Phytoplankton Bloom off Namibia
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Phytoplankton Bloom off Namibia

November 19, 2007

A flash of blue and green lit the waters off Namibia in early November 2007 as a phytoplankton bloom grew and faded in the Atlantic Ocean. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite captured this photo-like image on November 8, 2007. The bloom stretches from north to south along hundreds of kilometers, though it is brightest in the center of this image. Such blooms are common in the coastal waters off southwest Africa where cold, nutrient-rich currents sweep north from Antarctica and interact with the coastal shelf. At the same time, the trade winds push surface water away from the shore, allowing water from the ocean's floor to rise to the surface, bringing with it iron and other material. The suffusion of nutrients from both the currents and upwelling water creates an environment where tiny surface-dwelling ocean plants (phytoplankton) thrive.

Phytoplankton blooms are so common off Namibia that they often rob the water of oxygen. As the plants die, they sink to the ocean floor where bacteria consume them. There is so much plant material that the bacteria use all of the oxygen before they finish breaking down the plants, creating a dead-zone in the water where fish can't survive. Anaerobic bacteria, which don't require oxygen, take over in the decomposition process, releasing sulfur dioxide as a byproduct. The sulfur dioxide interacts with the ocean water to create solid sulfur and hydrogen sulfide, a poisonous gas, which eventually erupts to the surface killing fish. Though no eruption is readily apparent in this image, hydrogen sulfide eruptions are often visible in satellite imagery because the solid sulfur colors the water a milky blue-green.

The bloom shown here persisted over several days. It first became distinct on October 28 (though clouds covered the region on previous days, so it may have developed earlier) and was just beginning to fade on November 14. The daily images provided by the MODIS Rapid Response System show the bloom growing and fading throughout that period.

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