246
Chapter 9: The Atlantic Ocean
and Shannon (1985) suggest that upwelling in the south occurs mostly as a series of
short pulses, whereas in the north it is more continuous during the upwelling season.
That the southern upwelling should be more dominated by diatom growth than the
north is probably a consequence of the short time scale of the upwelling pulses. In the
north, high diatom concentrations (2–5 g chl liter
−1 ) occur within a shallow euphotic
zone of 20–35 m in which the colonial Phaeocystis lies preferentially close to the surface
while Coscinodiscus is deeper: this partition typically creates a bimodal chlorophyll profile
(Vordelwuebecke et al., 2000).
Especially in the north, coupling between herbivore biomass and plant growth is close
and this results in rapid silica depletion, as diatom frustules are incorporated in copepod
fecal pellets, then to contribute to the large deposits of biogenic silica that occur in the
shelf deposits off Namibia. In the south, more silica is recycled in the water column.
Consequently, it is probable that nitrate is the limiting nutrient in the south, off the Cape
peninsula, whereas silica limits plant growth in the north, off Namibia, as is reported to
be the case in the Peru-Chile Current (see HUMB).
Mesozooplankton of the Benguela system has low diversity relative to that of the
Agulhas Current to the east and there is little endemism among the individual biota,;
these characteristics were attributed by Gibbons and Hutchings (1996) to the episodic
and intermittent nature of the upwelling process; such pattern is inimical to stability and
diversity. They note also that a large fraction of total biomass is concentrated in a small
fraction of the total area of this province. They also remark that the identity of many of the
herbivore species could be predicted from studies elsewhere: thus, Calanoides carinatus is
the dominant copepod, associated with species of Paracalanus, Clausocalanus, Centropages,
and the rest. As there is a different emphasis in the dominant large algal cells between
neritic (diatoms) and offshore (dinoflagellates) and between northern (dinoflagellates)
and southern (diatoms) regions, similarly, for the mesozooplankton, there are different
regional patterns; offshore and to the north, there is high diversity and low biomass with
many gelatinous organisms, whereas inshore and to the south, crustaceans dominate a
zooplankton of low diversity and high biomass.
Despite such differences in relative abundance of different organisms, there is
very little replacement of species between northern and southern regions; the dominant foraminifera, siphonophora, hydromedusae, scyphomedusae, decapoda, copepoda,
amphipoda, and chaetognatha are similar in both. Only among the euphausiids is a
species-replacement important: Nyctiphanes capensis dominates in the north, Euphausia
lucens in the south. Within these species lists, Gibbons and Hutchings distinguish neritic,
frontal, and oceanic assemblages of differing diversity: neritic low, oceanic high, while
frontal plankton is a mixture of the other two groups and attains higher densities than
either. Within each assemblage, there is equally a vertical segregation that provides niche
partitioning among rather similar species, as between Euphausia hanseni and Nematoscelis
megalops, the former at the shelf break and the latter inshore; even at fronts, vertical segregation is maintained. Pioneer forms, such as Calanoides carinatus, display ontogenetic
migrations that place their resting stages in locations that ensure that upwelling water
will be sufficiently seeded with individuals capable of very rapid growth and reproduction
as to ensure, in turn, a sufficiency of resting individuals. This pattern has been proposed for other pioneer species in the Benguela, such as Sagitta friderici and Euphausia
lucens.
The Benguela current is the habitat—as are all the other eastern boundary currents—of
important pelagic clupeid stocks that consume calanoid copepods and diatoms. Clupeids
often exist as species pairs (one large and one small) in eastern boundary currents and
here Sardinops ocellatus and Engraulis capensis share this habitat with horse mackerel
(T. trachurus) and red-eye (Etrumeus whiteheadi). These shoaling fish occur preferentially
along the upwelling front over the outer continental shelf, and much of the patchiness of
Chapter 9: The Atlantic Ocean
and Shannon (1985) suggest that upwelling in the south occurs mostly as a series of
short pulses, whereas in the north it is more continuous during the upwelling season.
That the southern upwelling should be more dominated by diatom growth than the
north is probably a consequence of the short time scale of the upwelling pulses. In the
north, high diatom concentrations (2–5 g chl liter
−1 ) occur within a shallow euphotic
zone of 20–35 m in which the colonial Phaeocystis lies preferentially close to the surface
while Coscinodiscus is deeper: this partition typically creates a bimodal chlorophyll profile
(Vordelwuebecke et al., 2000).
Especially in the north, coupling between herbivore biomass and plant growth is close
and this results in rapid silica depletion, as diatom frustules are incorporated in copepod
fecal pellets, then to contribute to the large deposits of biogenic silica that occur in the
shelf deposits off Namibia. In the south, more silica is recycled in the water column.
Consequently, it is probable that nitrate is the limiting nutrient in the south, off the Cape
peninsula, whereas silica limits plant growth in the north, off Namibia, as is reported to
be the case in the Peru-Chile Current (see HUMB).
Mesozooplankton of the Benguela system has low diversity relative to that of the
Agulhas Current to the east and there is little endemism among the individual biota,;
these characteristics were attributed by Gibbons and Hutchings (1996) to the episodic
and intermittent nature of the upwelling process; such pattern is inimical to stability and
diversity. They note also that a large fraction of total biomass is concentrated in a small
fraction of the total area of this province. They also remark that the identity of many of the
herbivore species could be predicted from studies elsewhere: thus, Calanoides carinatus is
the dominant copepod, associated with species of Paracalanus, Clausocalanus, Centropages,
and the rest. As there is a different emphasis in the dominant large algal cells between
neritic (diatoms) and offshore (dinoflagellates) and between northern (dinoflagellates)
and southern (diatoms) regions, similarly, for the mesozooplankton, there are different
regional patterns; offshore and to the north, there is high diversity and low biomass with
many gelatinous organisms, whereas inshore and to the south, crustaceans dominate a
zooplankton of low diversity and high biomass.
Despite such differences in relative abundance of different organisms, there is
very little replacement of species between northern and southern regions; the dominant foraminifera, siphonophora, hydromedusae, scyphomedusae, decapoda, copepoda,
amphipoda, and chaetognatha are similar in both. Only among the euphausiids is a
species-replacement important: Nyctiphanes capensis dominates in the north, Euphausia
lucens in the south. Within these species lists, Gibbons and Hutchings distinguish neritic,
frontal, and oceanic assemblages of differing diversity: neritic low, oceanic high, while
frontal plankton is a mixture of the other two groups and attains higher densities than
either. Within each assemblage, there is equally a vertical segregation that provides niche
partitioning among rather similar species, as between Euphausia hanseni and Nematoscelis
megalops, the former at the shelf break and the latter inshore; even at fronts, vertical segregation is maintained. Pioneer forms, such as Calanoides carinatus, display ontogenetic
migrations that place their resting stages in locations that ensure that upwelling water
will be sufficiently seeded with individuals capable of very rapid growth and reproduction
as to ensure, in turn, a sufficiency of resting individuals. This pattern has been proposed for other pioneer species in the Benguela, such as Sagitta friderici and Euphausia
lucens.
The Benguela current is the habitat—as are all the other eastern boundary currents—of
important pelagic clupeid stocks that consume calanoid copepods and diatoms. Clupeids
often exist as species pairs (one large and one small) in eastern boundary currents and
here Sardinops ocellatus and Engraulis capensis share this habitat with horse mackerel
(T. trachurus) and red-eye (Etrumeus whiteheadi). These shoaling fish occur preferentially
along the upwelling front over the outer continental shelf, and much of the patchiness of