Atlantic Coastal Biome
237
Response of the Pelagic Ecosystems
This province provides us with an unusually simple model of how the characteristics
of a water column may structure a pelagic ecosystem and, assisted by its control of
the distribution of sediment types, may also determine the characteristic ecology of the
benthos and demersal fish. The simplicity of this model depends on the fact that the
shoal, but very abrupt, pycnocline typical of this province lies at midshelf depths over
the shelf, or even shoaler.
This province is characterized by a vertical arrangement of properties typical of oligotrophic situations, the depth of 1% irradiance being usually between 20 and 45 m
over the shelf. During boreal summer, studies at coastal stations from Sierra Leone to
Nigeria have recorded increases in mixed-layer chlorophyll and in phytoplankton cell
numbers and a decrease in water clarity consistent with phytoplankton blooms during
this season. The pattern is consistent with the various coastal upwelling regions discussed
earlier (Bainbridge, 1960a; Longhurst, 1964; Sevrin-Reyssac, 1993). The FOCAL meridional section at 4
W (Oudot, 1987) also associated the upsloping of the deep chlorophyll
maximum toward the coast in July with a shoreward increase in chlorophyll values. Off
the Central African coast, the nitracline shoals into the upper 20 m in July, inducing there
a seasonal shallow chlorophyll maximum (Binet, 1983).
The interpretation of sea surface chlorophyll images is relatively simple along the
northern Guinea coast, and these clearly confirm the existence and ecological consequences of upwelling cells on the central Guinea coast, east of Capes Palmas and Three
Points, and also the intermittent intrusion of chlorophyll-rich Canary Current water onto
the Bissau shelf. These images show that the seaward extent of the ecological consequence
of upwelling is much greater than had been anticipated. The most extreme case that I
have found is that of the July 1998 upwelling event east of Cape Palmas that induced a
300-km filament (<25 mg chl m
−3 ) that extended toward the southwest around the NE
quadrant of an equatorial meander centered at 2
N, just east of the Greenwich meridian.
A scan of the MODIS and SeaWiFS 30-day images will yield other examples of chlorophyll filaments far out over the deep ocean that are rooted in the coastal upwelling cells
situated near the two prominent capes. It will also reveal that upwelling cells are, as
we expect them to be from coastal observations, especially active from June to October
each year.
On the Central African coastline, surface chlorophyll is far more difficult to interpret,
principally because of the presence of the Congo effluent and of the Angola Dome
not far offshore: the shelf environment along this coast is strongly influenced by the
region of high chlorophyll values that is present in the Angola bight from Cape Lopez
to Mossamedes; coastal processes are with great difficulty isolated from processes on
the eastern, African side of the Angola Dome. The available chlorophyll images, not all
of which can be shown here because of licensing policies, will repay close examination,
although they cannot entirely resolve the difficulties; for instance, to what extent does
the image of the buoyant plume of the Congo (see Fig. 9.16) represent CDOM like
that of the Amazon? I can find no survey data representing transects across the plume
to resolve this issue. In April 2001, we see what appears to be a simple river plume
turned offshore by the then-prevailing winds from a source region between Cabinda and
Louanda, apparently at the Congo mouth. February 1998 images show a much larger
plume, rooted in approximately the same part of the Angola bight, which appears to be
a response to meanders in the equatorial current system. Its active nature appears to be
confirmed by the existence of cloud cover specifically associated with it, presumably in
response to lowered sea surface temperatures. More enigmatic is the coastwise plume
seen in images for September 2000, when both Congo effluent and terminal equatorial
enhancement appear to be separable. To add to the difficulties induced by the several
dynamic processes leading to chlorophyll enhancement in the adjacent open ocean, and
237
Response of the Pelagic Ecosystems
This province provides us with an unusually simple model of how the characteristics
of a water column may structure a pelagic ecosystem and, assisted by its control of
the distribution of sediment types, may also determine the characteristic ecology of the
benthos and demersal fish. The simplicity of this model depends on the fact that the
shoal, but very abrupt, pycnocline typical of this province lies at midshelf depths over
the shelf, or even shoaler.
This province is characterized by a vertical arrangement of properties typical of oligotrophic situations, the depth of 1% irradiance being usually between 20 and 45 m
over the shelf. During boreal summer, studies at coastal stations from Sierra Leone to
Nigeria have recorded increases in mixed-layer chlorophyll and in phytoplankton cell
numbers and a decrease in water clarity consistent with phytoplankton blooms during
this season. The pattern is consistent with the various coastal upwelling regions discussed
earlier (Bainbridge, 1960a; Longhurst, 1964; Sevrin-Reyssac, 1993). The FOCAL meridional section at 4
W (Oudot, 1987) also associated the upsloping of the deep chlorophyll
maximum toward the coast in July with a shoreward increase in chlorophyll values. Off
the Central African coast, the nitracline shoals into the upper 20 m in July, inducing there
a seasonal shallow chlorophyll maximum (Binet, 1983).
The interpretation of sea surface chlorophyll images is relatively simple along the
northern Guinea coast, and these clearly confirm the existence and ecological consequences of upwelling cells on the central Guinea coast, east of Capes Palmas and Three
Points, and also the intermittent intrusion of chlorophyll-rich Canary Current water onto
the Bissau shelf. These images show that the seaward extent of the ecological consequence
of upwelling is much greater than had been anticipated. The most extreme case that I
have found is that of the July 1998 upwelling event east of Cape Palmas that induced a
300-km filament (<25 mg chl m
−3 ) that extended toward the southwest around the NE
quadrant of an equatorial meander centered at 2
N, just east of the Greenwich meridian.
A scan of the MODIS and SeaWiFS 30-day images will yield other examples of chlorophyll filaments far out over the deep ocean that are rooted in the coastal upwelling cells
situated near the two prominent capes. It will also reveal that upwelling cells are, as
we expect them to be from coastal observations, especially active from June to October
each year.
On the Central African coastline, surface chlorophyll is far more difficult to interpret,
principally because of the presence of the Congo effluent and of the Angola Dome
not far offshore: the shelf environment along this coast is strongly influenced by the
region of high chlorophyll values that is present in the Angola bight from Cape Lopez
to Mossamedes; coastal processes are with great difficulty isolated from processes on
the eastern, African side of the Angola Dome. The available chlorophyll images, not all
of which can be shown here because of licensing policies, will repay close examination,
although they cannot entirely resolve the difficulties; for instance, to what extent does
the image of the buoyant plume of the Congo (see Fig. 9.16) represent CDOM like
that of the Amazon? I can find no survey data representing transects across the plume
to resolve this issue. In April 2001, we see what appears to be a simple river plume
turned offshore by the then-prevailing winds from a source region between Cabinda and
Louanda, apparently at the Congo mouth. February 1998 images show a much larger
plume, rooted in approximately the same part of the Angola bight, which appears to be
a response to meanders in the equatorial current system. Its active nature appears to be
confirmed by the existence of cloud cover specifically associated with it, presumably in
response to lowered sea surface temperatures. More enigmatic is the coastwise plume
seen in images for September 2000, when both Congo effluent and terminal equatorial
enhancement appear to be separable. To add to the difficulties induced by the several
dynamic processes leading to chlorophyll enhancement in the adjacent open ocean, and