Ocean 'dead zones' suffocating marine life

Posted: 17 June 2004

The number of oxygen-starved or "dead zones" in the world's oceans and seas is threatening already fragile fish stocks and millions of people who depend upon fisheries for food and livelihoods, the United Nations has warned.

In its report, the UN Environment Programme (UNEP) links these 'dead zones' to an excess of nutrients, mainly nitrogen from agricultural fertilizers. These fertilizers, often in combination with nutrients from sewage, are washed into the sea where they combine with nitrogen gases from traffic and industrial fumes which fall on coastal waters from the air. This can result in blooms of tiny marine organisms called phytoplankton.

Their rapid growth and decomposition uses up oxygen in the sea-water suffocating other marine life in the process. Sometimes the effects are mild, at other times they can be dramatic with fish fleeing the 'suffocating waters' and creatures, like clams, lobsters, oysters, snails and other slow moving, bottom living creatures, dying en masse. Global distribution of oxygen-depleted coastal zonesGlobal distribution of oxygen-depleted coastal zonesClick on thumbnail for larger version

The report estimates that there are nearly 150 dead zones worldwide and scientists warn that the number and size of oxygen-starved areas is on the rise with the total number detected rising every decade since the 1970s.

Some of these dead zones are relatively small, less than one square kilometre in size, whereas others are far larger at up to 70,000 square kilometres. Examples include Chesapeake Bay in the United States, the Baltic Sea, the Kattegat, the Black Sea and the northern Adriatic Sea. The most well known area of depleted oxygen is in the Gulf of Mexico, caused by nutrients or fertilizers washed down the Mississippi River.

northern Gulf of Mexico/Mississippi Delta showing deoxygenated (hypoxic) coastal water (light blue)
northern Gulf of Mexico/Mississippi Delta showing deoxygenated (hypoxic) coastal water (light blue)
Satellite image of the northern Gulf of Mexico/Mississippi Delta showing deoxygenated (hypoxic) coastal water (light blue). This is due to excessive nutrients being washed into the sea.Source: Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC, January 2003

Others have appeared off South America, China, Japan, south east Australia and New Zealand. There is concern that more oxygen-starved areas will emerge in coastal waters off parts of Asia, Latin America and Africa as industrialisation and more intensive agriculture increase the discharge of nutrients.

Threatened fisheries

According to the report, these dead zones are fast becoming a major threat to already threatened fisheries, including those in Europe, where certain areas in the Baltic sea were devoid of life for several months, as were parts of the Irish and Adriatic seas. The largest and oldest dead zone in the world lies in the Black Sea which supports only a few bacteria to a depth of 150 metres.

Robert Diaz, Professor of marine science at Maryland University and contributor to the UNEP report believes that global warming, with its likely increase in rainfall and temperatures, may aggravate the problem by increasing the levels of run-off from rivers into the seas. He warns that dead zones could become a bigger threat to fish stocks than over-fishing.

"Dead zones are especially dangerous to fisheries because they afflict coastal areas where many fish spawn and spend most of their lives before moving to deeper water," said UNEP officer Marion Cheatle. "It is getting noticeably worse."

Action needed

The report advises countries to take action to reduce the amounts of fertilizer and sewage run-off, which will help revive the the seas. Measures recommended include, for example, planting forests along rivers to 'soak up' excess nitrogen, improve 'precision agriculture' so that less fertilizer is wasted, better treatment of sewage to reduce the discharge of nutrients in to coastal waters, more widespread use of technologies that remove nitrogen compounds from vehicles fumes alongside the wider uptake of alternative energy sources that are not based on burning fossil fuels.

The report also encourages countries which share river basins to make agreements, such as the one between states which share river along River Rhine in Europe. This agreement has reduced by half the levels of nitrogen being discharged, and has cut the amount of nitrogen entering the North Sea by 37 per cent. "Humankind is engaged in a gigantic, global experiment as a result of the inefficient and often over-use of fertilizers, the discharge of untreated sewage and the ever rising emissions from vehicles and factories," said Klaus Toepfer, UNEP's Executive Director

"What is clear is that unless urgent action is taken to tackle the sources of the problem, it is likely to escalate rapidly," threatening the "hundreds of millions of people depend on the marine environment for food, for their livelihoods and for their cultural fulfillment," said Toepfer.

Janet Larsen of the Earth Policy Institute writes:

As summer comes to the Gulf of Mexico, it brings with it each year a giant "dead zone" devoid of fish and other aquatic life. Expanding over the past several decades, this area now can span up to 21,000 square kilometres, which is larger than the state of New Jersey.

A similar situation is found on a smaller scale in the Chesapeake Bay, where since the 1970s a large lifeless zone has become a yearly phenomenon, sometimes shrouding 40 per cent of the bay.

Worldwide, there are some 146 dead zones - areas of water that are too low in dissolved oxygen to sustain life. Since the 1960s, the number of deadzones has doubled each decade. Many are seasonal, but some of the low-oxygen areas persist year-round.

What is killing fish and other living systems in these coastal areas? A complex chain of events is to blame, but it often starts with farmers trying to grow more food for the world's growing population. Fertilizers provide nutrients for crops to grow, but when they are flushed into rivers and seas they fertilize microscopic plant life as well. In the presence of excessive concentrations of nitrogen and phosphorus, phytoplankton and algae can proliferate into massive blooms. When the phytoplankton die,they fall to the seafloor and are digested by microorganisms. This process removes oxygen from the bottom water and creates low-oxygen, or hypoxic, zones.

Baltic Sea

Most sea life cannot survive in low-oxygen conditions. Fish and other creatures that can swim away abandon dead zones. But they are still not entirely safe - by relocating they may become vulnerable to predators and face other stresses. Other aquatic life, like shellfish, that cannotmigrate in time suffocate in low-oxygen waters.

