Regional Anomalies in Nutrient Limitation
79
et al. (2001). There is now ample satellite evidence of phytoplankton blooms in this part
of the ocean other than those we might attribute to the island wake of the Galapagos.
Fiedler, Philbrick, and Chavez (1991) showed that both production rate and productivity in the eastern Pacific respond to the seasonal pattern of divergence (equatorial
and countercurrent) and coastal upwelling, forced by the consequent relative depths of
mixing and of light penetration. Now that we can routinely access 7- and 30-day images
of this region, both as surface chlorophyll and translated into productivity, it is very
clear that the observed pattern is a direct response to physical forcing. We are now quite
familiar with the consequences of the range of circulation processes that occur in this
region, responding to both seasonal and Niño/Niña cycles; the consequences of reduced
upwelling at the equator in the Niño situation are plain for all to see. Of course, we
anticipate that strong vertical motion will be induced in the Galapagos Islands wake;
this is now seen to be just one feature in the larger pattern of chlorophyll accumulation
characteristic of the eastern tropical Pacific.
A nutrient budget of the eastern Pacific high-S region, at 140
W and from 3
S to 6
N,
suggests that flux of iron and nitrate to the equatorial photic zone occurs in sufficiently
differing ratios that most of the nitrate remains unused. Coale et al. (1996a) assume a
molar C:Fe ratio of 167,000, a sustained upwelling velocity of 123 m d
−1 , and a subaerial
Fe supply rate of 5–25 nM m
−2 d
−1 . Since the observed new production rate here is
5–30 M C m
−2 d
−1 , iron must limit growth to <25% of the potential offered by nitrate
flux at this location, of which >80% remains unused. However, at 9
N, where upward
NO 3 flux is much lower, nitrogen was found to be the limiting element and is assimilated
down to very low concentrations indeed, as it is at the ADIOS station at about the same
longitude, but centrally in the gyral circulation at 26
N.
I have seen no discussion of the fact that, although a different nutrient is limiting
in each, both tropical Pacific and Atlantic have rather similar geographic patterns of
chlorophyll accumulation. In both oceans, just below the nutricline, nitrate and phosphate
take similar values, respectively 32–36 M kg
−1 NO 3 and 15–25 M kg
−1 PO 4 . However,
residual nitrate values in the euphotic zone of the two oceans are very different (Atlantic,
about 05 M kg
−1 ; Pacific, 2–10 M kg
−1 NO 3 ). Differences in euphotic zone phosphate
between the two oceans are also significant, but weaker than for nitrate. The simplest, and
perhaps correct, explanation for this difference in nutrient availability lies in the greater
subaerial Fe deposition in Atlantic low latitudes where euphotic zone nitrate is reduced
to near-limiting values by phytoplankton uptake. Thus, I suggest that in each ocean it
is the supply of an element by vertical flux across a nutricline that limits phytoplankton
growth. This is a very different matter from the paradigmatic and unthinking “It is
established that the high-nitrate, low-chlorophyll character of these waters is attributable
to the limited availability of iron in the photic zone” that we read too often.
Despite all this, it would be simplistic to suggest that the greater accumulation of
phytoplankton observed in satellite chlorophyll fields in Atlantic low latitudes is Fe-driven.
I have seen this suggestion made, but it ignores many other factors, of which the principal
are the different orientation of coastal margins in the two oceans and the fact that two
great rivers—Amazon and Congo—open into the Atlantic at near-equatorial latitudes.
These issues will be discussed in Chapter 9, so here it is only necessary to point out that
each represents a major input at very shallow depth of a mixed bag not only of nutrients,
but also of CDOM (see Chapter 1). Hu et al.’s (2004) analysis of SeaWiFS images revealed
a dominance of CDOM over chlorophyll in the low-salinity Amazon and Orinoco plumes
up to 2000–3000 km from the river mouths, both coastwise and across the ocean in the
NECC. We may assume that the same phenomenon occurs in the Congo plume because
this is the root of the chlorophyll enhancement that lies seasonally across the equatorial
currents. Of course, no rivers of the magnitude of these three enter the eastern tropical
Pacific. We must also remember the Niña-like thermocline uplift that occurs seasonally
79
et al. (2001). There is now ample satellite evidence of phytoplankton blooms in this part
of the ocean other than those we might attribute to the island wake of the Galapagos.
