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Mississippi Dead Zone
Sediment filled water meets the blue ocean in this photo at the outflow of the Neuse River after Hurricane Floyd. Recent reports indicate that the large region of low oxygen water often referred to as the 'Dead Zone' has spread across nearly 5,800 square miles of the Gulf of Mexico again in what appears to be an annual event. NASA satellites monitor the health of the oceans and spots the conditions that lead to a dead zone.

Image to right: This image shows the outflow of the Neuse River. It is an example of contrast seen in the Gulf of Mexico when sediment filled water meets the ocean.

The Dead Zone

Images comparing summer and winter in the Gulf of Mexico.

Image above: These images show how ocean color changes from winter to summer in the Gulf of Mexico. Summertime satellite observations of ocean color from MODIS/Aqua show highly turbid waters which may include large blooms of phytoplankton extending from the mouth of the Mississippi River all the way to the Texas coast. When these blooms die and sink to the bottom, bacterial decomposition strips oxygen from the surrounding water, creating an environment very difficult for marine life to survive in. Reds and oranges represent high concentrations of phytoplankton and river sediment.

Ships and Satellites Match Measurements

Image of NOAA ship data. The National Oceanic and Atmospheric Administration (NOAA) ships measured low oxygen water in the same location as the highly turbid water in the satellite images. Most studies indicate that fertilizers and runoff from human sources is one of the major stresses impacting coastal ecosystems. In this image, reds and oranges represent low oxygen concentrations.

Image to right: This image shows the data from the NOAA ship as described above.

River Outlets Support Life

Image showing rivers supporting life. Summer rains wash nutrients, dissolved organic matter and sediment out of the mouths of rivers, into the sea, sparking large phytoplankton blooms. South America presents two excellent examples of river outlets where phytoplankton tends to thrive. Along the northern part of the continent the mouth of the Orinoco River opens into the Caribbean. Along the Eastern side of South America, the mighty Amazon exits its thousand mile journey.

Image to left: Image shows how river outlets help to support marine life.

Creeping Dead Zones

Image still from animation of slow water cycle Enhanced phytoplankton blooms can create dead zones. Dead zones are areas of water so devoid of oxygen that sea life cannot live there. If phytoplankton productivity is enhanced by fertilizers or other nutrients, more organic matter is produced at the surface of the ocean. The organic matter sinks to the bottom, where bacteria break it down and release carbon dioxide. Bacteria thrive off excessive organic matter and absorb oxygen, the same oxygen that fish, crabs and other sea creatures rely on for life.

Image to right: This is a still from the animation showing a slow water cycle. Click on this still to view animation.

Rani Chohan
Goddard Space Flight Center