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(NaturalNews) A report issued last week by the United Nations Environment Programme (UNEP) has found that the number of oxygen-starved ocean "dead zones" has increased by more than a third over the last two years.

Dead zones form when microscopic marine plants called phytoplankton -- which act as the backbone of marine food chains -- explode in mass, attracting oxygen-consuming bacteria. The oxygen depletion, called hypoxia, kills off large quantities of fish, oysters, sea grasses and other forms of ocean life.

Marine scientists say phytoplankton levels naturally swell and die off, but abnormally large die-offs have been observed since the 1970s. The UNEP report says extremely high marine levels of certain nutrients -- such as phosphorus and nitrogen from fertilizers, sewage, animal wastes and other sources -- can cause dead zones.

Marine biologist Robert Diaz of the Virginia Institute of Marine Sciences, who helped compile UNEP's findings on dead zones by reviewing global scientific journals, says the increase may be partly because of better scientific reporting. However, Diaz said "there's no mistaking the consistent upward trend over the last 50 years."

Diaz said the report was unable to estimate the total size of worldwide dead zones, but estimated the area to be roughly 300,000 square kilometers. While the vast majority of dead zones occur during fall and summer, some persist year-round, such as the Baltic Sea's 80,000-sq/km zone.

The UNEP report predicts that because rivers are dumping increasingly high volumes of nitrogen into oceans, dead zones will only worsen. However, paleoceanographer Kjell Nordberg of Goteborg University in Sweden says not all dead zones are caused by human activities, as climate and ocean conditions can cause hypoxia.

Nordberg says the dead zones that are a result of humans can be helped by placing restrictions on sewage discharge and agricultural practices, which can improve marine oxygen levels.

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