Oxygen-starved "dead zones" will continue to threaten coastal ecosystems in what scientists are calling one of the world's most pressing environmental problems unless regulations are introduced to reduce nitrogen runoff from fertilizer and sewage outflows, marine biologists say.
The number of dead zones has doubled every 10 years since the 1960s and now ranks with overfishing and habitat loss as a global environmental problem, said Robert Diaz, a marine biologist and professor at the Virginia Institute of Marine Science, who coauthored the study published today in the journal Science.
Dead zones are concentrated mainly around the coasts of industrialized countries and cause the widespread killing of fish and other marine organisms. A handful of efforts are under way that could mitigate the effects of "hypoxic" (oxygen-depleted) dead zones, but the problem will get worse unless governments put stronger regulations in place, he said.
The main culprit is nitrogen from runoff of fertilizers, sewage outflows, and burning fossil fuels, Prof. Diaz said. Nutrient pollution from agricultural runoff fuels blooms of algae, a process known as eutrophication, that rot and consume oxygen.
"Dead zones are not local problems," Prof. 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."
In Canada, oxygen depletion in the lower St. Lawrence Estuary is killing cod and other fish species, Prof. Diaz said. In New Brunswick, dead zones resulting from nitrogen runoff from fish farms have caused massive fish kills and the transformation of complex and diverse coastal habitat into barren seascapes dominated by a few species, said Inka Milewski, a marine biologist with the Conservation Council of New Brunswick.
"They're multiplying at an alarming rate," she said.
In Lake Winnipeg, concentrations of nitrogen have approximately doubled in less than 10 years, causing periodic oxygen-depletion. Ms. Milewski conceded that the Canadian dead zones are not nearly as bad as the largest in North America in the Gulf of Mexico - the result of runoff from two-fifths of the continental United States. But she said the federal and provincial governments must regulate nitrogen emissions - similar to regulations already in place with phosphates - before the situation reaches a crisis.
Only Manitoba has regulations on nitrogen-based fertilizers, said Clyde Graham, a spokesman for the Canadian Fertilizer Institute. In every other province, the fertilizer industry works on a voluntary basis with government to reduce overuse and runoff.
"We think that there are many economic incentives for farmers to use fertilizer as effectively as possible," Mr. Graham said.
Some waters remain oxygen-depleted all year, but the problem usually occurs in Canada in the warmer months. Fish that swim into dead zones usually go belly up; the few that don't die either swim farther out to sea or are trapped in bays. The drastic changes to the ecosystem have caused entire species to leave some parts of the world and hurt many coastal fisheries, Prof. Diaz said. Bottom dwellers such as shrimp, clams and worms are forced to move to the surface away from the dead zones, making them stressed and more vulnerable to predators, he said.
Marine biologists have been sounding the alarm on hypoxic zones for decades. In 1995, a report estimated that there were around 60 worldwide, but Prof. Diaz's study, which quantified the problem by cross-referencing all the data available in the scientific community, estimates there are now 405, affecting a total area of more than 245,000 square kilometres.
The major increases have been in South America and Asia, where dead zones have been documented in recent years around major fisheries, he added.
Dead zones can be drastically reduced or eliminated with the proper regulations, Prof. Diaz said, noting that it happened when sanctions were put in place around the Mersey Estuary near Liverpool, England, and the Mondego Estuary in Portugal. But he said the changes needed to reverse the trend wouldn't have an immediate effect because it takes decades for fertilizer use to reach coastal ecosystems.
"The problem is so big now," he said. "Governments have to figure out a way to keep the nitrogen in the soil and make the crops green, not the sea."
OCEAN "DEAD ZONES"
A new global study of Earth's oceans shows a marked increase in the number of "dead zones" - areas of seafloor with too little oxygen to sustain most marine life.
WHY DEAD ZONES OCCUR:
1. Pollutants from the burning of fossil fuels rise into the atmosphere and fall back to Earth as rain
Farms use fertilizers, which eventually find their way into the sea
2. Excess nutrients from farm fertilizers and burning fossil fuels - primarily nitrogen and phosphorus - cause a rapid growth of algae in coastal waters
3. When the microscopic algae dies it falls to the ocean floor where it provides a rich food source for bacteria
4. The act of decomposition consumes dissolved oxygen from the surrounding waters, rendering them unable to sustain life
*AREAS OF WATER WITH TOO LITTLE OXYGEN FOR FISH AND OTHER ORGANISMS TO SURVIVE
SOURCE: SCIENCE MAGAZINE (AUGUST 15, 2008)