A massive dam straddling the river Danube in Romania has altered nutrient levels--and perhaps led to algal blooms--hundreds of kilometers away in the Black Sea. The findings, reported in tomorrow's issue of Nature, suggest that the ecological consequences of some of the world's 36,000 dams may be felt by large water bodies rather than simply the dammed rivers themselves and the surrounding lands.
A team led by Venugopalatan Ittekkot of the University of Hamburg in Germany analyzed data on nutrients such as silicates and nitrogen before and after the Iron Gates dam near the Romania-Yugoslavia border was built in the early 1970s. They found that dissolved silicates in the Danube carried to the sea had declined from about 800,000 tons a year in the late 1950s to about 300 tons a year today. Silicate concentrations in Black Sea waters near the Danube delta have declined similarly in the last 4 decades. Nitrogen concentrations, meanwhile, remained relatively steady.
The altered nutrient balance appears to have been a key factor behind an alarming rise in the number of phytoplankton blooms in the Black Sea in the past 20 years: Researchers registered 42 blooms in the 1980s, compared to just 12 in the 1960s. Toxic species such as Chromulina sp. also have appeared in the Black Sea since the dam was built. According to Ittekkot, the results show that the shift in nutrients has "altered the biogeochemistry not just of the river and the adjacent coastal waters, but also of the entire Black Sea Basin." But Ittekkot adds that other factors--such as overfishing and the introduction of exotic species--may also play a role in the blooms.
The work suggests that some environmental problems in large water bodies may be traced to faraway dams. That is "what makes the new paper particularly important," says John Milliman of the Virginia Institute of Marine Sciences in Gloucester Point. He says the study hints at possible catastrophic consequences if other major rivers feeding the Black Sea--such as the Dnieper and the Don--are ever dammed.