74
Chapter 5: Nutrient Limitation: The Example of Iron
deposition requires attention, because the N:P ratio of airborne dust particles is rather high
relative to Redfield. Thus, new production induced by aerosol nutrient input will tend to
drive a previously Fe-deficient phytoplankton toward P limitation (Baker et al., 2003).
It was thought until recently that 40–50% of subaerial Fe went rapidly into solution in
seawater provided this contained relatively little already in solution (Zhuang and Kester,
1990), but a much lower solubility is now assumed for geochemical models: 1–10%
by Fung et al. (2000) and 2% by Jickels and Spokes (in press). Many aerosol particles
become incorporated in rainwater or in snow, and this is an important mode of delivery
of trace metals and other essential molecules to the surface water mass (Jickels, 1995).
A significant fraction of this material is directly available to biota after deposition. In
squalls over the equatorial Pacific, rainwater may contain as much as 3 nM Fe kg
−1 that
may then be retained in a lens of lighter water at the surface (Hansen et al., 2001). Aerosol
fallout during the intermittent passage of Gobi dust across the North Pacific may induce
a doubling of mixed-layer phytoplankton biomass in the subsequent 2 weeks (Bishop
et al., 2002). In all these reports, Fe fertilization was assumed, but not observed, to have
occurred. However, since rainwater also delivers nitrogen and other macronutrients in
forms usable by plant cells, this fashionable assumption is in fact unproven; we can
anticipate that mesoscale blooms will occur after rainfall in a wide range of circumstances.
In fact, it has been suggested that as much as 10–15% of global primary production
above the thermocline may be induced by atmospheric deposition of useful nitrogen
(Prospero, 2002).
The available data on the distribution of Fe td within the ocean are sparse, but two
sources are sufficient to characterize global distribution in a preliminary manner. These
are (i) the 30 stations of the Moss Landing data set (Johnson et al., 1997) representing
North and Central Pacific, North Atlantic, Southern Ocean, and Arabian Sea, and (ii) the
Atlantic Meridional Sections (AMT) discussed by Bowie et al. (2002). These show that
the high concentrations observed near continental masses decay rapidly seaward, and also
that low-latitude Atlantic profiles are exceptional.
Over deep water, Fe profiles resembles those of other biologically active metals such
as Ni, Zn, Ge, Cd, and Y, being nutrient-like with a discontinuity at the pycnocline; this
is clear in the Moss Landing data set and also (for the North Pacific alone) in the vertical
profiles of the periodic table elements offered by Nozaki (2001). The discontinuity in
the generalized Fe profile, resulting from depletion in the photic zone, is designated the
ferricline. The 354 profiles in the Moss Landing archive are dominated by northeastern
Pacific data (2
S to 60
N) but are consistent with profiles from the North Atlantic, the
Southern Ocean, and the Arabian Sea, also shown by Johnson et al. Western equatorial
Pacific profiles of Mackey et al. (2002b) are similar, with very low mixed-layer values
above a strong ferricline at 150 m, near the pycnocline. The AMT sections show that
under the African dust veil (25
N–10
S) a secondary, near-surface maximum of Fe td is
imposed on profiles otherwise similar to the Pacific at similar latitudes (Bowie et al.,
2002). In the SE Atlantic, close to the American continent at >40
S, a surface layer is
again observed. Chester (2000) offers profiles with a secondary surface layer of higher
concentration from the central North Pacific gyre, presumably under the Asian dust veil
that, as he remarks, are “typical of a scavenged-type trace metal having a significant
external source.” The 6
W transect of the Antarctic Circumpolar Current (Löscher et al.,
1997b) showed that here, too, Fe td increases with depth and it is only in the region of
the Polar Front attributable to input from shelf water.
Although the relatively uniform Fe td field below the ferricline may be perturbed where a
water mass passes across shallows, or is adjacent to a continental mass, this does not negate
the generalizations about the deep Fe field of Johnson et al. Such an anomaly originates
in the Bismarck Sea, where a coastal undercurrent accumulates high concentrations of
Fe (2–5 nmol kg
−1 at 300 m) from iron-rich shelf sediments originating in the Sepik
Chapter 5: Nutrient Limitation: The Example of Iron
deposition requires attention, because the N:P ratio of airborne dust particles is rather high
relative to Redfield. Thus, new production induced by aerosol nutrient input will tend to
drive a previously Fe-deficient phytoplankton toward P limitation (Baker et al., 2003).
