80
Chapter 5: Nutrient Limitation: The Example of Iron
in the Atlantic (Longhurst, 1995), but only interannually in the Pacific. For all these
reasons, I believe it would be very rash to attribute the apparent difference in chlorophyll
accumulation in the tropical zones of the two oceans simply to their differing subaerial
Fe supply.
The high-S region of the subarctic Pacific is enigmatic and has engendered much
discussion. Why here, under dusty skies, where dry deposition rates are somewhat higher
than in the North Atlantic, should significant residual nitrate ≤18 M kg
−1 remain in
the euphotic zone at the end of summer? Why should there normally be no spring bloom
in the open ocean? This high-S region both is larger and has an unusually high residual
concentration of nitrate just below the photic zone; in the subarctic Atlantic, nitrate
concentration at 150 m is only ∼14 M, but in the subarctic Pacific is ∼30 M. This is
the highest value on this depth horizon than anywhere else in the ocean except at the
Antarctic Divergence.
The lack of a spring bloom here was originally attributed to the presence of large
calanoids (the pre-Martin “Major Grazer Hypothesis”; see Chapter 11) but then later to
Fe limitation of the growth of larger cells. The synthesis drawn from the results of the
SUPER investigations (Miller et al., 1991) remains valid today: that Fe limitation prevents
the development of a diatom bloom but does not restrain the high growth rate of small
phytoplankton cells that dominate the autotrophic biomass. These preferentially utilize
NH 4 , which is rapidly recycled in the mixed layer, while their cell numbers (and those
of bacteria) are held down by protozoa that have doubling times shorter than their food
organisms, and by small copepods. This “mixing and micrograzer” (or SUPER) hypothesis
requires that winter mixing be insufficiently deep as to sweep clear the micro- and
picophytoplankton and their protozoan herbivores. This synthesis forces our attention,
as it should, on the permanent stratification imposed by the low salinity of the surface
layer of the Subarctic Pacific. Restraint of winter mixing enables the survival through
winter of a community of small-celled phytoplankters and their protist consumers that
is not mixed down and eliminated—as it would be in the North Atlantic—thus enabling
a rapid resumption of active growth whenever sufficient irradiance is available.
Episodic diatom blooms that occur preferentially in ENSO years draw mixed-layer
silicate down to limiting levels while nitrate remains in excess (Wong and Matear, 1999).
These blooms have been attributed to episodes (and between-year differences) of Fe
deposition at the surface and “not through vertical mixing from a concentrated supply
at depth” (e.g., Miller, 1993). It was suggested that a dust-deposition event in the North
Pacific induced an observed doubling of phytoplankton biomass over a 14-day period and
at an appropriate spatial scale (Bishop et al., 2002). In this context, it will be recalled that
the SUPER hypothesis suggests that the relative lack of diatoms in the Subarctic Pacific
is due to deficiency in the subaerial Fe supply. However, we must surely note (as the
authors of the hypothesis apparently did not) that the North Atlantic lies under less dusty
skies than the North Pacific at similar latitudes: yet diatoms are prominent in the Atlantic
spring bloom. It seems inescapable, therefore, that it is rather the permanent stratification
of the subarctic Pacific than the surface flux of Fe that forces the fundamental difference
between the pelagic ecosystems of this region and those of the North Atlantic.
In fact, the SUPER hypothesis was crafted when understanding of Fe distribution in
the oceans was very weak, and prior to the publication of the Moss Landing data archive.
Now, of course, we know that the Vertex section along 140
W revealed nutrient-like
profiles with the anticipated additional enrichment below the ferricline close to Alaska
at 48–49
N; in retrospective analyses, where data are lacking for mixing or deposition
rates, or a nutrient budget is unavailable, vertical flux in either direction provides equally
plausible mechanisms.
