Atlantic Coastal Biome
245
persistent convergence between these upwelling cells and those off Luderitz to the
south.
Although the upwelling centers just described are the foci of enhanced algal growth
and surface chlorophyll, the Benguela Current is sufficiently dynamic that upwelling
occurs seasonally essentially the whole length of the coast.
Response of the Pelagic Ecosystems
Attention here, as in the other eastern boundary provinces, is necessarily directed at the
response of the pelagic ecosystem to physical processes associated with upwelling: particularly how autotrophic cells modify their nutrient demand and growth characteristics
when brought to the surface, thereby experiencing changes in irradiance and temperature.
Three sequential zones can be recognized in a Benguela upwelling cell, each corresponding to a stage in the evolution of the response of the plankton, both producers and
consumers, to the upwelling process: (i) an inshore zone where nitrate and silicate are
supplied and utilized by an algal bloom; (ii) a middle zone where one of these nutrients
becomes limiting; and (iii) an outer zone, associated with the meandering equatorward
jet current, where autotrophic cells become seriously nutrient limited and changes occur
in floristic composition. Sinking of aggregates and particles occurs principally in the
middle zone, and if this is over shelf depths, remineralization will be important. In general, the chlorophyll-enhanced water follows the outline of the shelf edge except where
it narrows to the south of Luderitz. During upwelling episodes there is often a nearcoastal band of relatively clear, recently upwelled water, with the maximum chlorophyll
concentration of the inshore zone in midshelf about 15–25 km offshore. Chlorophyll
biomass, after upwelling, forms a near-surface maximum (Shannon and Pillar, 1986)
and accumulates at the upwelling front, where <9 mg chl-a m
−3 is observed, associated
with high bacterial abundance. The relative abundance of autotrophic and heterotrophic
cells changes as upwelled water ages and heterotrophic activity comes progressively to
dominate.
The production and subsequent remineralization of organic material from phytoplankton and fecal pellets formed after the advection of nutrients into the photic layer in
upwelling cells is associated with the formation of oxygen-deficient water over the shelf
(Chapman and Shannon, 1985) so that, in the quiet periods between upwelling events, red
tides are common all along the Benguela. The main southeast Atlantic subsurface oxygen
minimum layer lies north of the Benguela-Angola front, between Cape Frio and Luanda,
that is associated with the cyclonic gyre of the Angola Dome. However, off Namibia,
especially Walvis Bay, and off Luderitz, as well as to the north of Cape Columbine, oxygen
deficiency may be induced in situ in shelf waters (Chapman and Shannon, 1985). Both
oxygen-deficient (< 20 ml liter
−1 ) and anoxic water that may even contain H 2 S may be
brought to the surface by coastal upwelling, and this has significant consequences for
shelf biota, including fish kills, especially at Walvis Bay.
The filaments and eddies (chlorophyll values of 3–10 mg chl m
−3 ) of upwelled water
may merge with the next upwelling center so that it is often visually impossible to
separate the production from each individual center in satellite color images. Under
these circumstances, the whole of the Benguela Current appears as a coastal band of
high chlorophyll. Investigation of a mature filament off Namibia revealed that it was
composed of cool, aged upwelled water in which utilization of NH 4 by nanoplankton
and picoplankton dominated nitrogen uptake, and in which salps were very abundant;
the filament lay between warm, oligotrophic water of a detached Agulhas ring and
oligotrophic Atlantic water (Shillington et al., 1990).
The biological response to upwelling differs significantly between the northern and
southern Benguelan regions. Comparing Namibia with the Cape Province, Chapman
245
persistent convergence between these upwelling cells and those off Luderitz to the
south.
Although the upwelling centers just described are the foci of enhanced algal growth
and surface chlorophyll, the Benguela Current is sufficiently dynamic that upwelling
occurs seasonally essentially the whole length of the coast.
Response of the Pelagic Ecosystems
Attention here, as in the other eastern boundary provinces, is necessarily directed at the
response of the pelagic ecosystem to physical processes associated with upwelling: particularly how autotrophic cells modify their nutrient demand and growth characteristics
when brought to the surface, thereby experiencing changes in irradiance and temperature.
Three sequential zones can be recognized in a Benguela upwelling cell, each corresponding to a stage in the evolution of the response of the plankton, both producers and
consumers, to the upwelling process: (i) an inshore zone where nitrate and silicate are
supplied and utilized by an algal bloom; (ii) a middle zone where one of these nutrients
becomes limiting; and (iii) an outer zone, associated with the meandering equatorward
jet current, where autotrophic cells become seriously nutrient limited and changes occur
in floristic composition. Sinking of aggregates and particles occurs principally in the
middle zone, and if this is over shelf depths, remineralization will be important. In general, the chlorophyll-enhanced water follows the outline of the shelf edge except where
it narrows to the south of Luderitz. During upwelling episodes there is often a nearcoastal band of relatively clear, recently upwelled water, with the maximum chlorophyll
concentration of the inshore zone in midshelf about 15–25 km offshore. Chlorophyll
biomass, after upwelling, forms a near-surface maximum (Shannon and Pillar, 1986)
and accumulates at the upwelling front, where <9 mg chl-a m
−3 is observed, associated
with high bacterial abundance. The relative abundance of autotrophic and heterotrophic
cells changes as upwelled water ages and heterotrophic activity comes progressively to
dominate.
The production and subsequent remineralization of organic material from phytoplankton and fecal pellets formed after the advection of nutrients into the photic layer in
upwelling cells is associated with the formation of oxygen-deficient water over the shelf
(Chapman and Shannon, 1985) so that, in the quiet periods between upwelling events, red
tides are common all along the Benguela. The main southeast Atlantic subsurface oxygen
minimum layer lies north of the Benguela-Angola front, between Cape Frio and Luanda,
that is associated with the cyclonic gyre of the Angola Dome. However, off Namibia,
especially Walvis Bay, and off Luderitz, as well as to the north of Cape Columbine, oxygen
deficiency may be induced in situ in shelf waters (Chapman and Shannon, 1985). Both
oxygen-deficient (< 20 ml liter
−1 ) and anoxic water that may even contain H 2 S may be
brought to the surface by coastal upwelling, and this has significant consequences for
shelf biota, including fish kills, especially at Walvis Bay.
The filaments and eddies (chlorophyll values of 3–10 mg chl m
−3 ) of upwelled water
may merge with the next upwelling center so that it is often visually impossible to
separate the production from each individual center in satellite color images. Under
these circumstances, the whole of the Benguela Current appears as a coastal band of
high chlorophyll. Investigation of a mature filament off Namibia revealed that it was
composed of cool, aged upwelled water in which utilization of NH 4 by nanoplankton
and picoplankton dominated nitrogen uptake, and in which salps were very abundant;
the filament lay between warm, oligotrophic water of a detached Agulhas ring and
oligotrophic Atlantic water (Shillington et al., 1990).
The biological response to upwelling differs significantly between the northern and
southern Benguelan regions. Comparing Namibia with the Cape Province, Chapman