The number of “dead zones” in the world’s oceans may have increased by a third in just two years, threatening fish stocks and the people who depend on them. Fertilizers, sewage, fossil fuel burning and other pollutants have led to a doubling in the number of oxygen-deficient coastal areas every decade since the 1960s.

The damage is caused by explosive blooms of tiny plants known as phytoplankton, which die and sink to the bottom, and then are eaten by bacteria which use up the oxygen in the water. Those blooms are triggered by too many nutrients — particularly phosphorous and nitrogen.

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