Environmental Engineering Reference
In-Depth Information
ments that indicate complete removal of hydrogen sulphide in the mats (Stief
and Bruchert, unpubl. data). Bruchert et al. [7] used data from 12 shelf stations
to calculate the percentage of the hydrogen sulphide flux relative to bacterial
sulphate reduction and found that between 4 and 51% of formed hydrogen
sulphide diffuses across the sediment-water interface. In the presence of
Beg-
giatoa
, no hydrogen sulphide enters the water column, while
Thiomargarita
may reduce the diffusive upward sulphide flux by up to 45% [7]. The different
physiological adaptations of these two bacterial species to hydrogen sulphide
fluxes and concentrations make them good indicators for differences in the
severity of bottom water anoxia and the potential occurrence of bottom water
sulphide.
5.3 Regional Distribution
Fig. 7a shows the areal distribution of bacterial sulphate reduction rates on
the Namibian shelf and slope. Areas with enhanced bacterial sulphate reduc-
tion correspond to areas of retention rather than to upwelling cells such as the
L uderitz upwelling cell. In the Luderitz cell upwelling is almost perennial [9,
10]. Since upwelling mixes the water column in the cell continuously, phyto-
plankton biomass is not as high as further north, where the water column can
become weakly stratified and water retention is higher [2]. As a consequence,
rates of organic matter accumulation are also smaller in the Luderitz cell. As
less phytoplankton reaches the seafloor, and bottom water ventilation is better
due to the upwelling of ESACW, rates of bacterial sulphate reduction in the
sediment are correspondingly lower.
Diffusive hydrogen sulphide fluxes are always a fraction of the bacterial
sulphate reduction rates (Fig. 7b). The highest fluxes are restricted to three
areas: (1) Between 25
◦
S and 24
◦
S extending approximately from Sylvia Hill to
Meob Bay, (2) from 23
◦
30'S to 22
◦
50'S, i.e, Conception Bay to Pelican Point
including the area of Walvis Bay, and (3) the area from Cape Cross to Henties
Bay. There is no significant agreement between the rates of bacterial sulphate
reduction and hydrogen sulphide fluxes. South of Walvis Bay, the rates of
sulphate reduction and the hydrogen sulphide fluxes roughly correlate, whereas
to the north, none of the areas with enhanced bacterial sulphate reduction are
evident in the hydrogen sulphide flux pattern. The highest hydrogen sulphide
fluxes only coincide in the area inside Walvis Bay and are generally restricted
to a very small area.
The distribution pattern of the large sulphur bacteria can provide answers to
these discrepancies.
Beggiatoa
and
Thiomargarita
are concentrated in different
areas, respectively (Fig. 8a and 8b).
Beggiatoa
is concentrated in the area north
of Palgrave Point and is found north to Cape Frio, whereas
Thiomargarita
is
most abundant in the area south of Palgrave Point and around Walvis Bay.