It was thought until recently that 40–50% of subaerial Fe went rapidly into solution in
seawater provided this contained relatively little already in solution (Zhuang and Kester,
1990), but a much lower solubility is now assumed for geochemical models: 1–10%
by Fung et al. (2000) and 2% by Jickels and Spokes (in press). Many aerosol particles
become incorporated in rainwater or in snow, and this is an important mode of delivery
of trace metals and other essential molecules to the surface water mass (Jickels, 1995).
A significant fraction of this material is directly available to biota after deposition. In
squalls over the equatorial Pacific, rainwater may contain as much as 3 nM Fe kg
−1 that
may then be retained in a lens of lighter water at the surface (Hansen et al., 2001). Aerosol
fallout during the intermittent passage of Gobi dust across the North Pacific may induce
a doubling of mixed-layer phytoplankton biomass in the subsequent 2 weeks (Bishop
et al., 2002). In all these reports, Fe fertilization was assumed, but not observed, to have
occurred. However, since rainwater also delivers nitrogen and other macronutrients in
forms usable by plant cells, this fashionable assumption is in fact unproven; we can
anticipate that mesoscale blooms will occur after rainfall in a wide range of circumstances.
In fact, it has been suggested that as much as 10–15% of global primary production
above the thermocline may be induced by atmospheric deposition of useful nitrogen
(Prospero, 2002).
The available data on the distribution of Fe td within the ocean are sparse, but two
sources are sufficient to characterize global distribution in a preliminary manner. These
are (i) the 30 stations of the Moss Landing data set (Johnson et al., 1997) representing
North and Central Pacific, North Atlantic, Southern Ocean, and Arabian Sea, and (ii) the
Atlantic Meridional Sections (AMT) discussed by Bowie et al. (2002). These show that
the high concentrations observed near continental masses decay rapidly seaward, and also
that low-latitude Atlantic profiles are exceptional.
Over deep water, Fe profiles resembles those of other biologically active metals such
as Ni, Zn, Ge, Cd, and Y, being nutrient-like with a discontinuity at the pycnocline; this
is clear in the Moss Landing data set and also (for the North Pacific alone) in the vertical
profiles of the periodic table elements offered by Nozaki (2001). The discontinuity in
the generalized Fe profile, resulting from depletion in the photic zone, is designated the
ferricline. The 354 profiles in the Moss Landing archive are dominated by northeastern
Pacific data (2
S to 60
N) but are consistent with profiles from the North Atlantic, the
Southern Ocean, and the Arabian Sea, also shown by Johnson et al. Western equatorial
Pacific profiles of Mackey et al. (2002b) are similar, with very low mixed-layer values
above a strong ferricline at 150 m, near the pycnocline. The AMT sections show that
under the African dust veil (25
N–10
S) a secondary, near-surface maximum of Fe td is
imposed on profiles otherwise similar to the Pacific at similar latitudes (Bowie et al.,
2002). In the SE Atlantic, close to the American continent at >40
S, a surface layer is
again observed. Chester (2000) offers profiles with a secondary surface layer of higher
concentration from the central North Pacific gyre, presumably under the Asian dust veil
that, as he remarks, are “typical of a scavenged-type trace metal having a significant
external source.” The 6
W transect of the Antarctic Circumpolar Current (Löscher et al.,
1997b) showed that here, too, Fe td increases with depth and it is only in the region of
the Polar Front attributable to input from shelf water.
Although the relatively uniform Fe td field below the ferricline may be perturbed where a
water mass passes across shallows, or is adjacent to a continental mass, this does not negate
the generalizations about the deep Fe field of Johnson et al. Such an anomaly originates
in the Bismarck Sea, where a coastal undercurrent accumulates high concentrations of
Fe (2–5 nmol kg
−1 at 300 m) from iron-rich shelf sediments originating in the Sepik