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Scientists studying the number of marine “dead zones” say the oxygen-starved waters have proliferated since the 1960s and now rank as one of the world's most pressing environmental problems.
The problem is concentrated mainly around the coasts of industrialized countries and is causing the widespread killing of fish and other marine organisms. A handful of efforts are under way that could mitigate the effects of “hypoxic” (oxygen-sapped) dead zones, but the problem will continue to get worse, said Robert Diaz, a marine biologist and professor at the Virginia Institute of Marine Science, who coauthored the study to be released Friday in the journal Science.
The root of the problem is the spread of nitrogen caused by runoff of fertilizers, sewage outflows, and nitrogen deposits from burning fossil fuels, Dr. Diaz said. Nutrient pollution from agricultural runoff fuels blooms of algae – a process known as eutrophication – that rot and consume oxygen levels as they decay.
“Dead zones are not local problems,” Dr. Diaz said. “They are occurring around the globe and have negative consequences in all locations. The surprising thing is that many of these zones are caused by the same set of processes and factors. It all links back to us, and what humans are doing.”
Some waters remain oxygen-depleted all year but most affected areas in Canada occur periodically in the warmer months. Fish that swim into dead zones usually go belly-up – the few that don't either swim farther out to sea or are trapped in bays.
The drastic changes to the ecosystem have eliminated entire species in some parts of the world and hurt many coastal fisheries, Dr. Diaz said. Bottom dwellers, such as some species of shrimp, clams and worms, are more vulnerable. They are forced to move to the surface away from the dead zones, stressing them and making them more vulnerable to prey, which reduces their populations, he said.
Marine biologists have been sounding the alarm on hypoxic zones for decades. In 1995, a report estimated that there were 60 worldwide, but Dr. Diaz's study, which quantified the problem by cross-referencing all the data available in the scientific community, estimated there are now 405 – affecting a total area of more than 245,000 square kilometres. They are now the “key stressor” on marine ecosystems, he said.
The estimate does not include India, where he said there probably are dozens of unstudied dead zones. The major increases have been in South America and Asia, where dead zones have been documented in recent years around major fisheries, he said.
The largest in North America is in the Gulf of Mexico, the result of runoff from two-fifths of the continental United States. The nitrogen content of the Mississippi River has more than doubled since 1965, and nitrate concentrations in the major rivers of the northeastern United States have increased three to 10 times since 1900.
The most obvious alternative is to move away from industrial farming and fertilizer use to organic practices, Dr. Diaz said. But while organic farming is growing in popularity with surging demand, it remains a small industry globally.
Dead zones can be drastically reduced or eliminated over a very short period of time, Dr. Diaz said, as happened when sanctions were put in place around the Mersey Estuary that runs near Liverpool, England, and the Mondego Estuary in Portugal.
“The problem is so big now,” Dr. Diaz said. “Governments have to figure out a way to keep the nitrogen in the soil and make the crops green, not the sea.”
