Ocean Dead Zones

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 prevalent 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.

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.

Other marine dead zones have apparently appeared in coastal waters of South America, China, Japan, and New Zealand. A 2008 study counted 405 dead zones worldwide.

Oregon Off the coast of Cape Perpetua, Oregon, there is also a dead zone with a 2006 reported size of 300 square miles (780 sq. km). This dead zone only exists during the summer, perhaps due to wind patterns.

Gulf of Mexico

Sediment from the Mississippi River carries fertilizer to the Gulf of Mexico. Off the coast of Cape Perpetua, Oregon, there is also a dead zone with a 2006 reported size of 300 square miles (780 sq. km). This dead zone only exists during the summer, perhaps due to wind patterns.

Currently, the most notorious dead zone is a 22,126 square kilometre (8,543 sq. mi) 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. The drainage of these nutrients are affecting important shrimp fishing grounds. This is equivalent to a dead zone the size of New Jersey.

There is some concern that the Deepwater Horizon oil spill from April to July 2010 may have significantly affected the dead zone. However, Terry Hazen, a microbial ecologist with the Lawrence Berkeley National Laboratory, has suggested that the oil released from the spill did not travel far enough west in appreciable quantities to affect the current size of the dead zone.

A dead zone off the coast of Texas where the Brazos River empties into the Gulf was also discovered in July 2007.


In the News ...

Huge 'Dead Zone' Predicted in Gulf of Mexico   Live Science - June 19, 2013
A very large dead zone, an area of water with no or very little oxygen, is expected to form in the Gulf of Mexico this year - a trend in recent years, according to the National Oceanic and Atmospheric Administration (NOAA). Meanwhile, models predict the dead zone in the Chesapeake Bay will be smaller than usual. The makings of a dead zone begin with nutrient pollution, primarily fertilizers and agricultural runoff. Once these excess nutrients reach the ocean, they fuel algae blooms. The algae then die and decompose in a process that consumes oxygen and creates lifeless areas where fish and other aquatic creatures can't survive. This zone can have serious impacts on commercial and recreational fisheries on the Gulf Coast.

Ocean Dead Zones Growing; May Be Linked to Warming National Geographic - May 2, 2008
The world's hypoxic zones swaths of ocean too oxygen-deprived to support fish and other marine organisms are rapidly expanding as sea temperatures rise, a new study suggests.

"Dead Zone" off Oregon Coast Is Growing, Study Says National Geographic - August 5, 2006
A "dead zone" of low-oxygen water has been appearing along the Oregon coast each summer since 2002, suffocating crabs and other creatures that can't swim or scuttle away. This year the zone is stretching longer and thicker than it ever has before, possibly reaching into the waters off Washington State.