Atlantic Trade Wind Biome
207
periods: January–March 2000, July–September 2001, and January–March 2002, the last
being shown in Color plate 8. On the contrary, the images for November 2001 show
general pigment enhancement of the tropical gyre with an imposed meandering plume of
blue water that corresponds in form and location to what we would expect of the Congo
plume; but the March 2003 image shows a well-developed plume extending west across
blue water in the complete absence of any general enhancement of the Angola Gyre.
This suggests that the plume remains more discrete than diffuse, and that is very largely
Ekman upwelling that causes the diffused chlorophyll enhancement observed in the gyre
south of the equatorial divergence. Figure 9.17 shows that regional surface salinity patterns confirm these interpretations of the chlorophyll field: both the Congo plume and
the freshwater from Niger and Cameroun rivers that discharges toward the west from
the Bight of Biafra are identifiable.
Finally, we must notice the fourth mechanism that may lead to pigment accumulation,
along the southern flank of the NECC-GC; we may attribute this simply to the same
processes that force a bloom in the transoceanic section of the NECC. As already noted,
in many of the serial surface chlorophyll images a very clear zone of oligotrophic water
is seen between the effects of the equatorial divergence and what we can attribute to
processes associated with the NECC-GC; this may occur in any month, but is especially
evident as the divergence bloom recedes during boreal winter months.
How far can the inferences made from satellite images be supported by in situ observations? During the FOCAL voyages, meridional chlorophyll sections were worked across
the equator at 4
23
, and 35
W (Herbland et al., 1987; Oudot and Morin, 1987). At
4
W the DCM shoals from 50 m to about 25 m at the equator while chlorophyll values
within it double. The same occurs, but less clearly, at 23
W. At 35
W, in the WTRA, the
DCM remained at about 60–70 m in both seasons with essentially unchanged values. The
nutricline and the DCM were found to co-occur in the upper few meters of the thermocline in both seasons, with nitrate at limiting concentrations in the mixed layer, except in
the equatorial divergence zone itself during the cool season. The cool water that lies along
and just south of the equator is indeed associated with enhanced (<5 M) nitrate levels
at the surface because the shoaling of the thermocline above the EUC coincides with the
occurrence of stronger turbulence there than to north or south (Hebert et al., 1991).
Meridional transects worked recently between 10
N − 10
S in the western part of
ETRA (Pérez et al., 2005) showed that although production rates were higher by a factor
of 6 near the equator, total cell biomass was remarkably similar along the entire length
of the transects (19–22 mg chl m
−2 ); chlorophyll enhancement near the equator was
strongly correlated with increased mixed-layer NO 3 (>10 M compared with < 01 M
to north and south). Cell size fractions were also remarkably uniform along each transect,
picophytoplankton contributing 80% of biomass in the oligotrophic zones and 60% at
the equatorial divergence, suggesting strong top-down control of primary production by
herbivores. Cell numbers of all fractions were higher in the divergence zone, peaking
at around 2
S, thus confirming the general observation noted earlier of a general bias
toward the south of higher chlorophyll biomass at the surface in the ETRA. Of course,
it has been known since the early 1980s that the seasonal blooms here are dominated by
small cells, though the data reported then did not yet reflect the revelation, later derived
from sea-going flow spectrometry, that cyanobacteria and prochlorophytes dominate
the chlorophyll biomass at the bottom of the photic zone. In 1985, Herbland reported
that in the nitrate-depleted mixed layer, cells <1 m dominate the chlorophyll biomass
(mean = 71%), whereas at the nitracline such cells comprise only 50% of autotrophic
biomass.
Finally, an SST climatology has been specified by Pérez et al. (2005) from the AMT
and other studies at sea that closely confirms the computations made from satellite data
as shown, for example, in Fig. 9.18: Pérez et al. show a seasonal cycle for both WTRA
207
periods: January–March 2000, July–September 2001, and January–March 2002, the last
being shown in Color plate 8. On the contrary, the images for November 2001 show
general pigment enhancement of the tropical gyre with an imposed meandering plume of
blue water that corresponds in form and location to what we would expect of the Congo
plume; but the March 2003 image shows a well-developed plume extending west across
blue water in the complete absence of any general enhancement of the Angola Gyre.
This suggests that the plume remains more discrete than diffuse, and that is very largely
Ekman upwelling that causes the diffused chlorophyll enhancement observed in the gyre
south of the equatorial divergence. Figure 9.17 shows that regional surface salinity patterns confirm these interpretations of the chlorophyll field: both the Congo plume and
the freshwater from Niger and Cameroun rivers that discharges toward the west from
the Bight of Biafra are identifiable.
Finally, we must notice the fourth mechanism that may lead to pigment accumulation,
along the southern flank of the NECC-GC; we may attribute this simply to the same
processes that force a bloom in the transoceanic section of the NECC. As already noted,
in many of the serial surface chlorophyll images a very clear zone of oligotrophic water
is seen between the effects of the equatorial divergence and what we can attribute to
processes associated with the NECC-GC; this may occur in any month, but is especially
evident as the divergence bloom recedes during boreal winter months.
How far can the inferences made from satellite images be supported by in situ observations? During the FOCAL voyages, meridional chlorophyll sections were worked across
the equator at 4
23
, and 35
W (Herbland et al., 1987; Oudot and Morin, 1987). At
4
W the DCM shoals from 50 m to about 25 m at the equator while chlorophyll values
within it double. The same occurs, but less clearly, at 23
W. At 35
W, in the WTRA, the
DCM remained at about 60–70 m in both seasons with essentially unchanged values. The
nutricline and the DCM were found to co-occur in the upper few meters of the thermocline in both seasons, with nitrate at limiting concentrations in the mixed layer, except in
the equatorial divergence zone itself during the cool season. The cool water that lies along
and just south of the equator is indeed associated with enhanced (<5 M) nitrate levels
at the surface because the shoaling of the thermocline above the EUC coincides with the
occurrence of stronger turbulence there than to north or south (Hebert et al., 1991).
Meridional transects worked recently between 10
N − 10
S in the western part of
ETRA (Pérez et al., 2005) showed that although production rates were higher by a factor
of 6 near the equator, total cell biomass was remarkably similar along the entire length
of the transects (19–22 mg chl m
−2 ); chlorophyll enhancement near the equator was
strongly correlated with increased mixed-layer NO 3 (>10 M compared with < 01 M
to north and south). Cell size fractions were also remarkably uniform along each transect,
picophytoplankton contributing 80% of biomass in the oligotrophic zones and 60% at
the equatorial divergence, suggesting strong top-down control of primary production by
herbivores. Cell numbers of all fractions were higher in the divergence zone, peaking
at around 2
S, thus confirming the general observation noted earlier of a general bias
toward the south of higher chlorophyll biomass at the surface in the ETRA. Of course,
it has been known since the early 1980s that the seasonal blooms here are dominated by
small cells, though the data reported then did not yet reflect the revelation, later derived
from sea-going flow spectrometry, that cyanobacteria and prochlorophytes dominate
the chlorophyll biomass at the bottom of the photic zone. In 1985, Herbland reported
that in the nitrate-depleted mixed layer, cells <1 m dominate the chlorophyll biomass
(mean = 71%), whereas at the nitracline such cells comprise only 50% of autotrophic
biomass.
Finally, an SST climatology has been specified by Pérez et al. (2005) from the AMT
and other studies at sea that closely confirms the computations made from satellite data
as shown, for example, in Fig. 9.18: Pérez et al. show a seasonal cycle for both WTRA