Dead zones range in size from small sections of coastal bays and estuaries to large seabeds spanning some 70,000 square kilometers. Most occur in temperate waters, concentrated off the east coast of the United States and in the seas of Europe. Others have appeared off the coasts of China, Japan, Brazil, Australia, and New Zealand.

The world's largest dead zone is found in the Baltic Sea, where a combination of agricultural runoff, deposition of nitrogen from burningfossil fuels, and human waste discharge has overfertilized the sea.

Similar problems have created hypoxic areas in the northern Adriatic Sea, the Yellow Sea, and the Gulf of Thailand. Offshore fish farming is another growing source of nutrient buildup in some coastal waters.

Forty-three of the world's known dead zones occur in US coastal waters. The one in the Gulf of Mexico, now the world's second largest, disrupts ahighly productive fishery that provides some 18 per cent of the US annual catch. Gulf shrimpers and fishers have had to move outside of the hypoxic area to find fish and shrimp. Landings of brown shrimp, the most economically important seafood product from the Gulf, have fallen from the record high in 1990, with the annual lows corresponding to the highly hypoxic years.

Fertilizer runoff

Excess nutrients from fertilizer runoff transported by the Mississippi River are thought to be the primary cause of the Gulf of Mexico's dead zone. Each year some 1.6 million tons of nitrogen now enter the Gulf from the Mississippi basin, more than triple the average flux measured between 1955 and 1970. The Mississippi River drains 41 per cent of the US landmass, yet most of the nitrogen originates in fertilizer used in the productive Corn Belt.

Worldwide, annual fertilizer use has climbed to 145 million tons, a tenfold rise over the last half-century. (See data )

This coincides with the increase in the number of dead zones around the globe. And not only has more usable nitrogen been added to the environment each year, but nature's capacity to filter nutrients has been reduced as wetlands are drained and as areas along riverbanks are developed. Over the last century, the world has lost half its wetlands.

In the United States, some of the key farming states like Ohio, Indiana, Illinois, and Iowa have drained 80 per cent of their wetlands. Louisiana, Mississippi, Arkansas, and Tennessee have lost over half of theirs. This lets even more of the excess fertilizer farmers apply flow down the Mississippi River to the gulf.

There is no one way to cure hypoxia, as the mix of contributing factors varies among locations. But the keys are to reduce nutrient pollution andto restore ecosystem functions. Fortunately, there are a few successes to point to. The Kattegat straight between Denmark and Sweden had been plagued with hypoxic conditions, plankton blooms, and fish kills since the 1970s. In 1986, the Norway lobster fishery collapsed, leading the Danish government to draw up an action plan. Since then, phosphorus levels in the water have been reduced by 80 per cent, primarily by cutting emissions from wastewater treatment plants and industry. Combined with the re-establishment of coastal wetlands and reductions of fertilizer use by farmers, this has limited plankton growth and raised dissolved oxygen levels.

Black Sea

The dead zone on the northwestern shelf of the Black Sea peaked at 20,000 square kilometres in the 1980s. Largely because of the collapse ofcentralised economies in the region, phosphorus applications were cut by 60 per cent and nitrogen use was halved in the Danube River watershed and fell similarly in other Black Sea river basins. As a result, the dead zone shrank. In 1996 it was absent for the first time in 23 years. Althoughfarmers sharply reduced fertilizer use, crop yields did not suffer proportionately, suggesting they had been using too much fertilizer before.

While phosphorus appears to have been the main culprit in the Black Sea, nitrogen from atmospheric sources--namely, emissions from fossil fuel burning--seems to be the primary cause of the dead zones in the North and Baltic seas. Curbing fuel use through efficiency nmprovements, conservation, and a move toward renewable energy can diminish this cause of the problem.

For the Gulf of Mexico, curbing nitrogen runoff from farms can shrink the dead zone. Applying fertilizer to match crop needs more precisely would allow more nutrients to be taken up by plants instead of being washed out to sea. Preventing erosion through conservation tillage and changing crop rotations, along with wetland restoration and preservation, can also play a part.

Innovative programmes such as the American Farmland Trust's Nutrient Best Management Practices Endorsement can reduce the common practice of using too much fertilizer. Farmers who follow recommendations for fertilizer application and cut their use are guaranteed financial coverage for potential shortfalls in crop yields. They save money on fertilizerpurchases and are insured against losses. Under test programs in the United States, fertilizer use has dropped by a quarter.

With carefully set goals and management, it is possible for some dead zones to shrink in as little as a year. For other hypoxic areas(especially in the Baltic, a largely enclosed sea with slower nutrient turnover), improvement may take longer, pointing to the need for earlyaction. For while dead zones shrink or grow depending on nutrient input and climatic conditions, the resulting fish dieoffs are not so easily reversed.

  • A low oxygen zone devoid of marine life has formed off the central Oregon Coast for the second time in three years. The event appears similar to one in 2002, when an area of ocean water with low oxygen content formed along the nearshore Oregon coast between Newport and Florence, causing a massive die-off of fish and invertebrate marine species....read more about this story on the website of ENS. Meanwhile, the BBC reports that a huge "dead zone" of water which has spread across the Gulf of Mexico may be contributing to an unusual spate of shark bites along the Texas coast. Researchers believe that the dead zone forces fish to seek better water, which may be a reason for the recent shark bites on Texas beaches...read more about this report at: BBC News on-line (Nature).

    Related links:

    The UNEP Global Environment Outlook (GEO) Year Book 2003 includes a report and graphics on 'Dead Zones'.

    Earth Policy Institute