Dead Zones
There are nearly 150 oxygen-starved or “dead zones” in the world’s oceans and seas, a new report by the United Nations Environment Programme (UNEP) shows.
These
‘dead zones’ are linked to an excess of nutrients, mainly nitrogen, that
originate from agricultural fertilizers, vehicle and factory emissions and
wastes. Experts claim that the number and size of deoxygenated areas is on
the rise with the total number detected rising every decade since the
1970s. They are warning that these areas are fast becoming major threats
to fish stocks.
The emergence of areas of artificially low oxygen levels is closely correlated with the use of synthetic fertilizers in agriculture. Nitrogen is a main ingredient of these fertilizers. Even when carefully managed, a lot of the fertilizer applied to crops is left in the soil and is easily washed into rivers and subsequently to the sea. The fertilizers, often in combination with nutrients from sewage, and nitrogen gases from traffic and industrial fumes falling on coastal water from the air, trigger blooms of tiny marine organisms called phytoplankton. Their rapid growth and decomposition uses up oxygen in the sea-water leading to depleted oxygen levels.
Sometimes the effects are mild; sometimes they can be dramatic with fish fleeing the ‘suffocating waters” and creatures, like clams, lobsters, oysters, and other slow moving, bottom living creatures, dying en masse.
Some of the earliest recorded dead zones were in places like Chesapeake Bay in the United States, the Baltic Sea, the Kattegat, the Black Sea and the northern Adriatic Sea. Others have been reported in Scandinavian fjords. The most well known area of depleted oxygen is in the Gulf of Mexico. Its occurrence is directly linked to nutrients or fertilizers brought to the Gulf by the Mississippi River. In some parts of the world, actions have been taken to reduce the amounts of fertilizer and sewage running off the land. An agreement for the River Rhine in Europe, in which countries agreed to reduce by half the levels of nitrogen being discharged, has cut by 37% the quantities of nitrogen entering the North Sea.
Experts believe that global warming, with its likely increase in rainfall and temperatures, may aggravate the problem. Research indicates that there may be large changes in rainfall patterns with a doubling of levels of carbon dioxide. This in turn could lead to a marked increase in the levels of run-off from rivers into the seas.
Actions to reduce the threats should focus on sources of the nitrogen overload. Numerous options are available to governments, partly as a result of new scientific understanding as to how nitrogen ‘cascades’ through the environment.
For example, forests and grasslands have a high ability to ‘soak up’ excess nitrogen and slow down its movement from the land to the rivers and the seas. Other actions include more widespread use of technologies that remove nitrogen compounds from vehicle fumes alongside the wider uptake of alternative energy sources that are not based on burning fossil fuels. Better treatment of sewage, both by high tech systems such as water treatment works and low tech systems, such as wetlands and reed bed networks, will not only reduce nutrient discharges to coastal waters, but will help the world meet the water and sanitation aims in the Millennium Development Goals.