Large dams worldwide are often vilified for causing problems ranging from erosion to forcing people out of their homes to make room for vast reservoirs.
Now research into a dam on the Danube is focusing attention on a potentially far-reaching threat the huge structures pose to seas and coastal ecosystems that rivers feed into.
A team of European scientists report that a dam known as the Iron Gates is keeping a mineral vital to certain types of algae from entering the Black Sea. The resulting nutrient mix favors algae that are causing blooms of plankton and leading to massive fish kills. Not only is the problem affecting ecosystems on the shoreline, they say, but apparently throughout the Black Sea.
Unless ways can be found to increase the amount of the missing mineral, silica, efforts to control the toxic blooms may fail, the team of scientists says.
"You're looking at one of the extreme examples of what happens when you dam up a river," says John Milliman, a geologist at the College of William and Mary's School of Marine Science at Gloucester Point, Va. The Black Sea is landlocked and thus receives nutrients chiefly from the rivers that feed it. Still, he says, the team's results are significant enough that similar studies should be undertaken in other parts of the world.
Toxic plankton blooms increase
Built between 1971 and 1972 on the Danube River along the border between Romania and what is now Serbia, Iron Gates supplies hydroelectric power to the region. Since it was built, deadly plankton blooms along the Black Sea coastline have grown in number and intensity.
Typically, these blooms have been attributed largely to increases in nitrates and phosphates.
Dams block the flow of these nutrients coming from upstream, but runoff from human activities downstream more than makes up for what the dams trap, says Venugopalan Ittekkot, a Hamburg University biogeochemist and a member of the research team.
Earlier studies by members of his group, however, showed that dams also block silica, the stuff of beach sand and windowpanes. That deficit, they found, is not restored downstream.
The pattern is evident on the Danube, which supplies 70 percent of the Black Sea's fresh water. Rainfall washes silica out of the mountains and into the river, Dr. Ittekkot explains. At the dam, where the waters grow still, suspended silica falls to the sediment on the reservoir's bottom and is trapped.
Comparing the outflows of silica at the mouth of the Danube today with those recorded in 1959-1960, Ittekkot and his colleagues found that the amount of silica the Danube deposits in the Black Sea may have fallen by as much as 70 percent, from 800,000 tons a year to as little as 230,000 tons today.
The new study's most significant contribution, Dr. Milliman notes, is the connection it makes between the silica shortfall and the nutrient and algae balance in the Black Sea.
Since the 1970s, blooms of algae known as diatoms, which are encased in tiny silicate shells, have increased 2.5 times. But blooms of non-diatoms, which thrive on nitrates and phosphates and are responsible for the most toxic and oxygen-depleting outbreaks, have increased six times.
Helpful diatoms need silica
Tellingly, the researchers calculate that diatoms could lock up the Danube's annual flow of dissolved inorganic nitrogen if only the diatoms had access to the amount of silica the Iron Gates blocks each year.
By withholding silica, the dam "has changed the plankton composition," Ittekkot says. "It has changed the food-web structure completely."
He adds that one implication of the team's research, reported in yesterday's issue of the weekly journal Nature, "is that a piecemeal approach to controlling the blooms won't work at all."
Limiting the amount of nitrates and phosphates entering the Danube is vital, "but it won't be enough," he adds.
A warning for other regions
The study, Milliman adds, could help explain the increased problems of plankton blooms and oxygen depletion noted in North American and Northern European coastal regions.
He adds that dams can be built in ways to minimize the buildup of sediment behind them.
"The Chinese have done interesting things, such as add sluiceways to let sediment out of reservoirs," he says.
But in the end, he says, "dams are built for economic reasons. Environmental concerns revolve around the dam's impact on its immediate neighborhood."
As scientists begin to undertake long-term studies to assess dams' effects farther downstream, their work will raise new issues.
"As you look more holistically at a dam's likely environmental impact, and then try to make an economic evaluation of the effects, you might realize that you shouldn't build that dam," Milliman says.