Hydrogen Sulphide Eruptions Off the Coast of Namibia
Friday, March 27, 2009 5:02:18 PM
If you think it is plankton bloom, you are wrong. It is a unique feature that has only been observed off the shore of Namibia. Here sporadic events are observed with hydrogen sulphide in the water column or even in the surface waters, which can be detected remotely from satellites. Cold waters well up from deep in the ocean, replenishing nutrients at the ocean surface, often resulting in a rapid increase in large colonies of microscopic ocean plants, phytoplankton, that grow in the nutrient rich water. As the plants use all of the nutrients, they die and sink to the sea floor where they decompose with the help of bacteria.
One specific kind of bacteria release toxic hydrogen sulfide gas into the sediments on the ocean floor. Eventually, the toxic gas periodically erupts from the sediments and bubbles up to the ocean surface; when it encounters more oxygen-rich water near the surface, a chemical reaction occurs that transforms the gas into pure sulfur. The mixture of the yellow sulfur and blue water make the water look green. Such an event is marked by massive fish die-offs and a strong smell that resembles rotten eggs.
The Benguela Upwelling Area off Namibia is one of the major upwelling systems of the earth. It is mainly a result of the cold Benguela current and the trade winds. Variations in the upwelling intensity are dependent on the strength and direction of the prevailing trade winds. Stronger trade winds result in increased upwelling. As coastal winds push surface waters offshore, cold, nutrient-rich waters from below replace them, stimulating plankton blooms that serve as food for many marine organisms, including fish. Normally this is excellent for the fishing industry, but of course hydrogen sulphide eruptions is destructive. As I mentioned in my post on Dead Zones (and Upwelling) an overdose of nutrients in a coastal upwelling zone can lead to dead zones. When upwelling intensifies, more nutrients rise to the surface, where plankton growth explodes to a degree where they are not all eaten but sink to deeper waters, where bacteria use all the available oxygen to decompose them - leading to hypoxia (low oxygen conditions). In South Africa shifts in the upwelling ecosystem have been documented since the early 1990s.
Below is a nice animation of the Benguela upwelling system taken from a NASA page on The Benguela Upwelling Zone, where you can find more information.