Three Gorges Dam is affecting ocean life

Oceanographic surveys suggest that China’s Three Gorges Dam is already influencing biological productivity in the East China Sea, even though the structure is still under construction.

The dam, on the Yangtze River, will be the world’s largest when it begins full-scale operations later this decade (SN: 5/24/03, p. 323: Available to subscribers at A Dam Shame? Project may slam China’s biodiversity). The first phase of filling the dam’s reservoir was completed in June 2003. That impoundment of water and the material it carries is affecting the food chain in the East China Sea, one of the world’s largest fisheries, says Louis A. Codispoti, a chemical oceanographer at the University of Maryland’s Horn Point Laboratory in Cambridge.

The surface waters of the East China Sea receive much less nutrient-rich sediment than they did before the dam was built, says Codispoti. Measurements taken just downstream of the dam a few months after it was filled suggest that the river there carries just 20 percent of the sediment that it did previously. That trend deprives diatoms, a group of marine microorganisms consumed by larger animals, of the silicon they need to build their shells. As a result, diatom populations have crashed. In August 2003, their numbers in the East China Sea were only 14 percent what they had been 5 years earlier.

Diatom populations showed a slight recovery in 2004, says Codispoti. However, a continued dearth of sediment may trigger a significant shift in the balance of organisms at the base of the food chain, he and his colleagues speculate in the April 16 Geophysical Research Letters.

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