9 The Upwelling Area Off Namibia, the Northern Part of the Benguela Current System
177
from north of the Angola-Benguela front transported below the halocline into the
northern Benguela system (Monteiro et al. 2006).
The SE trade winds generate Ekman offshore transport and the near bottom compensation current transported hydrogen sulphide-rich waters towards the coast. The
upwelling processes transport this water to the surface (e.g. Bailey 1991; Weeks
et al. 2002; Brüchert et al. 2004; Emeis et al. 2004) where the hydrogen sulphide
can be oxidised finally to elemental sulphur. This oxidation depends on water temperature, pH-value, dissolved oxygen content, salinity, the presence of chemical
catalysts and the biological system (Train 1979; Dohnalek and FitzPatrick 1983;
Hill 1984; Millero et al. 1987). The oxidation of hydrogen sulphide to elemental sulphur increases the light scattering and makes the water milky (Weeks et al. 2004a).
Besides the depletion of oxygen hydrogen sulphide has a strong influence on the
marine ecosystem due to the toxic effect on organisms (e.g. Evans 1967). This can
decrease the amount of benthos, fish stocks and other marine organisms or can cause
mass mortalities (Grindley and Sapeika 1969; Horstman 1981; Matthews and Pitcher
1996; Cockroft et al. 1999; Cockcroft 2001). Lobsters are leaving for example the
ocean and can be found in large numbers on the beach. The hydrogen sulphide gas is
also reaching the atmosphere causing “rodden egg” like smell and leading to erosion
effects on metallic objects. Potential regions for the occurrence of sulphur outbreaks
are oxygen minimum zones related to the eastern boundary currents in the ocean
with strong upwelling and high bio-productivity. The Namibian upwelling area is a
prominent example where the sulphur plumes were verified in surface water.
Because of the consequences for the marine ecosystem the identification and registration of colloidal sulphur events is important. The sporadic occurrence of the
events makes the study by land observations or ship cruises difficult. Satellite remote sensing is the only method to investigate the occurrence of sulphur plumes
systematically. Validation was performed by contacting colleagues from Swakopmund during such an event. The contact was realised from both side, when we found
indications in satellite data and when the colleagues recognised the “rodden egg”
like smell. An example is presented in a quasi-true colour image (Red-Green-Blue,
or RGB) of a MERIS scene on 10 April 2004 with a spatial resolution of 300 m
(Fig. 9.6). The plume is clearly visible in Saint Francis Bay. In early studies (Weeks
et al. 2004a) quasi-true colour images derived from SeaWiFS and combinations of
spectral channels were applied to identify sulphur plumes. Thus, sulphur plumes
could not be distinguished from other phenomena changing the water colour such as
coccolithophore blooms. Therefore, discrepancies occurred in the interpretation of
satellite images (Weeks et al. 2004a; Siegel et al. 2007).
A method to identify colloidal sulphur in the area off Namibia was developed on
the basis of detailed analyses of MERIS derived water-leaving reflectances (Ohde
et al. 2007). Sulphur patches in the area off Namibia have specific spectral reflectance
signatures. They significantly differ from the spectral characteristics of the typical
water types in this area and such as different algae blooms, river discharge, and
sediment resuspension. The algorithm derived from MERIS data of the first half of
the year 2004 eliminates all other events changing the water colour. Four conditions
and thresholds were implemented in the classification algorithm and have to be
executed consecutively to flag the pixels of sulphur spectra:
177
from north of the Angola-Benguela front transported below the halocline into the
northern Benguela system (Monteiro et al. 2006).
The SE trade winds generate Ekman offshore transport and the near bottom compensation current transported hydrogen sulphide-rich waters towards the coast. The
upwelling processes transport this water to the surface (e.g. Bailey 1991; Weeks
et al. 2002; Brüchert et al. 2004; Emeis et al. 2004) where the hydrogen sulphide
can be oxidised finally to elemental sulphur. This oxidation depends on water temperature, pH-value, dissolved oxygen content, salinity, the presence of chemical
catalysts and the biological system (Train 1979; Dohnalek and FitzPatrick 1983;
Hill 1984; Millero et al. 1987). The oxidation of hydrogen sulphide to elemental sulphur increases the light scattering and makes the water milky (Weeks et al. 2004a).
Besides the depletion of oxygen hydrogen sulphide has a strong influence on the
marine ecosystem due to the toxic effect on organisms (e.g. Evans 1967). This can
decrease the amount of benthos, fish stocks and other marine organisms or can cause
mass mortalities (Grindley and Sapeika 1969; Horstman 1981; Matthews and Pitcher
1996; Cockroft et al. 1999; Cockcroft 2001). Lobsters are leaving for example the
ocean and can be found in large numbers on the beach. The hydrogen sulphide gas is
also reaching the atmosphere causing “rodden egg” like smell and leading to erosion
effects on metallic objects. Potential regions for the occurrence of sulphur outbreaks
are oxygen minimum zones related to the eastern boundary currents in the ocean
with strong upwelling and high bio-productivity. The Namibian upwelling area is a
prominent example where the sulphur plumes were verified in surface water.
Because of the consequences for the marine ecosystem the identification and registration of colloidal sulphur events is important. The sporadic occurrence of the
events makes the study by land observations or ship cruises difficult. Satellite remote sensing is the only method to investigate the occurrence of sulphur plumes
systematically. Validation was performed by contacting colleagues from Swakopmund during such an event. The contact was realised from both side, when we found
indications in satellite data and when the colleagues recognised the “rodden egg”
like smell. An example is presented in a quasi-true colour image (Red-Green-Blue,
or RGB) of a MERIS scene on 10 April 2004 with a spatial resolution of 300 m
(Fig. 9.6). The plume is clearly visible in Saint Francis Bay. In early studies (Weeks
et al. 2004a) quasi-true colour images derived from SeaWiFS and combinations of
spectral channels were applied to identify sulphur plumes. Thus, sulphur plumes
could not be distinguished from other phenomena changing the water colour such as
coccolithophore blooms. Therefore, discrepancies occurred in the interpretation of
satellite images (Weeks et al. 2004a; Siegel et al. 2007).
A method to identify colloidal sulphur in the area off Namibia was developed on
the basis of detailed analyses of MERIS derived water-leaving reflectances (Ohde
et al. 2007). Sulphur patches in the area off Namibia have specific spectral reflectance
signatures. They significantly differ from the spectral characteristics of the typical
water types in this area and such as different algae blooms, river discharge, and
sediment resuspension. The algorithm derived from MERIS data of the first half of
the year 2004 eliminates all other events changing the water colour. Four conditions
and thresholds were implemented in the classification algorithm and have to be
executed consecutively to flag the pixels of sulphur spectra: