Along Namibian coast, off southwest Africa, a cold, deep current snakes northward past the Namib Desert carrying icy waters from deep in the Southern Ocean. Year-round southerly winds cause the warmer surface waters near the coast to be deflected westward away from shore, and the cold waters of the Benguela Current rise up from the depths to replace them.
In the ocean, the welling up of cold water has a positive influence on living organisms. As ocean organisms grow and reproduce in the surface waters of the ocean, they use up the nutrients there. Cold waters welling up from deep in the ocean replenish those nutrients and often result in a rapid increase in marine plant life, called a bloom. The individual, microscopic plants, called phytoplankton, live just a few days, and when they die their remains sink to the bottom of the ocean, where they build up in the mud on the coastal floor.
Bottom-dwelling bacteria chew through this rich belt of coastal mud, decomposing the phytoplankton remains. The result of this decomposition can be seen in this true-color Moderate Resolution Imaging Spectroradiometer (MODIS) image from the Aqua satellite, acquired on October 16, 2003, in which the coastal waters appear milky-green. (A similar event occurred on Jan. 9, 2003.)
Some bacteria consume oxygen as they break down the plant remains, giving off carbon dioxide and water as by-products. But eventually, all the oxygen is used up. At that point, other bacteria take over the decomposition. These bacteria use a form of sulfur when they decompose the organic matter, and give off hydrogen sulfide gas as a by- product. The hydrogen sulfide gas periodically bubbles up from the ocean bottom, and when it encounters more oxygen-rich water near the surface, a chemical reaction occurs that transforms the sulfide gas into pure sulfur. In the first stages of the reaction, the sulfur appears white, and in this image creates a milky-green green tinge to the water. When the transformation is more complete, the plume will look very green—a mixture of the yellow sulfur and blue water.
Ironically, the region’s high productivity is also one of its greatest threats, since the hydrogen sulfide gas resulting from such an explosion of life and its inevitable decay is highly toxic to the fish and other marine animals that feed off the phytoplankton. Periodic die-offs of whole populations of fish and other commercial seafood are ongoing concerns for the regional fishing industry.
Image courtesy Jacques Descloitres, MODIS Rapid Response Team at NASA GSFC