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The Danube Blues


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The Iron Gates dam spans the Danube along the border between Romania and Yugoslavia. Completed in 1972, the huge structure, which lies about 620 miles west of the river’s mouth at the Black Sea, was designed to provide the two countries with hydroelectric power. Unfortunately, it now seems that the dam has devastated marine life in the Black Sea.

Researchers at the Romanian Institute for Marine Research in Constanta have been monitoring the Black Sea for the past 20 years and have noted sporadic toxic algal blooms and fish kills since the dam’s construction. More recently, biogeochemist Venugopalan Ittekkot of the University of Hamburg in Germany has been trying to understand if or how the dam might be responsible. Now his ongoing study has shown that silicates--natural components of sand that enter the Danube as it winds to the sea--seem to be the key to the Black Sea’s problems.

The researchers found that the sea’s silicate concentration is now a mere sixtieth of what it was in 1969, while levels of nitrates--a component of fertilizer and sewage--have increased by a factor of 600. They believe the Iron Gates dam is blocking the flow of silicates. Moreover, single-celled plankton called diatoms live in the reservoir behind the dam and use silicates to build their bodies. When they die they sink to the bottom of the reservoir, where the silicate remains.

Meanwhile, beyond the dam, enormous amounts of untreated sewage and agricultural runoff rich in nitrates pour into the Danube from rivers in Bulgaria, Romania, and Ukraine, countries with poorly enforced environmental controls. The change in the ratio of silicate to nitrate has disrupted the entire Black Sea food chain, says Ittekkot.

The Black Sea diatoms don’t get the silicates they need to grow and are being outcompeted by toxic nitrate-using algae that poison fish. The zooplankton that feed on the diatoms--and the fish that in turn feed on the zooplankton--are deprived of their food source. And for a coup de grâce, the bacteria that break down the nitrate-fueled algal blooms suck oxygen from the water, killing more fish.

A similar chain of events can be expected elsewhere--with global implications, says Ittekkot. There are more than 36,000 dams in the world today. The spectacular drop in fish catch in the eastern Mediterranean in the early 1970s was probably due to the Aswân High Dam’s blocking the flow of silicates. Our results show there are long-term implications that might change the complete ecosystem structure of oceans, and that is the most dangerous thing, says Ittekkot. The social and economic implications of this will be enormous.

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