NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
Mouth of the Mississippi river, Landsat 7 satellite image. Draining much of North America, the Mississippi carries large quantities of sediment into the Gulf of Mexico and along the coast of Louisiana (light blue areas). Modern farming techniques mean much of this sediment contains phosphorus and nitrogen from fertilisers. These nutrient-rich effluents cause an explosion in the summer population of phytoplankton. When they die, the bacteria responsible for decomposition absorb oxygen in the water leading to a 'dead zone' on the ocean floor where few animals can live. Autumnal storms stir the water and clear the dead zone until the following year. Photographed in 1999.
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