82
Chapter 5: Nutrient Limitation: The Example of Iron
more available and deep mixing is restrained. Nevertheless, we must address the general
question of whether the constraints on phytoplankton growth in this ocean are due to
Fe deficiency, given the characteristic excess availability of macronutrients, or due to
Sverdrup dynamics appropriate to unusually deep wind-mixing and low solar irradiance.
Or, do both factors intervene—as in the case where Fe limitation responds to ambient
irradiance?
Several generic solutions have been proposed. Regional differences in the regulation
of algal growth by iron in the annular zones were invoked by de Baar et al. (1995) to
explain the observations: Fe-deficient water south of the STF, Fe-replete water in the
PFZ, and Fe-limiting water to the south of the PF. Fe-replete water in the coastwise East
Wind Drift completed the logic that “iron availability was the critical factor in allowing
blooms to occur.” Versions of this model are still quoted as the basic explanation of the
observation of higher chlorophyll within the PF than elsewhere. It responds well to the
observations that in the Southern Ocean, where episodic aeolian dust deposition is as low
as anywhere on earth (Watson, 2001), the supply of Fe to surface water must be almost
entirely from below, forced by autumn-winter mixing or by convergence and upwelling
in the frontal zones. Here, it should be remembered, Fe concentrations in deep water do
not diverge from what is normal at similar depths in other oceans. Surface deposition
is seasonal and restricted to the marginal ice zone, and near drifting fields of melting
icebergs.
The recent proposal of Erickson et al. (2003) appears to ignore the consensus that the
dominant supply of iron to the euphotic zone of the Southern Ocean must by vertical
flux across the pycnocline (Watson, 2001; de Baar et al., 1995) and is based rather on
what is called the Patagonian plume of atmospheric dust. Statistical techniques are used
to suggest that the zonal band of high chlorophyll accumulation along about 50
S from
the SW Atlantic around to the Pacific is causally correlated with dust deposition rate,
spatially and (perhaps) seasonally. I shall return to the modeling aspects of this suggestion
later, but it is important to understand that the term plume that has been applied to this
phenomenon is inexact. Global surveys of aerosol optical thickness by NOAA AVHRR
(e.g., Husar et al., 1997) show that the distribution of haze over the Southern Ocean is very
diffuse, not at all plumelike, and—relative to such haze in the northern hemisphere—very
light. Further, it is far more diffuse than the circumpolar, meandering bandlike signature
of chlorophyll enhancement in the ocean below. Moreover, deposition rates based on
pelagic sediment cores in the Southern Ocean (Rea, 1994) are lower by factors of 5–10
than those estimated by Duce et al. (1991) from haze observations.
In each of the zonal partitions of the Southern Ocean, to be discussed in Chapter 12,
the seasonal cycle of chlorophyll accumulation is unimodal. Biomass accumulation follows the seasonal irradiance cycle, as does the mean value for Z m so that chlorophyll
biomass is maximal in midsummer when Z m is most shoal. A return to net growth in
response to reduced mixing depth and increased irradiance is entirely in accordance with
Sverdrup. An argument will be made in Chapter 12 that the phytoplankton ecology of the
Southern Ocean can only be understood if its physical characteristics are fully considered:
unusually deep wind mixing, associated with characteristically low sun angles, requires
that we consider the probability that irradiance may be an important factor limiting
phytoplankton growth here. Mixing is much deeper between frontal zones than within
them and, at least in late summer, phytoplankton growth in the Antarctic Circumpolar
Current (ACC) south of the Polar Front is light-limited (van Oijen, 2004). In spring and
summer, however, Fe limitation is important here.
As I have already noted, the frontal zones appear generally to be enhanced in Fe compared with the deeper-mixed ACC water between them, although de Baar’s observations
of Fe enhancement in the Polar Front were made just east of the Drake Passage. This
is not a typical region and even if this condition appears to obtain around the entire
Chapter 5: Nutrient Limitation: The Example of Iron
more available and deep mixing is restrained. Nevertheless, we must address the general
question of whether the constraints on phytoplankton growth in this ocean are due to
Fe deficiency, given the characteristic excess availability of macronutrients, or due to
Sverdrup dynamics appropriate to unusually deep wind-mixing and low solar irradiance.
Or, do both factors intervene—as in the case where Fe limitation responds to ambient
irradiance?
Several generic solutions have been proposed. Regional differences in the regulation
of algal growth by iron in the annular zones were invoked by de Baar et al. (1995) to
explain the observations: Fe-deficient water south of the STF, Fe-replete water in the
PFZ, and Fe-limiting water to the south of the PF. Fe-replete water in the coastwise East
Wind Drift completed the logic that “iron availability was the critical factor in allowing
blooms to occur.” Versions of this model are still quoted as the basic explanation of the
observation of higher chlorophyll within the PF than elsewhere. It responds well to the
observations that in the Southern Ocean, where episodic aeolian dust deposition is as low
as anywhere on earth (Watson, 2001), the supply of Fe to surface water must be almost
entirely from below, forced by autumn-winter mixing or by convergence and upwelling
in the frontal zones. Here, it should be remembered, Fe concentrations in deep water do
not diverge from what is normal at similar depths in other oceans. Surface deposition
is seasonal and restricted to the marginal ice zone, and near drifting fields of melting
icebergs.
The recent proposal of Erickson et al. (2003) appears to ignore the consensus that the
dominant supply of iron to the euphotic zone of the Southern Ocean must by vertical
flux across the pycnocline (Watson, 2001; de Baar et al., 1995) and is based rather on
what is called the Patagonian plume of atmospheric dust. Statistical techniques are used
to suggest that the zonal band of high chlorophyll accumulation along about 50
S from
the SW Atlantic around to the Pacific is causally correlated with dust deposition rate,
spatially and (perhaps) seasonally. I shall return to the modeling aspects of this suggestion
later, but it is important to understand that the term plume that has been applied to this
phenomenon is inexact. Global surveys of aerosol optical thickness by NOAA AVHRR
(e.g., Husar et al., 1997) show that the distribution of haze over the Southern Ocean is very
diffuse, not at all plumelike, and—relative to such haze in the northern hemisphere—very
light. Further, it is far more diffuse than the circumpolar, meandering bandlike signature
of chlorophyll enhancement in the ocean below. Moreover, deposition rates based on
pelagic sediment cores in the Southern Ocean (Rea, 1994) are lower by factors of 5–10
than those estimated by Duce et al. (1991) from haze observations.
In each of the zonal partitions of the Southern Ocean, to be discussed in Chapter 12,
the seasonal cycle of chlorophyll accumulation is unimodal. Biomass accumulation follows the seasonal irradiance cycle, as does the mean value for Z m so that chlorophyll
biomass is maximal in midsummer when Z m is most shoal. A return to net growth in
response to reduced mixing depth and increased irradiance is entirely in accordance with
Sverdrup. An argument will be made in Chapter 12 that the phytoplankton ecology of the
Southern Ocean can only be understood if its physical characteristics are fully considered:
unusually deep wind mixing, associated with characteristically low sun angles, requires
that we consider the probability that irradiance may be an important factor limiting
phytoplankton growth here. Mixing is much deeper between frontal zones than within
them and, at least in late summer, phytoplankton growth in the Antarctic Circumpolar
Current (ACC) south of the Polar Front is light-limited (van Oijen, 2004). In spring and
summer, however, Fe limitation is important here.
As I have already noted, the frontal zones appear generally to be enhanced in Fe compared with the deeper-mixed ACC water between them, although de Baar’s observations
of Fe enhancement in the Polar Front were made just east of the Drake Passage. This
is not a typical region and even if this condition appears to obtain around the entire