Off the coast of southwest Africa, ocean currents, winds, and the underwater shelf interact to create compelling biology and chemistry. Plant-like phytoplankton often bloom in the nutrient-rich surface waters, while bacteria on the seafloor consume decaying plant and animal matter and occasionally release gas that bubbles to the surface.
Just off the coast of Namibia, the Benguela Current flows along the ocean surface. It moves north and west along the coast from South Africa and is enriched by iron and other nutrients from the Southern Ocean and from dust blowing off African coastal deserts. Easterly winds push surface waters offshore and promote upwelling near the coast, which brings up cold, nutrient-rich waters from the deeper ocean. These interactions can make the ocean come alive with color.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this image of the waters off Namibia on April 10, 2014. (Click here for another image of the area on April 13.)
Near the shore, yellow-green features in the water suggest the presence of sulfur. Studies have described how bacteria in oxygen-depleted bottom waters consume organic matter and produce prodigious amounts of hydrogen sulfide. As that gas bubbles up into more oxygen-rich water, the sulfur precipitates out and floats near the surface. It can give off a potent rotten-egg smell and pose a toxic threat to fish.
Further offshore, milky green water may be a bloom of one or several species of phytoplankton. As countless microscopic, plant-like organisms consume sunlight and nutrients, they also consume oxygen. Oxygen depletion can sometimes become so complete that it creates a “dead zone” that can suffocate other marine species. At the same time, the oxygen-depleted waters help sulfur-producing bacteria to thrive.
NASA image by Norman Kuring, NASA Ocean Color team. Caption by Michael Carlowicz.