Worldwide Coastal Dead Zones Are Multiplying

Worldwide Coastal Dead Zones Are Multiplying



August, 2008 - Around 1910, when

scientists began studying the marine areas of low oxygen known as dead

zones, there were only four of them worldwide.

Now, there are 405 dead zones in the world's coastal waters, covering a

total area of 95,000 square miles, according to the latest research

published today in the journal "Science."

A global study led by Virginia Institute of Marine Science Professor

Robert Diaz shows that the number of dead zones has increased by a third

between 1995 and 2007.

"Dead zones were once rare. Now they're commonplace. There are more of

them in more places," Diaz says. Worldwide, the number of dead zones has

roughly doubled each decade since the 1960s, his research shows.

Diaz and collaborator Rutger Rosenberg of the University of Gothenburg in

Sweden say that dead zones are now "the key stressor on marine ecosystems"

and "rank with over-fishing, habitat loss, and harmful algal blooms as

global environmental problems."

Dead zones occur when excess nutrients, primarily nitrogen and phosphorus,

enter coastal waters and help fertilize blooms of algae. When these

microscopic plants die and sink to the bottom, they provide a rich food

source for bacteria, which in the act of decomposition consume dissolved

oxygen from surrounding waters.

Major nutrient sources include agricultural fertilizers and the burning of

fossil fuels.



Diaz and Rosenberg write, "There's no other variable of such ecological

importance to coastal marine ecosystems that has changed so drastically

over such a short time as dissolved oxygen."

The largest dead zone in the United States today, at the mouth of the

Mississippi River, covers more than 8,500 square miles, an area roughly

the size of New Jersey.

A dead zone also underlies much of the main channel of Chesapeake Bay,

each summer occupying about 40 percent of its area and up to five percent

of its volume.

Geologic evidence shows that dead zones were not "a naturally recurring

event" in Chesapeake Bay or most other estuarine ecosystems, says Diaz.

The first dead zone in Chesapeake Bay was reported in the 1930s.

Scientists refer to water with too little oxygen for fish and other active

organisms as "hypoxic." Diaz says that many ecosystems experience a

progression in which periodic hypoxic events become seasonal and then, if

nutrient inputs continue to increase, persistent.

Earth's largest dead zone, in the Baltic Sea, is hypoxic year-round.

Chesapeake Bay experiences seasonal, summertime hypoxia through much of

its main channel.

Diaz and Rosenberg note that hypoxia tends to be overlooked until it

starts to affect organisms that people eat. A possible indicator of

hypoxia's adverse effects on an economically important finfish species in

Chesapeake Bay is the link between oxygen-poor bottom waters and a chronic

outbreak of a bacterial disease among striped bass.

Several Chesapeake Bay researchers, including VIMS fish pathologist

Wolfgang Vogelbein, believe that the high prevalence of mycobacteriosis,

found in more than 75 percent of the Bay stripers, occurs because they are

weakened by the stress of encountering the Bay's summertime dead zone.

When the dead zone forms, it forces the stripers from the cooler bottom

waters they prefer into warmer waters near the surface.

Diaz and Rosenberg say an even more fundamental effect of hypoxia is the

loss of energy from the Bay's food chain.

Without bottom-dwellers such as clams and worms, their predators lose an

important source of nutrition.

Diaz and VIMS colleague Linda Schaffner estimate that Chesapeake Bay now

loses about five percent of the Bay's total production of food energy to

hypoxia each year.

The Baltic Sea has lost about 30 percent of its food energy, which

contributes to the decline in its fisheries yields.

Diaz and Rosenberg say the key to reducing dead zones is "to keep

fertilizers on the land and out of the sea."

Farmers concerned with the high cost of buying and applying nitrogen to

their crops share that goal.

"They certainly don't want to see their dollars flowing off their fields

into the Bay," says Diaz. "Scientists and farmers need to continue working

together to develop farming methods that minimize the transfer of

nutrients from land to sea."





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