Dead zone

Dead zones are hypoxic (low-oxygen) areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. The term could as well apply to the identical phenomenon in large lakes. In March 2004, when the recently-established UN Environment Programme published its first Global Environment Outlook Year Book (GEO Year Book 2003) it reported 146 dead zones in the world oceans where marine life could not be supported due to depleted oxygen levels. Some of these were as small as a square kilometer, but the largest dead zone covered 70,000 square kilometers.

Causes of dead zones

Aquatic and marine dead zones can be caused by the process of eutrophication, triggered by an excess of plant nutrients (nitrogen and phosphorus) from fertilizers, sewage, combustion emissions from vehicles, power generators, and factories. In a cascade of effects, the nutrients trigger a bloom of phytoplankton at the bottom of the marine food chain, allowing zooplankton to proliferate. As phytoplankton and zooplankton die and sink below the photic zone where photosynthesis can occur, a bloom of natural bacterial degradation exhausts the water's dissolved oxygen.

Dead zones can also be produced by the natural event of river flooding. Large amounts of fresh water empty into the ocean forming a thick layer of fresh water atop the denser salt water, effectively forming a barrier between the ocean water and oxygen in the atmosphere. (Osterman, 2004)

Remains of organisms found within sediment layers near the mouth of the Mississippi River indicate four hypoxic events before the advent of artificial fertilizer. In these sediment layers, anoxia-tolerant species are the most prevalant remains found. The periods indicated by the sediment record correspond to historic records of high river flow recorded by instruments at Vicksburg, Mississippi.

Effects of dead zones

Low oxygen levels recorded along the Gulf Coast of North America have led to reproductive problems in fish involving decreased size of reproductive organs, low egg counts and lack of spawning activity.

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 3 or more hours per day were found to have smaller reproductive organs. The male gonads were 34 to 50% as large as males of similar size in bays where the oxygen levels were normal (6 to 8 ppm). Females were found to have ovaries that were half as large as those in normal oxygen levels. 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)

Another study by the University of Texas at Austin Marine Science Institute was done on the Atlantic croaker fish in Pensacola Bay, Florida. The study was of year-old croakers that live in an estuary that has summer-long hypoxic conditions. During the study, none of the fish spawned at the expected time, or later. Examination of sample fish determined that they lacked mature eggs or sperm. (Murphy, et. al., 2004)

Fish raised in laboratory created hypoxic conditions showed extremely low sex-hormone concentrations and increased elevation of activity in two genes triggered by the hypoxia-inductile factor (HIF) protein. Under hypoxic conditions, HIF pairs with another protein, ARNT. The two then bind to DNA in cells, activating genes in those cells.

Under normal oxygen conditions, ARNT combines with estrogen to activate genes. Hypoxic cells in a test tube didn't react to estrogen placed in the tube. HIF appears to render ARNT unavailable to interact with estrogen, providing a mechanism by which hypoxic conditions alter reproduction in fish. (Johanning, 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 mineralization and recycling that occurs among benthic life-forms is stifled.

Locations of dead zones

In the 1970s, marine dead zones were first noted in areas where intensive economic use stimulated "first-world" scientific scrutiny: in the U.S. East Coast's Chesapeake Bay, in Scandinavia's strait called the Kattegat, which is the mouth of the Baltic Sea and in other important Baltic Sea fishing grounds, in the Black Sea, (which may have been anoxic in its deepest levels for millennia, however) and in the northern Adriatic.

Currently the most notorious dead zone is a 20,000 square kilometer 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, the Midwest, affecting important shrimp fishing grounds.

Other marine dead zones have appeared in coastal waters of South America, China, Japan, southeast Australia, and even in the waters of New Zealand, which have a reputation for being some of the planet's most pristine near-coastal marine environments.

Reversal of dead zones

Dead zones are not irreversible. The Black Sea dead zone, previously the largest dead zone 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, the U.N. has advocated other cleanups by reducing large industrial emissions. From 1985-2000, 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.

References

• Osterman, L.E., et al. 2004. Reconstructing an 180-yr record of natural and anthropogenic induced hypoxia from the sediments of the Louisiana Continental Shelf. Geological Society of America meeting. Nov. 7-10. Denver. http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_75830.htm Abstract].
• Landry, C.A., S. Manning, and A.O. Cheek. 2004. Hypoxia suppresses reproduction in Gulf killifish, Fundulus grandis. e.hormone 2004 conference. Oct. 27-30. New Orleans.
• Murphy, C. . . . P. Thomas, et al. 2004. Modeling the effects of multiple anthropogenic and environmental stressors on Atlantic croaker populations using nested simulation models and laboratory data. Fourth SETAC World Congress, 25th Annual Meeting in North America. Nov. 14-18. Portland, Ore. Abstract (http://abstracts.co.allenpress.com/pweb/setac2004/document/?ID=42930).
• Johanning, K., et al. 2004. Assessment of molecular interaction between low oxygen and estrogen in fish cell culture. Fourth SETAC World Congress, 25th Annual Meeting in North America. Nov. 14-18. Portland, Ore. Abstract (http://abstracts.co.allenpress.com/pweb/setac2004/document/?ID=41689).

External links

• UN Geo Yearbook 2003 report on nitrogen and dead zones (http://www.unep.org/geo/yearbook/087.htm)
• MSNBC report on dead zones (http://www.msnbc.msn.com/id/4624359/), March 29, 2004
• NASA on dead zones (Satellite pictures) (http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/dead_zones.html)
• Gulf of Mexico Dead Zone - multimedia (http://www.smm.org/deadzone/top.html)

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