Regional Anomalies in Nutrient Limitation
81
meltwater from Alaska or to increased transport of dust from Asian sources. Episodic
breakdown of stratification seems not to have been considered, but would produce the
same effect. Given the conditions now understood to characterize the subarctic Pacific,
it should be no surprise that Fe-fertilization of the mixed layer here should produce a
diatom bloom, as has been observed.
The Southern Ocean is the most talked-about high-S region. Of course, much of
the recent attention is due to the suggestion that the artificial addition of Fe in tanker
quantities to surface waters might perhaps promote sufficient algal growth to sequester
significant amounts of CO 2 from the atmosphere. This idea has been enthusiastically
taken up by some oceanographers and many “ecologists”—in the political sense of that
term—and has perhaps assisted in obtaining support for extensive investigation of the
role of iron in supporting phytoplankton growth. These entrepreneurial suggestions have
drawn substantial criticism from Miller (2004) and others.
Because the Southern Ocean is the focus of Chapter 12, what follows serves only to put
this ocean in the context of the present discussion: a wider analysis of how phytoplankton
production is controlled—and consumed—is presented in that chapter. So here, we need
deal with only a very few salient points of the arguments in favor of iron limitation of
phytoplankton growth. In particular, it may be useful to use this region to illustrate the
complex relationships between irradiance and iron as limiting factors for the growth of
phytoplankton—or, more strictly, their production of carbohydrates. A useful primer to
this complex problem is that of Tim van Oijen (2004): I recommend it highly.
During the initiation of blooms, phytoplankton must utilize NO 3 as their principal
nitrogen source, because of the relative absence of NH 4 at this stage of the production
cycle. Because Fe is required in the cellular processes involved in the reduction of NO 3
−
to NH 4
+ their requirement for Fe is higher (by as much as a factor of 1.6) than during
later stages of a bloom (Maldonado et al., 1999). Then, the cellular Fe requirement
of phytoplankton varies inversely with irradiance, because the photosynthetic apparatus
has a higher Fe demand at low light levels. Thus, phytoplankton in regions of shallow
mixing are capable of growth at ambient Fe levels that would preclude growth in deeper
mixed layers where the irradiance experienced during each diurnal cycle would be lower.
Then, as noted earlier, small cells can obtain sufficient Fe from lower concentrations
than can large cells such as diatoms: the half-saturation constant for cell growth of
phytoplankton responds to cell size across several orders of magnitude, ranging from
<0001 to 120 nM kg
−1 Fe (Blain et al., 2001).
It is customary to restrict analyses of the role of iron in the ecology of this ocean to
regions poleward of the Sub-Antarctic Front where, it is said, the “real” Southern Ocean
begins. However, it will be more instructive to consider the whole ocean south of 35
S,
as is done in Chapter 12. This includes the entire region having remnant nitrate at the
surface, as well as both of the annular zones of high chlorophyll that dominate the open
Southern Ocean (see Color plate 20); it is highly probable that these two linear regions of
seasonally high chlorophyll have some functional commonality. Each is a complex frontal
zone that encompasses several degrees of latitude, and each has characteristic conditions
that result in higher chlorophyll biomass within the front than in the intervening zones.
The Southern Ocean presents us with an apparently simple question, posed by the
observation that it habitually exhibits regions of clear, blue water where phytoplankton
biomass and primary production must take low values. In fact, satellite chlorophyll
fields suggest that the widely held view that most of the Southern Ocean does not
accumulate chlorophyll is an overstatement: episodic seasonal blooms are widespread and
accumulate chlorophyll biomass to levels characteristic of other high-latitude regions.
Differential cellular demand for ambient Fe under varying conditions, discussed earlier,
generally support the observation that small cells dominate the Fe-depleted areas of the
Southern Ocean and that diatoms are restricted largely to the frontal zones where Fe is
81
meltwater from Alaska or to increased transport of dust from Asian sources. Episodic
breakdown of stratification seems not to have been considered, but would produce the
same effect. Given the conditions now understood to characterize the subarctic Pacific,
it should be no surprise that Fe-fertilization of the mixed layer here should produce a
diatom bloom, as has been observed.
The Southern Ocean is the most talked-about high-S region. Of course, much of
the recent attention is due to the suggestion that the artificial addition of Fe in tanker
quantities to surface waters might perhaps promote sufficient algal growth to sequester
significant amounts of CO 2 from the atmosphere. This idea has been enthusiastically
taken up by some oceanographers and many “ecologists”—in the political sense of that
term—and has perhaps assisted in obtaining support for extensive investigation of the
role of iron in supporting phytoplankton growth. These entrepreneurial suggestions have
drawn substantial criticism from Miller (2004) and others.
Because the Southern Ocean is the focus of Chapter 12, what follows serves only to put
this ocean in the context of the present discussion: a wider analysis of how phytoplankton
production is controlled—and consumed—is presented in that chapter. So here, we need
deal with only a very few salient points of the arguments in favor of iron limitation of
phytoplankton growth. In particular, it may be useful to use this region to illustrate the
complex relationships between irradiance and iron as limiting factors for the growth of
phytoplankton—or, more strictly, their production of carbohydrates. A useful primer to
this complex problem is that of Tim van Oijen (2004): I recommend it highly.
During the initiation of blooms, phytoplankton must utilize NO 3 as their principal
nitrogen source, because of the relative absence of NH 4 at this stage of the production
cycle. Because Fe is required in the cellular processes involved in the reduction of NO 3
−
to NH 4
+ their requirement for Fe is higher (by as much as a factor of 1.6) than during
later stages of a bloom (Maldonado et al., 1999). Then, the cellular Fe requirement
of phytoplankton varies inversely with irradiance, because the photosynthetic apparatus
has a higher Fe demand at low light levels. Thus, phytoplankton in regions of shallow
mixing are capable of growth at ambient Fe levels that would preclude growth in deeper
mixed layers where the irradiance experienced during each diurnal cycle would be lower.
Then, as noted earlier, small cells can obtain sufficient Fe from lower concentrations
than can large cells such as diatoms: the half-saturation constant for cell growth of
phytoplankton responds to cell size across several orders of magnitude, ranging from
<0001 to 120 nM kg
−1 Fe (Blain et al., 2001).
It is customary to restrict analyses of the role of iron in the ecology of this ocean to
regions poleward of the Sub-Antarctic Front where, it is said, the “real” Southern Ocean
begins. However, it will be more instructive to consider the whole ocean south of 35
S,
as is done in Chapter 12. This includes the entire region having remnant nitrate at the
surface, as well as both of the annular zones of high chlorophyll that dominate the open
Southern Ocean (see Color plate 20); it is highly probable that these two linear regions of
seasonally high chlorophyll have some functional commonality. Each is a complex frontal
zone that encompasses several degrees of latitude, and each has characteristic conditions
that result in higher chlorophyll biomass within the front than in the intervening zones.
The Southern Ocean presents us with an apparently simple question, posed by the
observation that it habitually exhibits regions of clear, blue water where phytoplankton
biomass and primary production must take low values. In fact, satellite chlorophyll
fields suggest that the widely held view that most of the Southern Ocean does not
accumulate chlorophyll is an overstatement: episodic seasonal blooms are widespread and
accumulate chlorophyll biomass to levels characteristic of other high-latitude regions.
Differential cellular demand for ambient Fe under varying conditions, discussed earlier,
generally support the observation that small cells dominate the Fe-depleted areas of the
Southern Ocean and that diatoms are restricted largely to the frontal zones where Fe is