because it remains a powerful concept to understand the
processes that limit primary production.
Among the six major constitutive elements (C, H, N, O,
P, and S) that are essential to phytoplankton activity and
biomass synthesis, nitrogen and phosphorus (Box 14.2) are
often limiting because of their uptake by microorganisms. In
contrast, they are abundant in deep waters where most of the
organic matter exported from the euphotic layer is
mineralized. With the exception of coastal areas where
nutrient can be supplied by continents, vertical mixing,
atmospheric transport, and upwellings are the main
mechanisms that supply nutrients to the euphotic zone,
thereby canceling out the nutrient limitation of primary
production. These physical mechanisms do not take place
in summer when the high surface temperature combined
with moderate winds, generates a strong thermal stratification of the water column that prevents vertical mixing.
Under these conditions primary production is strongly limited by nutrients despite high levels of incident light.
Other minerals may also limit primary production. This is
the case of silicon (Box 14.1), a frustule constituent of
diatoms whose contribution to primary production and carbon export toward the deep ocean is very important
(Box 14.1). Iron is weakly abundant in oceans where its
importance was only recognized in the 1980s. It is present
as a co-factor of enzymes responsible for the reduction of
nitrates and fixation of atmospheric dinitrogen (cf.
Sect. 14.3). Iron sources are numerous (atmospheric dust,
deep waters, and continental rocks) but large oceanic zones
away from continents (equatorial zones, sub-arctic Pacific,
and Southern Ocean) exhibit iron concentrations which are
limiting for primary production. This explains the paradox
of the so-called high nutrients but low chlorophyll (HNLC)
areas where the phytoplankton biomass is low despite high
nitrate, phosphate, and silicon concentrations.
Fate of the Net Primary Production
Regarding the fate of net primary production (NPP) it is
necessary to distinguish three scenarios (Fig. 14.12):
1. All NPP is consumed by heterotrophs in the euphotic
zone. Primary production and respiration balance each
other and then there is no organic matter available for
export to the deep ocean. This situation, that is largely
under the control of the microbial network is the most
common in the open ocean;
2. The total respiration exceeds NPP. In contrast, there is
necessarily an input of allochthonous material to sustain
heterotrophic respiration. The system is therefore open.
The microbial assemblage will control the export of
organic matter to the deep ocean;
3. A portion of photo-autotrophs and/or of heterotrophs
escapes remineralization and is exported. The vertical
flux of the sinking particulate and dissolved organic matter corresponds to the exported primary production*,
most of which is remineralized in the water column
before reaching the sediments. Except in upwelling
areas, the products of this remineralization cannot return
back to the surface when the water column depth
becomes greater than the ventilation depth (i.e., permanent pycnocline*). CO 2 and nutrients accumulate in
the deep ocean, creating a strong vertical gradient. This
is one of the mechanisms of the biological pump,
the other being the long-term storage of carbon in
sediments.
Ammonium and other reduced forms of nitrogen (urea
and amino acids) are excreted when the NPP is
remineralized in the euphotic layer.
However, remineralization that takes place in the deep
ocean produces nitrate that can be brought back to the euphotic zone by vertical mixing and upwelling. There are therefore
two dinitrogen sources for phytoplankton: one autochthonous
source (i.e., ammonium) which leads to regenerated primary production*, and one allochthonous source (nitrate
and atmospheric dinitrogen fixed by diazotrophy, cf.
Sect. 14.3.2), which leads to new primary production*.
The f ratio* represents the fraction of total primary
production due to the regenerated production. It should be
noted that a complication was not sufficiently evaluated at
the definition of these terms: nitrification (cf. Sect. 14.3.4)
can occur in the photic layer, particularly in coastal waters,
and this can lead to the presence of autochthonous nitrate.
Another term often used is the e ratio* that is the fraction of
total primary production that is exported.
Under steady-state conditions, new primary production,
exported primary production and the net community
production* should be in balance. However, in situ
measurements only rarely express a steady state of the productive oceanic system.
Global Production
Remote sensing is the only tool that gives access to global
primary production, i.e., on a planetary scale. Satellites
provide ocean color images from which are derived
parameters used by numerical models to estimate net primary production:
1. The amount of phytoplanktonic chlorophyll;
2. The photosynthetically active radiation (wavelength
range of 400–700 nm) reaching the ocean surface;
3. Temperature.
14 Biogeochemical Cycles
529