1 The African Marginal and Enclosed Seas: An Overview
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1000 km), which propagate to the south along the coast and around the Cape of Good
Hope, depending on the prevailing wind systems. These upwelling pulses, providing
a period of upwelling followed by a period of stratification and relatively calm waters,
can induce very high productivity. In the Benguela system, phytoplankton blooms
usually lag upwelling events by 1–4 days, and continue for 4–10 days. In order for
zooplankton to have a continuous food supply, the blooms must not occur too far
apart. Pulses of upwelling in the Benguela system regularly have duration of 10 days,
an optimal period for biological production. It is estimated that the Benguela system
is 30–60 times more productive per unit area than the global ocean average. While
upwelling generates abundant primary and secondary production in the upper water
column and near the coast, hypoxic areas called oxygen minimum zones can occur
in deeper waters with limited oxygen exchange, at the coastal shelf and upper coastal
slope. The Benguela oxygen minimum zone starts around a depth of 100 m and is
a few hundred meters thick. Bacteria that use sulphur rather than oxygen reside in
the oxygen minimum zone. The most abundant fishes in the Benguela system are
sardines (Sardinops ocelata) and anchovies (Engraulis capensis). The sardine was
intensely fished beginning in the 1950s, with peak catches in 1968, but has declined
since then to the point that the anchovy fishery has taken over. Similar to the Pacific
Ocean El Niño, about once per decade warm, nutrient poor surface waters enter the
northern part of the Benguela upwelling system off the Namibia coast. During the
so-called Benguela Niño, the Angola Current moves southward, from 15
◦ S to 25
◦ S,
extending 150 km offshore and to 50 m depth. Temporal proximity to the Pacific El
Niño, heavy rains and changes in fish abundance have been observed. However, the
causes and effects of the Benguela Niño are not well understood. The phenomenon
has been ascribed to the effect of winds in the western equatorial Atlantic Ocean that
propagates as subsurface temperature anomalies to the African coast; but the importance of local winds in its development off the coast of Namibia and Angola has
also been documented. This local occurrence and the remote signal from the equatorial regions form the basis of a generation mechanism in which both processes can
reinforce each other.
1.3 Indian Ocean African Waters
The Indian Ocean, connecting the marginal basins east of the African continent, is the
third largest of the Earth’s oceanic divisions. It is the youngest of the major oceans,
but its origin and evolution are the most complicated. Its formation is a consequence
of the breakup, about 150 million years ago, of the southern supercontinent Gondwana, followed by the movement to the northeast of the Indian subcontinent (started
about 125 million years ago), which began colliding with Eurasia about 50 million
years ago; by the western movement of Africa and the separation of Australia from
Antarctica some 53 million years ago. By 36 million years ago, the Indian Ocean
had taken on its present configuration.
The Indian Ocean occupies about 14.4 % of the planetary surface, representing
an area of approximately 73,556,000 km
2 , with an average depth of 3,890 m, and a
9
1000 km), which propagate to the south along the coast and around the Cape of Good
Hope, depending on the prevailing wind systems. These upwelling pulses, providing
a period of upwelling followed by a period of stratification and relatively calm waters,
can induce very high productivity. In the Benguela system, phytoplankton blooms
usually lag upwelling events by 1–4 days, and continue for 4–10 days. In order for
zooplankton to have a continuous food supply, the blooms must not occur too far
apart. Pulses of upwelling in the Benguela system regularly have duration of 10 days,
an optimal period for biological production. It is estimated that the Benguela system
is 30–60 times more productive per unit area than the global ocean average. While
upwelling generates abundant primary and secondary production in the upper water
column and near the coast, hypoxic areas called oxygen minimum zones can occur
in deeper waters with limited oxygen exchange, at the coastal shelf and upper coastal
slope. The Benguela oxygen minimum zone starts around a depth of 100 m and is
a few hundred meters thick. Bacteria that use sulphur rather than oxygen reside in
the oxygen minimum zone. The most abundant fishes in the Benguela system are
sardines (Sardinops ocelata) and anchovies (Engraulis capensis). The sardine was
intensely fished beginning in the 1950s, with peak catches in 1968, but has declined
since then to the point that the anchovy fishery has taken over. Similar to the Pacific
Ocean El Niño, about once per decade warm, nutrient poor surface waters enter the
northern part of the Benguela upwelling system off the Namibia coast. During the
so-called Benguela Niño, the Angola Current moves southward, from 15
◦ S to 25
◦ S,
extending 150 km offshore and to 50 m depth. Temporal proximity to the Pacific El
Niño, heavy rains and changes in fish abundance have been observed. However, the
causes and effects of the Benguela Niño are not well understood. The phenomenon
has been ascribed to the effect of winds in the western equatorial Atlantic Ocean that
propagates as subsurface temperature anomalies to the African coast; but the importance of local winds in its development off the coast of Namibia and Angola has
also been documented. This local occurrence and the remote signal from the equatorial regions form the basis of a generation mechanism in which both processes can
reinforce each other.
1.3 Indian Ocean African Waters
The Indian Ocean, connecting the marginal basins east of the African continent, is the
third largest of the Earth’s oceanic divisions. It is the youngest of the major oceans,
but its origin and evolution are the most complicated. Its formation is a consequence
of the breakup, about 150 million years ago, of the southern supercontinent Gondwana, followed by the movement to the northeast of the Indian subcontinent (started
about 125 million years ago), which began colliding with Eurasia about 50 million
years ago; by the western movement of Africa and the separation of Australia from
Antarctica some 53 million years ago. By 36 million years ago, the Indian Ocean
had taken on its present configuration.
The Indian Ocean occupies about 14.4 % of the planetary surface, representing
an area of approximately 73,556,000 km
2 , with an average depth of 3,890 m, and a