238
Chapter 9: The Atlantic Ocean
despite seasonal cloud cover during the appropriate seasons, the images do suggest that
upwelling cells occur along the coast both north and south of Cape Lopez.
Of course, over most of the shelf in this province, most of the time, upwelling is
exceptional and the water column is stratified, with phytoplankton biomass concentrated
in a DCM at the depth of the pycnocline, encountering the continental shelf at 15–45 m.
Most attention has been given to the production cycle in locations where upwelling events
occur, so that for the larger part of the open coast where upwelling does not occur,
it is much more difficult to locate a satisfactory account of the production calendar.
From data on water clarity and color at Lagos, obtained on a standard monthly transect
(Longhurst, 1964), results at midshelf beyond any influence of creek effluents, suggested
that a seasonal bloom occurs during the wet season, when strong onshore winds occur,
during boreal summer months; this is supported by zooplankton biomass data obtained
on the same transect. Copepod, cladoceran, and appendicularian numbers were one to
several orders of magnitude greater in August and September than during the remainder
of the year. This seasonal cycle suggests that the bloom on the open nonupwelling shelf of
Nigeria, or off Sierra Leone and Guinea, is initiated by nutrient flux and later terminated
by nutrient limitation.
In some of the larger embayments, such as the wide Sierra Leone estuary where tidal
streams are strong, the seasonal production cycle is the reverse of that of the open sea,
and algal cells are probably limited by light rather than by nutrients. Phytoplankton
growth (about 850 mg C m
−2 d
−1 ) is maximal in the boreal winter dry season, when
estuarine water is relatively clear, and it is greatly reduced (<75 mg C m
−2 d
−1 ) in the
rainy season of boreal summer when inshore water carries a heavy load of silt. Furthermore, during the dry season, phytoplankton biomass is greater at neap than at spring
tides, and thus when the coastal water is clearest (Bainbridge, 1960b). Herbivore consumption of this coastal bloom is small relative to autotrophic production. In the wet
season, the apparent mesozooplankton demand is computed at about 91% of the relatively small daily primary production, but in the dry season only 5% of the larger
bloom is required to sustain the zooplankton. Because in this case the demand of the
benthic community is even less (about 1%), much of the inshore dry season diatom
bloom must be exported from the system or buried in the coastal mud banks as organic
matter (Longhurst, 1983). However, the persistence of large diatoms in inshore blooms
province supports a rich population of phytoplankton-feeding clupeids (Ethmalosa dorsalis) in the inshore waters. These are the ecological equivalent of the Indian oil sardine
(see INDW).
A peculiarity of the zooplankton of the Gulf of Guinea are seasonal “blooms” of
cladocera (mainly Penilia avirostris, but also Evadne tergestina) that sporadically dominate the biomass in nonupwelling situations, this being observed at all seasons. Thus,
off Sierra Leone from October to February (Bainbridge, 1960b) and off western Nigeria
in September (Longhurst, 1964) cladocera may briefly dominate the zooplankton population, especially inshore. The zooplankton profile is typical of tropical seas; at night,
biomass in the 0–25 m zone is 5–10 times greater than below 25 m, whereas by day
the biomass is equally distributed from 0 to 50 m. Genera that perform these minor
diel migrations are chiefly Nanocalanus, Paracalanus, and Neocalanus. Except over water
>500 m deep, the major diel migrants Metridia and Pleuromamma are absent.
It is remarkable that even the very minor upwelling cells along the Guinea coast
should support populations of the paradigmatic large copepod of the major upwellings
in eastern boundary currents: Calanoides carinatus. This species has been recorded extensively over the outer shelf during boreal summer, and preferentially during upwelling
episodes, from Ghana to Angola (Petit and Courties, 1976). In fact, as Bainbridge (1960a)
demonstrated, the global biomass of zooplankton along the shelf from Fernando Po
to Ghana is structured by the relative abundance of Calanoides rather than by the
Chapter 9: The Atlantic Ocean
despite seasonal cloud cover during the appropriate seasons, the images do suggest that
upwelling cells occur along the coast both north and south of Cape Lopez.