Fiedler, Philbrick, and Chavez (1991) showed that both production rate and productivity in the eastern Pacific respond to the seasonal pattern of divergence (equatorial
and countercurrent) and coastal upwelling, forced by the consequent relative depths of
mixing and of light penetration. Now that we can routinely access 7- and 30-day images
of this region, both as surface chlorophyll and translated into productivity, it is very
clear that the observed pattern is a direct response to physical forcing. We are now quite
familiar with the consequences of the range of circulation processes that occur in this
region, responding to both seasonal and Niño/Niña cycles; the consequences of reduced
upwelling at the equator in the Niño situation are plain for all to see. Of course, we
anticipate that strong vertical motion will be induced in the Galapagos Islands wake;
this is now seen to be just one feature in the larger pattern of chlorophyll accumulation
characteristic of the eastern tropical Pacific.
A nutrient budget of the eastern Pacific high-S region, at 140
W and from 3
S to 6
N,
suggests that flux of iron and nitrate to the equatorial photic zone occurs in sufficiently
differing ratios that most of the nitrate remains unused. Coale et al. (1996a) assume a
molar C:Fe ratio of 167,000, a sustained upwelling velocity of 123 m d
−1 , and a subaerial
Fe supply rate of 5–25 nM m
−2 d
−1 . Since the observed new production rate here is
5–30 M C m
−2 d
−1 , iron must limit growth to <25% of the potential offered by nitrate
flux at this location, of which >80% remains unused. However, at 9
N, where upward
NO 3 flux is much lower, nitrogen was found to be the limiting element and is assimilated
down to very low concentrations indeed, as it is at the ADIOS station at about the same
longitude, but centrally in the gyral circulation at 26
N.
I have seen no discussion of the fact that, although a different nutrient is limiting
in each, both tropical Pacific and Atlantic have rather similar geographic patterns of
chlorophyll accumulation. In both oceans, just below the nutricline, nitrate and phosphate
take similar values, respectively 32–36 M kg
−1 NO 3 and 15–25 M kg
−1 PO 4 . However,
residual nitrate values in the euphotic zone of the two oceans are very different (Atlantic,
about 05 M kg
−1 ; Pacific, 2–10 M kg
−1 NO 3 ). Differences in euphotic zone phosphate
between the two oceans are also significant, but weaker than for nitrate. The simplest, and
perhaps correct, explanation for this difference in nutrient availability lies in the greater
subaerial Fe deposition in Atlantic low latitudes where euphotic zone nitrate is reduced
to near-limiting values by phytoplankton uptake. Thus, I suggest that in each ocean it
is the supply of an element by vertical flux across a nutricline that limits phytoplankton
growth. This is a very different matter from the paradigmatic and unthinking “It is
established that the high-nitrate, low-chlorophyll character of these waters is attributable
to the limited availability of iron in the photic zone” that we read too often.
Despite all this, it would be simplistic to suggest that the greater accumulation of
phytoplankton observed in satellite chlorophyll fields in Atlantic low latitudes is Fe-driven.
I have seen this suggestion made, but it ignores many other factors, of which the principal
are the different orientation of coastal margins in the two oceans and the fact that two
great rivers—Amazon and Congo—open into the Atlantic at near-equatorial latitudes.
These issues will be discussed in Chapter 9, so here it is only necessary to point out that
each represents a major input at very shallow depth of a mixed bag not only of nutrients,
but also of CDOM (see Chapter 1). Hu et al.’s (2004) analysis of SeaWiFS images revealed
a dominance of CDOM over chlorophyll in the low-salinity Amazon and Orinoco plumes
up to 2000–3000 km from the river mouths, both coastwise and across the ocean in the
NECC. We may assume that the same phenomenon occurs in the Congo plume because
this is the root of the chlorophyll enhancement that lies seasonally across the equatorial
currents. Of course, no rivers of the magnitude of these three enter the eastern tropical
Pacific. We must also remember the Niña-like thermocline uplift that occurs seasonally