What are dead zones in a sea really, and what is the impact it has on the sea? How does it affect marine life? Where are they found?

Dead Zones are areas where the bottom water (the water at the sea floor) is anoxic, meaning that it has very low (or completely zero) concentrations of dissolved oxygen. These dead zones are occurring in many areas along the coasts of major continents, and they are spreading over larger areas of the sea floor. Because very few organisms can tolerate the lack of oxygen in these areas, they can destroy the habitat in which numerous organisms make their home.

The cause of anoxic bottom waters is fairly simple: the organic matter produced by phytoplankton at the surface of the ocean (in the euphotic zone) sinks to the bottom (the benthic zone) where it is subject to breakdown by the action of bacteria, a process known as bacterial respiration. The problem is, while phytoplankton use carbon dioxide and produce oxygen during photosynthesis, bacteria use oxygen and give off carbon dioxide during respiration. The oxygen used by bacteria is the oxygen dissolved in the water, and that’s the same oxygen that all of the other oxygen-respiring animals on the bottom (crabs, clams, shrimp, and a host of mud-loving creatures) and swimming in the water (zooplankton, fish) require for life to continue.

The "creeping dead zones" are areas in the ocean where it appears that phytoplankton productivity has been enhanced, or natural water flow has been restricted, leading to increasing bottom water anoxia. If phytoplankton productivity is enhanced, more organic matter is produced, more organic matter sinks to the bottom and is respired by bacteria, and thus more oxygen is consumed. If water flow is restricted, the natural refreshing flow of "oxic" waters (water with normal dissolved oxygen concentrations) is reduced, so that the remaining oxygen is depleted faster.

Many of the areas where increasing bottom water anoxia has recently been observed are near the mouths of major river systems. While Satellite images can’t see the bottom of the ocean, they can see the surface, where sediments from rivers mix with ocean waters. The images shown here are Satellite images (SeaWiFS) observations of the Mississippi River delta, the Yangtze River mouth in China (The Yangtze River mouth is not currently identified as an area with an associated dead zone, but such conditions could develop there in the future), and the Pearl River mouth in China, near Hong Kong.

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