by Dorothy Hoffman

Created on: September 09, 2010   Last Updated: September 14, 2010

Coastal dead zones (also known as hypoxia, or oxygen-depleted zones) have occurred periodically throughout the world, and were observed long before human activities became a major causal factor. According to a recent report issued by NOAA (National Oceanic and Atmospheric Administration), however, incidents of hypoxia have increased nearly 30-fold since 1960. The study documented dead zones in about half of the 647 waterways assessed.

In 2004, the UN’s Environment Programme identified 146 dead zones throughout the world, ranging in size from less than a half a square mile to some 27,000 square miles, and a 2008 study found 405 hypoxic zones in the world’s oceans. These dead zones result from large deposits of nutrients that deplete the oxygen in the water to levels that can’t sustain life; the fish and other marine animals that dwell along the sea bottom die off.

In the Gulf of Mexico, a dead zone off the Louisiana coast stretches over more than 7000 square miles –the second largest dead zone in the world. On the surface, this area looks much like the surrounding waters, but below the surface the absence of normal marine life is quite striking.

Hypoxia can be caused by natural phenomena such as coastal upwelling and changing wind and current patterns; however, dead zones today are largely an effect of human activities along waterways feeding into the seas and other bodies of water. Modern agricultural practices, air pollution, and sewage discharges from urban and suburban areas are the primary culprits. Runoff from large industrial sites, farmlands, and human and animal waste from urban sewage systems contain high levels of nitrogen and phosphorus. In the water, these nutrients produce eutrophication, or oxygen depletion. Large algae blooms often form in stagnant, oxygen-depleted waters, choking off the marine life, and the dissolved oxygen is further depleted when algae decay.

The Mississippi River, which empties into the Gulf of Mexico, carries massive amounts of nitrogen-rich nutrients and toxins from hundreds of urban and suburban communities and farmland all along its course. The oxygen depleting effects of these discharges and the algae blooms that form are particularly bad during the hot summer months. Violent hurricanes, heavy rainfall, and flooding also contribute to hypoxia. Because global warming produces both rising temperatures and more extreme weather events, we can expect even more frequent and massive dead zones and fish kills with climate change. The loss of marine life on the scale seen in the Gulf of Mexico obviously has a devastating effect on seafood industries and, consequently, on the world’s food supply.

Many environmentalists now fear the overlap of the massive oil plume from the BP spill with Gulf’s dead zone could greatly exacerbate the environmental disaster. It may be years before we know how the oil and toxic dispersants may affect the aquatic ecosystem in the Gulf.

Fortunately, dead zones aren’t irreversible. One of the largest dead zones, in the Black Sea, was cleaned up in the decade following the collapse of the Soviet Union, when use of agricultural pesticides and fertilizer was unaffordable to many farmers. The United Nations has recently begun to promote actions to eliminate ocean dead zones. Laws are being enacted in some developed nations to prevent runoff of damaging human and animal waste and toxic chemicals from reaching the earth’s waterways. Policy changes from the mid-1980s through 2000 in European countries along the Rhine River, for example, have significantly reduced the nitrogen levels of the North Sea dead zone.

References

Northern Gulf of Mexico

Dead Zone

Wikipedia

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