86
Chapter 5: Nutrient Limitation: The Example of Iron
so that the near-surface concentration of NO 3 increased from 0.2 to 07 nM kg
−1 after
one episode. Nevertheless, it was thought that the dust-fall released the autotrophic
community from Fe limitation, rather than from NO 3 limitation. Similarly, on the West
Florida Shelf, deposition pulses of African dust may provide an explanation of blooms of
Trichodesmium whose nitrogenase enzyme system has a high iron demand; after a dust
event, background iron levels increase from <05 to 16nM kg
−1 , and Trichodesmium
colonies increase a hundredfold (Lenes et al., 2001).
To what extent these observations can be generalized is unknown, but such events
can be expected only in regions where the prevailing winds carry heavy dust loads:
this occurs commonly under the Atlantic trade winds and the North Pacific westerlies,
but not elsewhere. Their frequency is unknown but probably follows the relative rate
of dry deposition in each region. It has also been suggested that the flux of both iron
and nitrogen at Midway 180
W 30
N and Bermuda 60
W 32
N may represent
a major part of nutrient flux to the photic zone, and sufficient to induce a significant
growth response (Donaghey et al., 1991). Suggestions have also been made that the strong
increases in deposition of both desert dust and pollutant aerosols after the mid-20th
century are currently modifying the structure of the base of the pelagic ecosystem by
inducing anomalous population growth both of pathogenic microbes and of diazotrophic
cyanobacteria (e.g., Hayes et al., 2001).
Observations such as these support the view that there must be regional consequences
for phytoplankton growth of the deposition of aerosols at the sea surface and that we
should expect that the most significant deposition occurs at the sea surface of aerosol
particles, whether of desert dust or industrial haze. Accepting this view, we may conclude
that the high productivity of parts of the Atlantic, especially in low latitudes, is one result
of this process. Unfortunately, this is very difficult to demonstrate and, indeed, has yet
to be done: nevertheless, I have already seen statements making this connection. The
problem, of course, as discussed earlier is that it is difficult to isolate the deposition effect
from the many other factors that control phytoplankton growth and accumulation. The
physical forcing processes that are involved differ strongly between oceans, and between
comparable biogeochemical provinces in each ocean: this makes meaningful comparison
of aerosol deposition effects very difficult.
Recourse may be had to an estimate of phytoplankton productivity, partitioned
among 50-odd biogeochemical provinces (Longhurst et al., 1995), that has been revised
for this volume, using the same algorithms, with data both from SeaWiFS and from
MODIS for 2002–2005. In only 8 provinces does total phytoplankton production exceed
400 gC m
−2 y
−1 and, of these, 3 are in Atlantic low latitudes: CNRY (Canary Current
upwelling), GUIA (Amazon shelf and plume), and GUIN (tropical West African coast).
The other high-productivity provinces are the coast of China, the NE, SE and NW Atlantic
shelf regions, the Alaska shelf, and the NW Arabian Sea. In each of these 8 provinces,
as in the remainder, serial surface chlorophyll images clearly suggest that the pattern of
productivity matches that of physical processes, rather than the more diffuse pattern of
aerosol deposition.
It may be significant to the question of Fe deposition that the Canary Current upwelling
province, lying directly below the Saharan dust plume, has almost twice the productivity
of each of the other three eastern boundary current upwelling provinces: 710 gC m
−2 y
−1
compared with 269–396 m
−2 y
−1 . However, these four upwelling provinces are not directly
comparable, and the Canary Current region is unique in the great width of its shelf.
