Growing global populations, mainly concentrated along coastlines, and the resulting increase in untreated sewage is endangering human health and wildlife, as well as livelihoods from fisheries to tourism, according to the "State of the Marine Environment" report.
"An estimated 80 percent of marine pollution originates from the land," said Achim Steiner, United Nations undersecretary-general and UNEP's executive director.
"And this could rise significantly by 2050 if, as expected, coastal populations double in just over 40 years time and action to combat pollution is not accelerated," Steiner said in a statement.
The report is compiled from a wide variety of government, academic and other sources by UNEP's Global Program of Action for the Protection of the Marine Environment from Land-Based Sources.
In many developing countries, between 80 percent and nearly 90 percent of sewage entering the coastal zones is estimated to be raw and untreated. These wastes contain bacteria and viruses that can contaminate marine species such as shellfish that are consumed by people, Nuttall said.
Studies in the Caribbean Sea have also shown that sewage encourages the spread of disease in corals, ultimately destroying them. About 80 percent of Caribbean coral has been lost to disease in the past 20 years, report researchers at the University of North Carolina in the United States.
Some cities in the developed world also dump their sewage directly into waterways.
More than one half of wastewater entering the Mediterranean Sea is untreated, as is 60 percent of the wastewater discharged into the Caspian Sea, the UNEP report found.
Unlike the United States and countries in the European Union, Canada has no national standards for sewage treatment for cities. Montreal dumps billions of liters of untreated sewage into the St. Lawrence River, while the postcard-perfect tourist city of Victoria, British Columbia, dumps all of its waste directly into the Pacific Ocean.
Such waste can contain high levels of toxic chemicals, heavy metals and excreted pharmaceuticals. The latter pose risks that are only beginning to be understood. Emerging research shows negative impacts on marine life from residues of birth control medications and antidepressant drugs like Prozac, even at extremely low concentrations of less than one part per billion.
"The big unknown" is what effect these pharmaceutical residues might have on chronically exposed plants, animals and people, Christian Daughton, chief of the environmental chemistry branch at the U.S. Environmental Protection Agency, has been reported as saying.
Expensive treatment plants are not the only solution to untreated sewage wastes -- coastal wetlands, salt marshes and mangroves can also do the job, Nuttall explained.
"It's important for governments to conserve and rehabilitate these natural features and take their value into consideration in their urban planning," he said.
Plastic is an even more visible environmental concern, killing more than a million seabirds and 100,000 mammals and sea turtles each year, according to previous UN reports.
Plastic bags, bottle tops and polystyrene foam coffee cups are often found in the stomachs of dead sea lions, dolphins, sea turtles and birds. Seagulls in the North Sea had an average of 30 pieces of plastic in their stomachs, according to a Dutch study in 2004.
The volume of plastic debris was estimated at 8 million pieces a day in 1982 and is unquestionably much higher today, perhaps double or triple that number. About 20 percent of the plastic in the oceans comes from ships or offshore platforms; the rest is blown or washed off the land, according to the U.S. Commission on Ocean Policy.
Plastic debris is now found everywhere, even the remotest regions of Antarctica.
Truly pristine locales no longer exist, writes David K.A. Barnes of the British Antarctic Survey in a recent paper.
"Some surveys have involved the first known visit by man to very 'remote' shores, but our miracle material had long since beaten us there," he wrote.
In parts of the Southern Ocean, marine debris has tripled in volume in the past decade. Barnes has also shown that marine debris is transporting exotic species to locales they could never have reached normally, changing the ecology of some regions.
Most plastics do not biodegrade, they just break up into ever-smaller particles. British scientists have discovered that microscopic pieces of plastic can be found everywhere in the oceans, even inside plankton, the foundation of the marine food chain.
"The problem of marine litter has steadily grown worse, despite national and international efforts to control it," acknowledges the UNEP report.
The report's findings will be officially presented to governments attending a review of the decade-old Global Program of Action initiative taking place in Beijing from Oct. 16-20.
There have been some improvements, the report notes. Levels of oily waste discharged from industry and cities have, since the mid 1980s, been cut by close to 90 percent. Marine contamination from toxic persistent organic pollutants like DDT and discharges of radioactive waste has also been sharply reduced.
However, larger challenges lie ahead, such as global warming and sea level rise.
"So we have a long way to go politically, technically and financially if we are to hand over healthy and productive seas and oceans to the next generation," Steiner said.
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