230
Chapter 9: The Atlantic Ocean
upwelling core, to expand or contract spatially with it. To verify this thesis, Blasco et al.
(1981) extracted PCA components from the occurrence of the most abundant 104 species
of diatoms, dinoflagellates, and coccoliths along serial cross-shelf sections in this region.
Three components accounted for almost 40% of the total phytoplankton variability. Two
of these were related to the upwelled water mass as it gradually moves offshore, although
PC2 has highest scores inshore and close to the bottom. The third component, PC3, has
highest scores during periods of relaxation of upwelling and is associated with the coastal
warm water current in the bight south of Cape Blanc. This arrangement conforms with
earlier informal suggestions that the phytoplankton species associations are arranged in
linear, coastwise bands.
Most of the current information about transfers within the pelagic ecosystem for the
Canary Current is at the diatom-copepod level, although a recent study off Cape Ghir
at 31
N found very high bacterial consumption of phytoplankton in the upper water
column. About 50% of the total production of 10–25 gC m
−2 d
−1 was consumed in this
way, and 02–04 gC m
−2 d
−1 could be accounted for by the activity of copepods (Head
et al., 2002). Another study in this region found that the dominant two large herbivorous
copepods (Calanoides carinatus and Centropages chierchae) fed preferentially on the small
centric diatoms, together with some pennate species, which are characteristic of such
situations. However, in offshore situations where upwelling blooms were not occurring,
the same species of copepods mostly had empty guts, a fact clearly related to the dominant
phytoplankton assemblage of microflagellates and very large species of diatoms.
We should especially note here the occurrence of Calanoides, a specialist of low-latitude
upwelling systems. We may expect, by analogy with what occurs elsewhere, that during
nonupwelling periods the population of C. carinatus, dominated by stage C5 copepodites,
will descend into deep water. Otherwise, over deep water, the mesozooplankton profile
closely resembles those for the eastern tropical Pacific: biomass is highest in the upper
100 m (not well specified in profiles from this region) and a secondary maximum at 200–
400 m represents diel migrants at their daytime residence depths. Over shallow water,
in the same region, a sharp chlorophyll maximum at about 10–15 m containing many
diatoms was associated with maximum abundance of a wide range of herbivore genera:
Corycaeus, Oithona, Oncaea, Euterpina, Muggaeia, Oikopleura, and others.
In the southern region off Senegal, two species groups of macrozooplankton dominate
the biomass. One (copepod-rich) is associated with cool upwelled water masses as their
contained phytoplankton evolves, the other (thaliacean-rich) with warm tropical surface
water. During the boreal winter upwelling period, the former comes to dominate the
biomass, whereas, during the summer quiet period, the latter dominates. At transitional
seasons between the two states, biomass is relatively low. In fact, the contrary transport
of the coastal current from the north in winter, and the offshore “countercurrent”
(as it is sometimes termed) of NECC water from the south leads to a very complex
distribution of plankton species off Senegal and Mauretania (e.g., Weikert, 1984). This is
further complicated by the intercalation of eddies originating in different water masses.
From the south, in NECC water, come Calanoides carinatus, Nannocalanus minor, and
Penilia avirostris, whereas Calanus helgolandicus is a clear indicator species, associated
with upwelling, of water of northern origin. With Calanus, we may associate Mecynocera
clausii, Candacia armata, and Pleuromamma piseki as typical NW African coastal species.
The fish component of the pelagic ecosystem is rather more diverse than in some
other eastern boundary currents, because of the unusual meridional extent of the Canary
Current system. The principal species, excluding large predators, are anchovy (Engraulis
enchrasicholus), sardine (Sardina pilchardus), sardinellas (Sardinella aurita and S. maderensis), horse mackerel (Trachurus trachurus and T. trechae), and Spanish mackerel (Scomber
japonicus). A peculiarity of this province is the relatively recent invasion of the African
coastline by the “European” sardine (S. pilchardus). Until the 1920s, this fish was restricted
Chapter 9: The Atlantic Ocean
upwelling core, to expand or contract spatially with it. To verify this thesis, Blasco et al.