Then, tropical West Africa (GUIN) has the only coastline anywhere that is aligned close
to, and parallel with, an equatorial current system. The Amazon shelf (GUIA) receives
the total nutrient flux of the largest river on any continent. So, it is altogether too
early to ascribe the relatively high production of some Atlantic regions to the effect of
African dust with any confidence. For what it is worth, which isn’t much, statistical
Chapter 5: Nutrient Limitation: The Example of Iron
so that the near-surface concentration of NO 3 increased from 0.2 to 07 nM kg
−1 after
one episode. Nevertheless, it was thought that the dust-fall released the autotrophic
community from Fe limitation, rather than from NO 3 limitation. Similarly, on the West
Florida Shelf, deposition pulses of African dust may provide an explanation of blooms of
Trichodesmium whose nitrogenase enzyme system has a high iron demand; after a dust
event, background iron levels increase from <05 to 16nM kg
−1 , and Trichodesmium
colonies increase a hundredfold (Lenes et al., 2001).
To what extent these observations can be generalized is unknown, but such events
can be expected only in regions where the prevailing winds carry heavy dust loads:
this occurs commonly under the Atlantic trade winds and the North Pacific westerlies,
but not elsewhere. Their frequency is unknown but probably follows the relative rate
of dry deposition in each region. It has also been suggested that the flux of both iron
and nitrogen at Midway 180
W 30
N and Bermuda 60
W 32
N may represent
a major part of nutrient flux to the photic zone, and sufficient to induce a significant
growth response (Donaghey et al., 1991). Suggestions have also been made that the strong
increases in deposition of both desert dust and pollutant aerosols after the mid-20th
century are currently modifying the structure of the base of the pelagic ecosystem by
inducing anomalous population growth both of pathogenic microbes and of diazotrophic
cyanobacteria (e.g., Hayes et al., 2001).
Observations such as these support the view that there must be regional consequences
for phytoplankton growth of the deposition of aerosols at the sea surface and that we
should expect that the most significant deposition occurs at the sea surface of aerosol
particles, whether of desert dust or industrial haze. Accepting this view, we may conclude
that the high productivity of parts of the Atlantic, especially in low latitudes, is one result
of this process. Unfortunately, this is very difficult to demonstrate and, indeed, has yet
to be done: nevertheless, I have already seen statements making this connection. The
problem, of course, as discussed earlier is that it is difficult to isolate the deposition effect
from the many other factors that control phytoplankton growth and accumulation. The
physical forcing processes that are involved differ strongly between oceans, and between
comparable biogeochemical provinces in each ocean: this makes meaningful comparison
of aerosol deposition effects very difficult.
Recourse may be had to an estimate of phytoplankton productivity, partitioned
among 50-odd biogeochemical provinces (Longhurst et al., 1995), that has been revised
for this volume, using the same algorithms, with data both from SeaWiFS and from
MODIS for 2002–2005. In only 8 provinces does total phytoplankton production exceed
400 gC m
−2 y
−1 and, of these, 3 are in Atlantic low latitudes: CNRY (Canary Current
upwelling), GUIA (Amazon shelf and plume), and GUIN (tropical West African coast).
The other high-productivity provinces are the coast of China, the NE, SE and NW Atlantic
shelf regions, the Alaska shelf, and the NW Arabian Sea. In each of these 8 provinces,
as in the remainder, serial surface chlorophyll images clearly suggest that the pattern of
productivity matches that of physical processes, rather than the more diffuse pattern of
aerosol deposition.
It may be significant to the question of Fe deposition that the Canary Current upwelling
province, lying directly below the Saharan dust plume, has almost twice the productivity
of each of the other three eastern boundary current upwelling provinces: 710 gC m
−2 y
−1
compared with 269–396 m
−2 y
−1 . However, these four upwelling provinces are not directly
comparable, and the Canary Current region is unique in the great width of its shelf.
Then, tropical West Africa (GUIN) has the only coastline anywhere that is aligned close
to, and parallel with, an equatorial current system. The Amazon shelf (GUIA) receives
the total nutrient flux of the largest river on any continent. So, it is altogether too
early to ascribe the relatively high production of some Atlantic regions to the effect of
African dust with any confidence. For what it is worth, which isn’t much, statistical