Environmental Engineering Reference
In-Depth Information
8. Summary
Temporally and spatially variable circulation patterns control the devel-
opment and persistence of an isolated, stagnant bottom boundary layer that
contains low concentrations of oxygen and nitrate. Preconditions are low wind
stress and distance from the Angola-Benguela Front in order to deplete nutrient-
rich SACW of nitrate.
Aerobic respiration in the water column consumes the major amount of
oxygen supplied by upwelling and primary production. Sediments account for
9 to 25 % of the oxygen consumption. The major source of hydrogen sulphide
is the diatomaceous mud belt, where excess organic matter deposition supports
bacterial hydrogen sulphide and methane production. Water column bacterial
sulphate reduction was found to contribute little to the development of water
column hydrogen sulphide.
For the development of bottom water sulphidic conditions, bacterial sulphide
oxidation at the sediment-water interface must be inhibited or ineffective. These
conditions are only met after complete depletion of nitrate in the bottom waters.
The large sulphur bacteria
Beggiatoa
spp. would represent an effective barrier
against hydrogen sulphide. Nitrate-consuming processes in the water column
such as anammox contribute to the development of complete nitrate depletion.
The development of bottom water sulphidic conditions is accelerated by
advective transport mediated by gas ebullition and during catastrophic eruption
events. The structure of the Namibian shelf and slope sediments indicate that
this mechanism is active in an area between 23
◦
S and 24
◦
S in water depths less
than 100 m.
The temporal and spatial variability of water column hydrogen sulphide
occurrences suggest that several mechanisms are active on the shelf. The in-
shore area may be affected more strongly by gas ebullition, whereas the central
shelf probably experiences mainly diffusive supply of hydrogen sulphide to the
bottom waters. Possible linkages between the gas emanations and wind-driven
circulation remain the subject of further research.
Acknowledgements
We would like to thank Lev Neretin for the invitation to contribute to this
topic and the two reviewers for their constructive comments. Tim Ferdelman
provided unpublished data on sulphate reduction rates from 1997, and Daniela
Riechmann provided bacterial counts from the METEOR cruise M48-2. We are
grateful for scientific discussions with Anja van der Plas, Bo Barker Jørgensen,
Tim Ferdelman, Kay-Christian Emeis, Helle Ploug, Marcel Kuypers, Gaute
Lavik, and Heide Schulz. Gerd Bening, Chibola Chiklililwa, Kirsten Imhoff,
Swantje Lilienthal, Martina Meyer, Gerald Nickel, Andrea Schipper, the late
Bernd Schulz, Heidi Skrypzeck, Monika Tr umper, and Tamara Zemskaya pro-