Despite this, the episodes of high diatom flux that have been observed in sediment cores
here were ascribed by McDonald, Pedersen, and Crusius (1999) to the influx of Fe-rich
Chapter 5: Nutrient Limitation: The Example of Iron
in the Atlantic (Longhurst, 1995), but only interannually in the Pacific. For all these
reasons, I believe it would be very rash to attribute the apparent difference in chlorophyll
accumulation in the tropical zones of the two oceans simply to their differing subaerial
Fe supply.
The high-S region of the subarctic Pacific is enigmatic and has engendered much
discussion. Why here, under dusty skies, where dry deposition rates are somewhat higher
than in the North Atlantic, should significant residual nitrate ≤18 M kg
−1 remain in
the euphotic zone at the end of summer? Why should there normally be no spring bloom
in the open ocean? This high-S region both is larger and has an unusually high residual
concentration of nitrate just below the photic zone; in the subarctic Atlantic, nitrate
concentration at 150 m is only ∼14 M, but in the subarctic Pacific is ∼30 M. This is
the highest value on this depth horizon than anywhere else in the ocean except at the
Antarctic Divergence.
The lack of a spring bloom here was originally attributed to the presence of large
calanoids (the pre-Martin “Major Grazer Hypothesis”; see Chapter 11) but then later to
Fe limitation of the growth of larger cells. The synthesis drawn from the results of the
SUPER investigations (Miller et al., 1991) remains valid today: that Fe limitation prevents
the development of a diatom bloom but does not restrain the high growth rate of small
phytoplankton cells that dominate the autotrophic biomass. These preferentially utilize
NH 4 , which is rapidly recycled in the mixed layer, while their cell numbers (and those
of bacteria) are held down by protozoa that have doubling times shorter than their food
organisms, and by small copepods. This “mixing and micrograzer” (or SUPER) hypothesis
requires that winter mixing be insufficiently deep as to sweep clear the micro- and
picophytoplankton and their protozoan herbivores. This synthesis forces our attention,
as it should, on the permanent stratification imposed by the low salinity of the surface
layer of the Subarctic Pacific. Restraint of winter mixing enables the survival through
winter of a community of small-celled phytoplankters and their protist consumers that
is not mixed down and eliminated—as it would be in the North Atlantic—thus enabling
a rapid resumption of active growth whenever sufficient irradiance is available.
Episodic diatom blooms that occur preferentially in ENSO years draw mixed-layer
silicate down to limiting levels while nitrate remains in excess (Wong and Matear, 1999).
These blooms have been attributed to episodes (and between-year differences) of Fe
deposition at the surface and “not through vertical mixing from a concentrated supply
at depth” (e.g., Miller, 1993). It was suggested that a dust-deposition event in the North
Pacific induced an observed doubling of phytoplankton biomass over a 14-day period and
at an appropriate spatial scale (Bishop et al., 2002). In this context, it will be recalled that
the SUPER hypothesis suggests that the relative lack of diatoms in the Subarctic Pacific
is due to deficiency in the subaerial Fe supply. However, we must surely note (as the
authors of the hypothesis apparently did not) that the North Atlantic lies under less dusty
skies than the North Pacific at similar latitudes: yet diatoms are prominent in the Atlantic
spring bloom. It seems inescapable, therefore, that it is rather the permanent stratification
of the subarctic Pacific than the surface flux of Fe that forces the fundamental difference
between the pelagic ecosystems of this region and those of the North Atlantic.
In fact, the SUPER hypothesis was crafted when understanding of Fe distribution in
the oceans was very weak, and prior to the publication of the Moss Landing data archive.
Now, of course, we know that the Vertex section along 140
W revealed nutrient-like
profiles with the anticipated additional enrichment below the ferricline close to Alaska
at 48–49
N; in retrospective analyses, where data are lacking for mixing or deposition
rates, or a nutrient budget is unavailable, vertical flux in either direction provides equally
plausible mechanisms.
Despite this, the episodes of high diatom flux that have been observed in sediment cores
here were ascribed by McDonald, Pedersen, and Crusius (1999) to the influx of Fe-rich