Areas in the ocean, bays, harbors or lakes that have little or no oxygen dissolved in the water (hypoxic zones) are often referred to as “dead zones” because most marine life either dies, or, if they can, leaves the area. Habitats that would normally be teeming with life become, essentially, biological deserts. Oceanographers have been noting the growth of such areas since at least the 1970s.
Hypoxic zones [the word anoxic (no oxygen) is also used] can occur naturally, but there has been an alarming increase in such areas created or enhanced by human activity. There are many physical, chemical, and biological factors that combine to create dead zones, but nutrient pollution from fertilizers is the primary cause of those zones created by humans. Excess nutrients that run off land or are piped as wastewater into rivers or the ocean can stimulate an overgrowth of algae, which then sinks and decomposes in the water. The decomposition process consumes oxygen and depletes the supply available to healthy marine life. A more detailed description of this process can be found in the next section.
Dead zones occur in many areas of the country, particularly along the East Coast, the Gulf of Mexico, and the Great Lakes, but there is no part of the country or the world that is immune. The second largest dead zone in the world is located in the U.S., in the northern Gulf of Mexico.
In March 2004, when the UN Environment Programme published its first Global Environment Outlook Year Book, it reported 146 dead zones in the world's oceans where marine life could not be supported due to depleted oxygen levels. Although some of these were less than a square mile, the largest dead zone covered 27,000 square miles. A 2008 study counted 405 dead zones worldwide.
The cause of anoxic bottom waters is fairly simple: more oxygen is consumed by the bacteria working to breakdown organic matter at the bottom of the ocean than is produced by phytoplankton during photosynthesis at the surface. Normally photosynthesis produces enough oxygen so that there is enough for 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). But those bacteria working overtime on the organic matter produced by phytoplankton consume all the oxygen so there isn’t any left over for shrimp and fish. Oh, by the way – those bacteria consuming organic matter also reduce the excess oxygen coming out of the water into the air – that source represents 50 to 70 percent of the oxygen we breathe!
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.
The apparent cause of the increasing number of dead zones is agriculture, specifically fertilizer. While fertilizer is necessary for many agricultural crops, it can also run off the fields into streams and rivers. When the fertilizer reaches the ocean, it just becomes more nutrients for the phytoplankton, so they do what they do best: they grow and multiply. Which leads to more organic matter reaching the bottom, more bacterial respiration, and more anoxic bottom water.
Climate has a significant impact on the growth and decline of ecological dead zones. During spring months, as rainfall increases, more nutrient-rich water flows down the mouth of the Mississippi River. At the same time, as sunlight increases during the spring, algal growth in the dead zones increases dramatically. In fall months, tropical storms begin to enter the Gulf of Mexico and break up the dead zones, and the cycle repeats again in the spring.
Besides the direct mortality of both animals at the bottom and in the water, low oxygen levels recorded along the Gulf Coast have led to reproductive problems in fish involving decreased size of reproductive organs, low egg counts and lack of spawning.
In a study of the Gulf killifish by the Southeastern Louisiana University done in three bays along the Gulf Coast, fish living in bays where the oxygen levels in the water dropped to 1 to 2 parts per million (ppm) for three or more hours per day were found to have smaller reproductive organs. The number of eggs in females living in hypoxic waters were only one-seventh the number of eggs in fish living in normal oxygen levels. (Landry, et al., 2004)
It might be expected that fish would flee this potential suffocation, but they are often quickly rendered unconscious and doomed. Slow moving bottom-dwelling creatures like clams, lobsters and oysters are unable to escape. All colonial animals are extinguished. The normal re-mineralization and recycling that occurs among benthic life-forms is stifled.
Notable dead zones in the United States include the northern Gulf of Mexico region, surrounding the outfall of the Mississippi River, portions of the Chesapeake Bay, and certain coastal regions of the Pacific Northwest, all of which have experienced recurring events over the last several years.
The most notorious dead zone is an approximately 8,500 square mile region in the Gulf of Mexico, where the Mississippi River dumps high-nutrient runoff from its vast drainage basin, which includes the heart of U.S. agribusiness in the Midwest. The drainage of these nutrients is affecting important shrimp fishing grounds. Louisiana and other Gulf states are impacted and concerned, but doing something about it will require the cooperation of several states whose land drains into the Mississippi River.
The waters of the Chesapeake Bay are impacted by runoff from large concentrated animal feeding operations, other agricultural runoff, and wastewater treatment plant discharges. When 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.
Off the coast of Cape Perpetua, Oregon, there is also a dead zone with a 2006 reported size of 300 square miles. This dead zone only exists during the summer, perhaps due to wind patterns.
Dr. Robert Diaz of the Virginia Institute of Marine Science (VIMS) has created a map of dead zones throughout the world (a version of this map also appeared in the March 2000 issue of Discover magazine). Diaz estimates that the number of such sites could double within a decade.
Dead zones are reversible. The Black Sea dead zone, previously the largest in the world, largely disappeared between 1991 and 2001 after fertilizers became too costly to use following the collapse of the Soviet Union and the demise of centrally planned economies in Eastern and Central Europe. Fishing has again become a major economic activity in the region.
While the Black Sea "cleanup" was largely unintentional and involved a drop in hard-to-control fertilizer usage, progress has been made more intentionally in other counties. From 1985 to 2000, the North Sea dead zone had nitrogen reduced by 37% when policy efforts by countries on the Rhine River reduced sewage and industrial emissions of nitrogen into the water. Other cleanups have taken place in the United States along the Hudson River and San Francisco Bay.
"Dead zone" is a more common term for hypoxia, which refers to a reduced level of oxygen in the water National Ocean Service, NOAA
Science Focus: Dead Zones Goddard Earth Sciences, Data and Information Services Center, NASA
Dead Zones Wikipedia