318
Chapter 10: The Indian Ocean
suggests that these events produce very prominent chlorophyll features at the retroflection
of the Agulhas Current and that they are quite variable between years. Each event appears
to be initiated by the appearance of a narrow zone of high chlorophyll along the coast
from East London (32
) almost down to Cape Agulhas at 35
S, evidently forced by
inshore upwelling. This feature very quickly begins to stream offshore along the eastern
side of the Agulhas Bank. Such observations from the satellite images are consistent with
other reports that a quasipermanent ridge of cool, upwelled water (12–18
C) lies toward
the southwest from Mossel Bay and extends back along the coast past Port Elizabeth.
This is on the coastal side of the narrow stream of warm subtropical water that is very
clearly seen in serial satellite SST images of this region.
When it reaches the tip of the Bank, there is a tendency for the stream of highchlorophyll water to continue offshore toward the southwest but, in most cases, the
nascent eddy is captured in the flow of the Subtropical Convergence Zone and then
passes off to the east as a mesoscale eddy. Algal growth within these warm-core rings
is limited by convective instability, whereas algal growth outside them is light limited.
Maximum chlorophyll concentration around the edges is consistent with ring-induced
stability (Dower and Lucas, 1993). On the western edge of the Agulhas Bank, uplifting
of the thermocline is episodic, associated with wind events that, during summer, may be
associated with a linear bloom along the shelf edge. This is contiguous with the upwelling
events in the Benguela Current and is perhaps best considered as part of that system.
Despite this very active production system, the pelagic ecosystem is not that of
an upwelling coast. The dominant copepod, Calanus aghulensis, comprises 50–80% of
all mesozooplankton and is a member of the C. helgolandicus group (Huggett and
Richardson, 2000). The principal copepod along the upwelling coast of the Benguela
region is, instead, Calanoides carinatus.
Calanus aghulensis is a diel migrant whose relatively rapid generation time (20 d at
155
C) is more sensitive to food availability than to temperature; diel vertical migration
is linked to food abundance, and ontogenetic layering occurs at daytime depths. Relative
population abundance is greatest around the periphery of the tongue of cold water
discussed earlier, and the area occupied by this species extends from 32
S on the east
coast to about 29
S, at the Orange River, to the west. This copepod is the main resource
on which the Cape anchovy (Engraulis capensis) builds its population (Richardson et al.,
1997); the spawning and subsequent recruitment of anchovies is strongly dependent
on the size of the population of Calanus aghulensis. Although it has been postulated
that population fluctuations of Engraulis and Calanus should be mutually dependent,
such balances are very difficult to analyze. Here, as off California, a negative functional
relationship between population sizes of Sardinella occelatus and Engraulis has also been
proposed—further complicated by the effects on each of varying fishing pressures. It has
also been suggested that on the Agulhas Bank the transfer of biomass from primary and
secondary production to fish biomass is, relative to regions such as Georges Bank, rather
inefficient.
Regional Benthic and Demersal Ecology
The unusually strong southward flow of the Agulhas Current induces bed-load motion
on the shelf and the formation of linear shoals at around 50-m depth, running parallel
to the coastline; these evolve geologically into elevated masses of sandstone that bear
some of the most southerly coral reefs in this ocean, off KwaZulu-Natal at 26–27
S. The
mass of these reefs is due, therefore, not to biogenic accretion of carbonate but to the
sandstone hillocks that originate in the submerged sand dunes on the inner shelf. Only
a few of these reefs break the surface. The warmth of the subtropical water (22–26
C,
seasonally) of the Agulhas Current permits their growth so far to the south.
Chapter 10: The Indian Ocean
suggests that these events produce very prominent chlorophyll features at the retroflection
of the Agulhas Current and that they are quite variable between years. Each event appears
to be initiated by the appearance of a narrow zone of high chlorophyll along the coast
from East London (32
) almost down to Cape Agulhas at 35
S, evidently forced by
inshore upwelling. This feature very quickly begins to stream offshore along the eastern
side of the Agulhas Bank. Such observations from the satellite images are consistent with
other reports that a quasipermanent ridge of cool, upwelled water (12–18
C) lies toward
the southwest from Mossel Bay and extends back along the coast past Port Elizabeth.
This is on the coastal side of the narrow stream of warm subtropical water that is very
clearly seen in serial satellite SST images of this region.
When it reaches the tip of the Bank, there is a tendency for the stream of highchlorophyll water to continue offshore toward the southwest but, in most cases, the
nascent eddy is captured in the flow of the Subtropical Convergence Zone and then
passes off to the east as a mesoscale eddy. Algal growth within these warm-core rings
is limited by convective instability, whereas algal growth outside them is light limited.
Maximum chlorophyll concentration around the edges is consistent with ring-induced
stability (Dower and Lucas, 1993). On the western edge of the Agulhas Bank, uplifting
of the thermocline is episodic, associated with wind events that, during summer, may be
associated with a linear bloom along the shelf edge. This is contiguous with the upwelling
events in the Benguela Current and is perhaps best considered as part of that system.
Despite this very active production system, the pelagic ecosystem is not that of
an upwelling coast. The dominant copepod, Calanus aghulensis, comprises 50–80% of
all mesozooplankton and is a member of the C. helgolandicus group (Huggett and
Richardson, 2000). The principal copepod along the upwelling coast of the Benguela
region is, instead, Calanoides carinatus.
Calanus aghulensis is a diel migrant whose relatively rapid generation time (20 d at
155
C) is more sensitive to food availability than to temperature; diel vertical migration
is linked to food abundance, and ontogenetic layering occurs at daytime depths. Relative
population abundance is greatest around the periphery of the tongue of cold water
discussed earlier, and the area occupied by this species extends from 32
S on the east
coast to about 29
S, at the Orange River, to the west. This copepod is the main resource
on which the Cape anchovy (Engraulis capensis) builds its population (Richardson et al.,
1997); the spawning and subsequent recruitment of anchovies is strongly dependent
on the size of the population of Calanus aghulensis. Although it has been postulated
that population fluctuations of Engraulis and Calanus should be mutually dependent,
such balances are very difficult to analyze. Here, as off California, a negative functional
relationship between population sizes of Sardinella occelatus and Engraulis has also been
proposed—further complicated by the effects on each of varying fishing pressures. It has
also been suggested that on the Agulhas Bank the transfer of biomass from primary and
secondary production to fish biomass is, relative to regions such as Georges Bank, rather
inefficient.
Regional Benthic and Demersal Ecology
The unusually strong southward flow of the Agulhas Current induces bed-load motion
on the shelf and the formation of linear shoals at around 50-m depth, running parallel
to the coastline; these evolve geologically into elevated masses of sandstone that bear
some of the most southerly coral reefs in this ocean, off KwaZulu-Natal at 26–27
S. The
mass of these reefs is due, therefore, not to biogenic accretion of carbonate but to the
sandstone hillocks that originate in the submerged sand dunes on the inner shelf. Only
a few of these reefs break the surface. The warmth of the subtropical water (22–26
C,
seasonally) of the Agulhas Current permits their growth so far to the south.