(1981) extracted PCA components from the occurrence of the most abundant 104 species
of diatoms, dinoflagellates, and coccoliths along serial cross-shelf sections in this region.
Three components accounted for almost 40% of the total phytoplankton variability. Two
of these were related to the upwelled water mass as it gradually moves offshore, although
PC2 has highest scores inshore and close to the bottom. The third component, PC3, has
highest scores during periods of relaxation of upwelling and is associated with the coastal
warm water current in the bight south of Cape Blanc. This arrangement conforms with
earlier informal suggestions that the phytoplankton species associations are arranged in
linear, coastwise bands.
Most of the current information about transfers within the pelagic ecosystem for the
Canary Current is at the diatom-copepod level, although a recent study off Cape Ghir
at 31
N found very high bacterial consumption of phytoplankton in the upper water
column. About 50% of the total production of 10–25 gC m
−2 d
−1 was consumed in this
way, and 02–04 gC m
−2 d
−1 could be accounted for by the activity of copepods (Head
et al., 2002). Another study in this region found that the dominant two large herbivorous
copepods (Calanoides carinatus and Centropages chierchae) fed preferentially on the small
centric diatoms, together with some pennate species, which are characteristic of such
situations. However, in offshore situations where upwelling blooms were not occurring,
the same species of copepods mostly had empty guts, a fact clearly related to the dominant
phytoplankton assemblage of microflagellates and very large species of diatoms.
We should especially note here the occurrence of Calanoides, a specialist of low-latitude
upwelling systems. We may expect, by analogy with what occurs elsewhere, that during
nonupwelling periods the population of C. carinatus, dominated by stage C5 copepodites,
will descend into deep water. Otherwise, over deep water, the mesozooplankton profile
closely resembles those for the eastern tropical Pacific: biomass is highest in the upper
100 m (not well specified in profiles from this region) and a secondary maximum at 200–
400 m represents diel migrants at their daytime residence depths. Over shallow water,
in the same region, a sharp chlorophyll maximum at about 10–15 m containing many
diatoms was associated with maximum abundance of a wide range of herbivore genera:
Corycaeus, Oithona, Oncaea, Euterpina, Muggaeia, Oikopleura, and others.
In the southern region off Senegal, two species groups of macrozooplankton dominate
the biomass. One (copepod-rich) is associated with cool upwelled water masses as their
contained phytoplankton evolves, the other (thaliacean-rich) with warm tropical surface
water. During the boreal winter upwelling period, the former comes to dominate the
biomass, whereas, during the summer quiet period, the latter dominates. At transitional
seasons between the two states, biomass is relatively low. In fact, the contrary transport
of the coastal current from the north in winter, and the offshore “countercurrent”
(as it is sometimes termed) of NECC water from the south leads to a very complex
distribution of plankton species off Senegal and Mauretania (e.g., Weikert, 1984). This is
further complicated by the intercalation of eddies originating in different water masses.
From the south, in NECC water, come Calanoides carinatus, Nannocalanus minor, and
Penilia avirostris, whereas Calanus helgolandicus is a clear indicator species, associated
with upwelling, of water of northern origin. With Calanus, we may associate Mecynocera
clausii, Candacia armata, and Pleuromamma piseki as typical NW African coastal species.
The fish component of the pelagic ecosystem is rather more diverse than in some
other eastern boundary currents, because of the unusual meridional extent of the Canary
Current system. The principal species, excluding large predators, are anchovy (Engraulis
enchrasicholus), sardine (Sardina pilchardus), sardinellas (Sardinella aurita and S. maderensis), horse mackerel (Trachurus trachurus and T. trechae), and Spanish mackerel (Scomber
japonicus). A peculiarity of this province is the relatively recent invasion of the African
coastline by the “European” sardine (S. pilchardus). Until the 1920s, this fish was restricted