9 The Upwelling Area Off Namibia, the Northern Part of the Benguela Current System
169
forming the Angola-Benguela front. In the south it extends to the warm Agulhas current coming from the Indian Ocean, passing South Africa and entering the Atlantic
Ocean. The coastal Benguela current propagates northward along the African coast
and passes the upwelling area off Namibia. Prevailing south-east trade winds are
blowing the whole year with spatial and temporal variations. The winds induce Ekman offshore transport of surface water, a compensation current in the near bottom
layer resulting in upwelling of cold nutrient-rich waters along the coast. Due to the
physical and biological characteristics the entire area can be divided into two regions.
The northern region which mainly include the Namibian waters is characterised by
seasonal warming during late summer and early autumn (Boyd et al. 1987; Shannon
et al. 1987) and intense upwelling throughout the year (Boyd and Agenbag 1985;
Campillo-Campbell and Gordoa 2004). The upwelling is stronger during the cooler
months. The seasonal effect caused by the intrusion of warm saline water from the
equatorial region reduces the upwelling to the Lüderitz area and increases the permanent SST latitudinal gradient (Gordoa et al. 2000; Campillo-Campbell and Gordoa
2004). The upwelling filaments may extend up to 625 km offshore (Lutjeharms and
Meeuwis 1987; Lutjeharms and Stockton 1987) and over 1000 km off Lüderitz (Lutjeharms et al. 1991). The region around Lüderitz is the most active upwelling cell
(Boyd and Agenbag 1985; Shannon 1985; Agenbag and Shannon 1988).
The Namibian waters are very productive because of the continuous nutrient input
into the euphotic zone by upwelling processes. The phytoplankton development is
highly variable in space and time and occurs in a succession starting with plankton groups absorbing visible light. Algae groups which are dominantly scattering
light may develop in persistent shallow surface filaments (Siegel et al. 2004). The
absorbing phytoplankton is dominated by diatoms and dinoflagellates and the scattering phytoplankton bloom consist of coccolithophores (Siegel et al. 2007). After the
blooms dead organisms sink down and are decomposed by bacteria under consumption of dissolved oxygen. Denitrification and sulphate reduction lead to hydrogen
sulphide in the lower water layers particularly during calm conditions. Onset of trade
winds cause Ekman offshore transport again and the compensation current transports
the hydrogen sulphide enriched waters onshore. The upwelling processes along the
coast carry this water into the surface layer where hydrogen sulphide can be oxidised
finally to elemental sulphur changing the water colour milky turquoise.
An example of Advanced Very High Resolution Radiometer (AVHRR) SST and
Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll-a shows the highly
variable spatial distributions (Fig. 9.2). The core of the upwelling area is located in
front of Lüderitz with minimum temperature lower than 10
◦ C. The cold upwelled
water is distributed offshore in filaments with a meridional extent of up to 400 km.
The temperature of the open ocean waters outside the influence of upwelling is about
25
◦ C. The chlorophyll maps show the highest concentrations north of the main
upwelling area due to transport of nutrient enriched water by the northward directed
Benguela current and the response time of phytoplankton.
The large extent of the upwelling area, the high spatial and temporal variability
connected with phytoplankton blooms favor remote sensing methods to investigate
these processes synoptically.
169
forming the Angola-Benguela front. In the south it extends to the warm Agulhas current coming from the Indian Ocean, passing South Africa and entering the Atlantic
Ocean. The coastal Benguela current propagates northward along the African coast
and passes the upwelling area off Namibia. Prevailing south-east trade winds are
blowing the whole year with spatial and temporal variations. The winds induce Ekman offshore transport of surface water, a compensation current in the near bottom
layer resulting in upwelling of cold nutrient-rich waters along the coast. Due to the
physical and biological characteristics the entire area can be divided into two regions.
The northern region which mainly include the Namibian waters is characterised by
seasonal warming during late summer and early autumn (Boyd et al. 1987; Shannon
et al. 1987) and intense upwelling throughout the year (Boyd and Agenbag 1985;
Campillo-Campbell and Gordoa 2004). The upwelling is stronger during the cooler
months. The seasonal effect caused by the intrusion of warm saline water from the
equatorial region reduces the upwelling to the Lüderitz area and increases the permanent SST latitudinal gradient (Gordoa et al. 2000; Campillo-Campbell and Gordoa
2004). The upwelling filaments may extend up to 625 km offshore (Lutjeharms and
Meeuwis 1987; Lutjeharms and Stockton 1987) and over 1000 km off Lüderitz (Lutjeharms et al. 1991). The region around Lüderitz is the most active upwelling cell
(Boyd and Agenbag 1985; Shannon 1985; Agenbag and Shannon 1988).
The Namibian waters are very productive because of the continuous nutrient input
into the euphotic zone by upwelling processes. The phytoplankton development is
highly variable in space and time and occurs in a succession starting with plankton groups absorbing visible light. Algae groups which are dominantly scattering
light may develop in persistent shallow surface filaments (Siegel et al. 2004). The
absorbing phytoplankton is dominated by diatoms and dinoflagellates and the scattering phytoplankton bloom consist of coccolithophores (Siegel et al. 2007). After the
blooms dead organisms sink down and are decomposed by bacteria under consumption of dissolved oxygen. Denitrification and sulphate reduction lead to hydrogen
sulphide in the lower water layers particularly during calm conditions. Onset of trade
winds cause Ekman offshore transport again and the compensation current transports
the hydrogen sulphide enriched waters onshore. The upwelling processes along the
coast carry this water into the surface layer where hydrogen sulphide can be oxidised
finally to elemental sulphur changing the water colour milky turquoise.
An example of Advanced Very High Resolution Radiometer (AVHRR) SST and
Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll-a shows the highly
variable spatial distributions (Fig. 9.2). The core of the upwelling area is located in
front of Lüderitz with minimum temperature lower than 10
◦ C. The cold upwelled
water is distributed offshore in filaments with a meridional extent of up to 400 km.
The temperature of the open ocean waters outside the influence of upwelling is about
25
◦ C. The chlorophyll maps show the highest concentrations north of the main
upwelling area due to transport of nutrient enriched water by the northward directed
Benguela current and the response time of phytoplankton.
The large extent of the upwelling area, the high spatial and temporal variability
connected with phytoplankton blooms favor remote sensing methods to investigate
these processes synoptically.