| The Benguela
Current Large Marine Ecosystem (BCLME) is one
of the world's most productive marine environments,
not only in terms of fishery resources but also
mineral deposits. It is also one of the four major
coastal upwelling ecosystems in the world which
lie at the eastern boundaries of the oceans. Its
distinctive bathymetry, hydrography, chemistry
and trophodynamics combine to make it one of the
most productive ocean areas in the world.
The Benguela Current forms
the eastern boundary of the south Atlantic intercontinental
gyre. It sweeps northwards within 150km
of the west coast, bringing cold Antarctic water
into warmer subtropical regions. It flows
in a northwesterly direction along the west coast
of southern Africa and the seabed. It is
one of the four major coastal upwelling ecosystems
in the world which lie at the eastern boundaries
of the oceans. Its distinctive bathymetry,
hydrography, chemistry and trophodynamics combine
to make it one of the most productive ocean areas
in the world.
North of Walvis Bay the
current flow moves offshore away from the coast.
A southward undercurrent reportedly flows at deeper
levels on the continental slope and also nearer
the coast at depths greater than 30 metres.
The speed of the Benguela Current varies between
10 and 30 cm per second depending on the location
off the coast, wind velocity and direction, and
the time of year.
During the upwelling process,
surface water is transported in an offshore direction
by a combination of the effects of the prevailing
equator-ward winds and the rotation of the earth
(Coriolis force). This results in the movement
of deeper cooler bottom water into the upper layers
at the coast.
The rate and intensity
of upwelling fluctuates with seasonal variations
in wind patterns. Bottom topography and
the sea-ward extent of the continental shelf also
influence the upwelling process with high energy
areas found where the shelf is narrowest and the
wind the strongest.
The most intense and important
upwelling region off the Namibian coast is located
in the south near Lüderitz. There are
smaller less intense upwelling cells at Cape Fria,
Pelgrave Point and Conception Bay. Typical
surface temperatures and salinity values in coastal
upwelling areas such as off Lüderitz range
from 11 to 14 °C and 34.8 to 35.2 parts per
thousand respectively.
The marine environment
off Namibia and the dynamics of the Benguela Current
are controlled by seasonal changes in the south
Atlantic high-pressure system. Southerly
winds which blow off Namibia through-out the year
are strongest in winter and spring. In the
Lüderitz area, these winds are strongest
in spring and summer, whereas at Cape Fria, they
tend to be most intense in spring and autumn.
Hot, dry “berg winds” (mountain winds
from the east or the north east in autumn and
winter) also influence the marine environment
by suppressing local upwelling and occasionally
transporting large quantities of dust and sand
far out to sea.
In summer and autumn,
the southerly winds relax off central and northern
Namibia and upwelling becomes weak. The
warm and more saline Angolan Current moves south
and mixes with the cooler water of the Benguela
Current leading to stable stratified conditions
over the continental shelf. The surface
water temperatures rise to between 17 and 22 C
and salinities are usually within the range of
35.5 to 35.9 parts per thousand. These frontal
areas where the two currents mix have high plankton
production and are important spawning and nursery
grounds for pelagic fish.
Not only is the Benguela
at a critical location in terms of the global
climate system, but its marine and coastal environments
are also potentially extremely vulnerable to any
future climate change or increasing variability
in climate - with obvious consequences for long-term
sustainable management of the coast and marine
resources.
Extract from Namibia Environment
Volume 1, article page 51-55 “Namibia's
Marine Environment” by Mick O'Toole and
BCLME website: http://www.bclme.org.na
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