Atlantic Coastal Biome
233
strong upwelling cells, the effects of which are concealed in these mean values. The P rate
increases continuously from the December minimum to an annual maximum in March–
April with an unusually prominent secondary maximum in September–November due to
upwelling processes. Accumulation of cells approximately matches changing P rates yearround. Consumer biomass (or sinking rate) appears to increase rapidly after midsummer
when chlorophyll biomass takes annual minimum values despite high summer upwelling
P rates. This would be consistent with rise of seasonal vertical migrant consumers.
Guinea Current Coastal Province (GUIN)
Extent of the Province
The GUIN province extends from Cape Roxo (12
N) eastward along the coastline of
tropical West Africa (the Guinea Coast hereafter) to the Bight of Biafra, and thence south
along the Congolese and Angolan coast to Cape Frio (18
S), the Central African coast
hereafter. Along the east-west Guinea Coast the seaward boundary is the shelf-edge front
within the NECC-GC (at 2–3
N 4
W). Along the coast from Cameroon to the Angola
Bight, faute de mieux (see below), the line of the Angola/Benguela Front is used, though
this (enclosing the large offshore cyclonic gyre) really carries us too far seaward from the
coastal boundary zone.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf of the Guinea Coast is narrow (about 25 km) except from Cape
Roxo to Sherbro Island in the extreme west, where a width of about 250 km is reached.
Lesser extensions of the shelf occur off Ghana and between the Niger delta and the large
island of Fernando Po. Deep canyons cut across the shelf off the Ivory Coast (the “Trou
sans Fond”) and at the mouth of the Congo River.
Especially on the western and central shelves of the Guinea Coast, linear relic coral
banks occur deep on the shelf, usually close to the shelf break. Otherwise, the shelf is
generally flat and the coastline straight, but the nature of the sediments varies significantly
according to coastal geology and the distribution of river effluents. Two major (and
many minor) rivers open onto the shelf: the Niger, through its wide deltaic region on
the eastern Guinea Coast, and the Congo, through a narrow mouth over the Congo
canyon. The wide western shelf has the form of a dissected plateau and carries the relic
submerged delta of the Bissau River, whose flow is much reduced in the present era;
emergent parts of this feature form the offshore Bissagos archipelago. On this segment
of the shelf, sediments are presently accumulating only inshore, so that muddy deposits
dominate at depths shoaler than the permanent thermocline, below which the bottom is
sandy out to the linear fossil coral banks of Pleistocene age on the outer shelf.
East of Sherbro Island and as far as Cape Palmas, the narrow shelf is largely mudcovered with a narrow coastal strip of sand in water <20 m deep. In the central upwelling
region, the extent of sandy substrate is much greater and may extend out to the shelf edge
as it does to the east of Sassandra. To the east of Lagos, and increasingly into the Bight
of Biafra, the sedimentary flux of the Niger River loads the shelf with extensive deposits
of soft mud so that here sandy ground is rare, even close inshore. Off the eastern Niger
delta, from Bonny to the Rio del Rey, black mud is widely distributed and is associated
with anoxic bottom water.
On the Central African coast, the Congo River canyon, and the Walvis Ridge just
south of Cape Frio are the dominant topographic features; the 200-m contour lies a little
farther offshore than on the Guinea Coast with very little change in shelf width the entire
length of this segment of the coastline.
233
strong upwelling cells, the effects of which are concealed in these mean values. The P rate
increases continuously from the December minimum to an annual maximum in March–
April with an unusually prominent secondary maximum in September–November due to
upwelling processes. Accumulation of cells approximately matches changing P rates yearround. Consumer biomass (or sinking rate) appears to increase rapidly after midsummer
when chlorophyll biomass takes annual minimum values despite high summer upwelling
P rates. This would be consistent with rise of seasonal vertical migrant consumers.
Guinea Current Coastal Province (GUIN)
Extent of the Province
The GUIN province extends from Cape Roxo (12
N) eastward along the coastline of
tropical West Africa (the Guinea Coast hereafter) to the Bight of Biafra, and thence south
along the Congolese and Angolan coast to Cape Frio (18
S), the Central African coast
hereafter. Along the east-west Guinea Coast the seaward boundary is the shelf-edge front
within the NECC-GC (at 2–3
N 4
W). Along the coast from Cameroon to the Angola
Bight, faute de mieux (see below), the line of the Angola/Benguela Front is used, though
this (enclosing the large offshore cyclonic gyre) really carries us too far seaward from the
coastal boundary zone.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf of the Guinea Coast is narrow (about 25 km) except from Cape
Roxo to Sherbro Island in the extreme west, where a width of about 250 km is reached.
Lesser extensions of the shelf occur off Ghana and between the Niger delta and the large
island of Fernando Po. Deep canyons cut across the shelf off the Ivory Coast (the “Trou
sans Fond”) and at the mouth of the Congo River.
Especially on the western and central shelves of the Guinea Coast, linear relic coral
banks occur deep on the shelf, usually close to the shelf break. Otherwise, the shelf is
generally flat and the coastline straight, but the nature of the sediments varies significantly
according to coastal geology and the distribution of river effluents. Two major (and
many minor) rivers open onto the shelf: the Niger, through its wide deltaic region on
the eastern Guinea Coast, and the Congo, through a narrow mouth over the Congo
canyon. The wide western shelf has the form of a dissected plateau and carries the relic
submerged delta of the Bissau River, whose flow is much reduced in the present era;
emergent parts of this feature form the offshore Bissagos archipelago. On this segment
of the shelf, sediments are presently accumulating only inshore, so that muddy deposits
dominate at depths shoaler than the permanent thermocline, below which the bottom is
sandy out to the linear fossil coral banks of Pleistocene age on the outer shelf.
East of Sherbro Island and as far as Cape Palmas, the narrow shelf is largely mudcovered with a narrow coastal strip of sand in water <20 m deep. In the central upwelling
region, the extent of sandy substrate is much greater and may extend out to the shelf edge
as it does to the east of Sassandra. To the east of Lagos, and increasingly into the Bight
of Biafra, the sedimentary flux of the Niger River loads the shelf with extensive deposits
of soft mud so that here sandy ground is rare, even close inshore. Off the eastern Niger
delta, from Bonny to the Rio del Rey, black mud is widely distributed and is associated
with anoxic bottom water.
On the Central African coast, the Congo River canyon, and the Walvis Ridge just
south of Cape Frio are the dominant topographic features; the 200-m contour lies a little
farther offshore than on the Guinea Coast with very little change in shelf width the entire
length of this segment of the coastline.