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Ocean 'dead zones' growing

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Courtesy of Jeff Schmaltz/MODIS Land Rapid Response Team/ GSFC/NASA

(Read caption) The Baltic Sea blooms with algae twice yearly, once in the spring and once in the late summer. This Moderate Resolution Imaging Spectroradiometer image shows the deep green swirls of the summer bloom around the Swedish island of Gotland. The summer bloom is usually caused by toxic blue-green algae growing on the surface of the water. The spring bloom is made up of nontoxic, cold-loving plants called diatoms and dinoflagellates. Though these blooms occur naturally, agricultural and industrial runoff provides additional nutrients that lead to larger, denser blooms.

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Dead zones – areas of oxygen-depleted bottom waters – are spreading at an alarming rate in coastal waters, killing off huge amounts of marine life, a new study has found.

In a paper published today in Science, Robert Diaz, a biological oceanographer at the Virginia Institute of Marine Science, and Rutger Rosenberg, a marine ecologist at Sweden's Göteborg University, identified more than 400 dead zones worldwide, affecting an area of more than 95,000 square miles, an area roughly the size of Oregon. The number and size of these dead zones are far greater than previously estimated.

While some dead zones occur naturally, many are caused or exacerbated by chemical fertilizer runoff, fossil fuels, and rain. The fertilizer, which is rich in nitrogen compounds, is washed away from farmlands into rivers and ends up in the ocean. Burning fossil fuels produces airborne nitrogen oxides, which the rain washes into the ocean.

The nitrogen compounds feed massive algae blooms. When the algae dies, it sinks to the ocean floor where it is consumed by microbes, which also consume oxygen in the process. As the oxygen is depleted, creating a condition called hypoxia, marine life that can flee does, and life that cannot – some fish but also clams, crustaceans, and other bottom dwellers – die of asphyxiation. At that point, microbes that live in oxygen-free environments begin to thrive and produce hydrogen sulfide, a poisonous gas. Most dead zones are seasonal, as the algae thrives in warm water.

According to the authors, the number of these dead zones has doubled every decade since the 1960s.

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