Dead Zones

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.

The apparent cause of the creeping dead zones is agriculture, specifically fertilizer. In some instances there are run offs that filters water which can help to stop pollutants in water systems, but with all of the agricultural crops we produce it is hard to prevent run off from the fields into the streams and rivers of a watershed. Once in the main waterways we cannot filter the fertilizer out before it reaches the ocean and it just becomes more nutrients for the phytoplankton. The phytoplankton do what they do best in the ocean water: they grow and multiply. Which leads to more organic matter reaching the bottom, more bacterial respiration, and more anoxic bottom water.

These effects can be magnified by catastrophe. When the heavy rains of Hurricane Floyd caused extensive flooding in North Carolina in September 1999, the heavy load of nutrients (from dead animals, flooded animal waste ponds, and numerous other sources) reached the sounds that lie between the coast and the Outer Banks, oxygen levels in the water plummeted. The picture at the top of the page shows the heavy load of sediments flowing into Pamlico Sound. SeaWiFS captured a remarkable image on September 23, 1999, when the sediment-laden water was carried into the Gulf Stream. In this image, note the turbidity in the sounds and the deep brown color at the river mouths. In some areas of the Neuse River, the water actually turned red.

In Europe, the flow of water into and out of the Baltic Sea is naturally restricted by the islands and narrow channels around Denmark. Thus, any increase in nutrients which augments biological productivity can be a problem — and that’s what is being observed in the Baltic. The situation at the mouths of major rivers is similar: the area covered by anoxic bottom water appears to be increasing every year.