VOA慢速英语:'Dead Zones' Expand in Coastal Waters Around the World

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This is SCIENCE IN THE NEWS in VOA Special English. I'm Bob Doughty.

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And I'm Faith Lapidus. This week, we will tell about two studies. Both involve the world's oceans. One study examined the health of coastal waters for fish and other sea life. The other study examined the quality of air in coastal cities.

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A recent study has raised serious questions about the health of the world's coastal waters. Coastal waters are important feeding grounds for fish and other sea life. But the waters of many coastal areas are being starved of oxygen. The result is that increasingly large areas of ocean are becoming "dead zones" with almost no sea life.

Robert Diaz was the lead writer of the study. He says historically good fishing areas are becoming dead zones. Mister Diaz is a professor at the Virginia Institute of Marine Science at the College of William and Mary. He worked with Rutger Rosenberg of the University of Gothenburg in Sweden. Their study appeared last month in Science, a publication of the American Association for the Advancement of Science.

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The scientists found that an increasing amount of chemical nutrients have been entering coastal waters. The nutrients are mainly nitrogen and phosphorus. They come from crop fertilizers and wastewater that pollute streams and rivers, which flow to the sea.

Nitrogen can also come from the atmosphere. Agriculture, the burning of fossil fuels and natural processes can produce nitrogen compounds. For example, nitrous oxide is a gas linked to climate change. Nitrogen then enters the ocean through rainfall and other atmospheric processes. Mister Diaz says one fourth of the nitrogen affecting coastal areas comes from the atmosphere.

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The nutrients cause an explosion in populations of phytoplankton, small organisms that use light to make food. Phytoplankton create oxygen on their own. But as they die, they sink to bottom waters where bacteria break down the organisms.

The bacteria use up oxygen in the water. This creates areas at the bottom of coastal waters that have little or no dissolved oxygen. Fish, crabs and other sea life cannot live in water that lacks oxygen. They must either move to more oxygenated waters or die.

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Sometimes the oxygen level in the water drops so quickly that it kills all sea life. Professor Diaz tells VOA this happened near the coast of New York in nineteen seventy-six. The damage to the coastal fishing industry was estimated at five hundred million dollars.

The study shows that the problem of oxygen-poor water or hypoxia is growing every year. Professor Diaz began studying dead zones in the nineteen eighties. He has found that the number of dead zones around the world has doubled every ten years since the nineteen sixties.

The study found more than four hundred dead zones around the world. They affect more than two hundred forty-five thousand square kilometers of ocean.

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Dead zones are generally seasonal. They increase in size during the warm summer months when there is more light for phytoplankton and algae. But after becoming dead zones, it is very difficult for coastal waters to regain good levels of oxygen. Some areas, such as parts of the Baltic Sea, have become permanent dead zones.

The Gulf of Mexico has a huge dead zone extending from the mouth of the Mississippi River. It has doubled in size in the past twenty years to about twenty-two thousand square kilometers. Hypoxic waters in the Gulf threaten an important shrimp and crab fishing industry.

Close to Washington, D.C., up to forty percent of the bottom waters of the Chesapeake Bay become a dead zone in the summer.

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The world's largest dead zone is in the Baltic Sea. Professor Diaz says about half of the Baltic dead zone contains no oxygen. This creates another problem. Different bacteria live in water that completely lacks dissolved oxygen. They are known as anaerobic bacteria because they do not need oxygen to live. Some of these bacteria produce poisonous hydrogen sulfide gas.

Professors Diaz and Rosenberg say no other environmental problem facing coastal waters has changed so much over such a short period of time. They say the only way to reduce dead zones is to keep fertilizers on the land and out of the sea. They say it is important that scientists and farmers work together to find ways to reach this goal.

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You are listening to SCIENCE IN THE NEWS, a program in VOA Special English. With Faith Lapidus, I'm Bob Doughty in Washington.

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Coastal cities are often considered good places to live. When many people think of such cities, they think of healthful, fresh air from the sea. But scientists at the University of California at San Diego found that this idea can be far from correct.

A report on the subject recently was published in Proceedings of the National Academy of Sciences. University of California Professor Mark Thiemens led the research team. His team's report blames ships that release dirty smoke by burning low-cost, high-sulfur fuel. The report says both ships at sea and those using their engines in port to produce electricity are to blame.

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The researchers worked at the Scripps Institution of Oceanography in La Jolla, California. They directly measured smoke released from the smokestack of a ship. They also tested air at the end of the Scripps Pier. The area is just north of San Diego, which is both a major city and port. The Scripps Pier is also close to waters with heavy shipping traffic and the city of Los Angeles. Los Angeles has the third largest port in the world.

Primary sulfate is produced when a ship burns fuel called bunker oil. Bunker oil contains a large amount of sulfur. Most sulfur released by ships burning bunker oil is a gaseous pollutant -- sulfur dioxide. After a time, sulfur dioxide becomes sulfate in the atmosphere. The researchers say this primary sulfate may be only a small part of what ships produce.

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The researchers found that ship smoke could cause up to forty-four percent of the sulfate particulates found in coastal California's atmosphere. They say the levels rose to nearly fifty percent on days when ships burning high sulfur fuel in the nearby ports were a major influence.

Professor Thiemens said no one had really expected that ships would be responsible for so many particulates. He said the extremely small particles are especially threatening. They measure only one and one half microns in size. A micron is one millionth of a meter.

The researchers say the particulates can travel long distances because they stay in the atmosphere longer than other pollutants. Professor Thiemens noted that the air of Los Angeles influenced air quality in San Diego. That distance is almost one hundred eighty kilometers.

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Primary sulfur particulates also can threaten human health. When people breathe, the particulates stay in their lungs. Other scientists have said that up to sixty thousand people around the world die each year from dirty ship smoke.

Gerardo Dominguez was a lead writer of the report. He developed a chemical test that showed differences between ship smoke and gases from trucks, cars and other causes. The researchers also used another test that let them identify sulfur molecules in the atmosphere. Professor Thiemens had developed that method earlier.

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Other pollutants from ships include nitrogen oxide and carbon dioxides. Earlier this year, a United Nations report said gases from ships produce more than one billion tons of carbon dioxide every year. Ships are not governed under the Kyoto Protocol, the agreement that establishes limits for gases linked to climate change. But international rules requiring ship fuels that burn cleanly are to become effective in twenty fifteen.

The state of California has already approved new fuel requirements. They are to begin next July. All tankers, supply and passenger ships will have to use cleaner-burning fuels within forty kilometers of the coast.

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This SCIENCE IN THE NEWS program was written by Mario Ritter and Jerilyn Watson. Our producer was Brianna Blake. I'm Faith Lapidus.

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And I'm Bob Doughty. Read and listen to our programs at VOAspecialenglish.com. Join us again at this time next week for more news about science in Special English on the voice of America.