Of course, over most of the shelf in this province, most of the time, upwelling is
exceptional and the water column is stratified, with phytoplankton biomass concentrated
in a DCM at the depth of the pycnocline, encountering the continental shelf at 15–45 m.
Most attention has been given to the production cycle in locations where upwelling events
occur, so that for the larger part of the open coast where upwelling does not occur,
it is much more difficult to locate a satisfactory account of the production calendar.
From data on water clarity and color at Lagos, obtained on a standard monthly transect
(Longhurst, 1964), results at midshelf beyond any influence of creek effluents, suggested
that a seasonal bloom occurs during the wet season, when strong onshore winds occur,
during boreal summer months; this is supported by zooplankton biomass data obtained
on the same transect. Copepod, cladoceran, and appendicularian numbers were one to
several orders of magnitude greater in August and September than during the remainder
of the year. This seasonal cycle suggests that the bloom on the open nonupwelling shelf of
Nigeria, or off Sierra Leone and Guinea, is initiated by nutrient flux and later terminated
by nutrient limitation.
In some of the larger embayments, such as the wide Sierra Leone estuary where tidal
streams are strong, the seasonal production cycle is the reverse of that of the open sea,
and algal cells are probably limited by light rather than by nutrients. Phytoplankton
growth (about 850 mg C m
−2 d
−1 ) is maximal in the boreal winter dry season, when
estuarine water is relatively clear, and it is greatly reduced (<75 mg C m
−2 d
−1 ) in the
rainy season of boreal summer when inshore water carries a heavy load of silt. Furthermore, during the dry season, phytoplankton biomass is greater at neap than at spring
tides, and thus when the coastal water is clearest (Bainbridge, 1960b). Herbivore consumption of this coastal bloom is small relative to autotrophic production. In the wet
season, the apparent mesozooplankton demand is computed at about 91% of the relatively small daily primary production, but in the dry season only 5% of the larger
bloom is required to sustain the zooplankton. Because in this case the demand of the
benthic community is even less (about 1%), much of the inshore dry season diatom
bloom must be exported from the system or buried in the coastal mud banks as organic
matter (Longhurst, 1983). However, the persistence of large diatoms in inshore blooms
province supports a rich population of phytoplankton-feeding clupeids (Ethmalosa dorsalis) in the inshore waters. These are the ecological equivalent of the Indian oil sardine
(see INDW).
A peculiarity of the zooplankton of the Gulf of Guinea are seasonal “blooms” of
cladocera (mainly Penilia avirostris, but also Evadne tergestina) that sporadically dominate the biomass in nonupwelling situations, this being observed at all seasons. Thus,
off Sierra Leone from October to February (Bainbridge, 1960b) and off western Nigeria
in September (Longhurst, 1964) cladocera may briefly dominate the zooplankton population, especially inshore. The zooplankton profile is typical of tropical seas; at night,
biomass in the 0–25 m zone is 5–10 times greater than below 25 m, whereas by day
the biomass is equally distributed from 0 to 50 m. Genera that perform these minor
diel migrations are chiefly Nanocalanus, Paracalanus, and Neocalanus. Except over water
>500 m deep, the major diel migrants Metridia and Pleuromamma are absent.
It is remarkable that even the very minor upwelling cells along the Guinea coast
should support populations of the paradigmatic large copepod of the major upwellings
in eastern boundary currents: Calanoides carinatus. This species has been recorded extensively over the outer shelf during boreal summer, and preferentially during upwelling
episodes, from Ghana to Angola (Petit and Courties, 1976). In fact, as Bainbridge (1960a)
demonstrated, the global biomass of zooplankton along the shelf from Fernando Po
to Ghana is structured by the relative abundance of Calanoides rather than by the