Fresh surface waters decline in Black Sea.

Fresh surface waters decline in Black Sea

A shallow freshwater layer atop the otherwise salty Black Sea supports fisheries vital to four nations. However, new observations suggest this oxygen-rich zone has thinned by about 30 percent in the past 13 years. Scientists disagree whether the process could threaten fish and whether dams around the Black Sea may have withheld enough fresh water to contribute to the change, discovered during five research cruises this spring and summer.

The expeditions, organized by U.S. and Turkish scientists and involving researchers from seven countries, offered Westerners their first opportunity since 1975 to sample the Black Sea's water and sediments. The sediment samples are scheduled to arrive in the United States by early November aboard the returning ship R/V Knorr.

Shipboard scientists found that the oxygen-rich layer, which in 1975 extended as far down from the surface as about 150 meters, now reaches no deeper than about 110 meters. They also observed that hydrogen sulfide no longer exists at the bottom of the oxygen-rich zone or directly beneath it. Instead, a band of water as thick as 40 meters and containing virtually no oxygen or hydrogen sulfide now spreads throughout the sea beneath the freshwater zone.

Scientists attribute the thinning of the surface layer, and perhaps its separation from the hydrogen sulfide region, to a change in the balance between saline and fresh water entering the 2-kilometer-deep sea. Salty Mediterranean water flows into the sea through the bosporus strait, while fresh water enters it from Turkish and Eastern European rivers, many of them dammed.

Whether dams, nature or a combination of both has altered the water balance remains an unknown variable that could determine the fate of Bulgarian, Rumanian, Soviet and Turkish fisheries. "If the change is due to the damming of rivers, we may not have seen the end results," says expedition organizer James W. Murray of the University of Washington in Seattle. But Bernward J. Hay of the Woods Hole (Mass.) Oceanographic Institution doubts dams could account for the drop in freshwater volume. Hay, who led one of the research cruises, says normal variations in climate and the sea's biological productivity could account for the change in water balance.

New clues to the cause could come from the sediment cores. Scientists hope the sediments -- deposited at different times in the past -- will indicate when oxygen depletion killed off marine life at various depths. Such evidence could help them determine if the recent decrease corresponds to a trend that predates dam construction. However, only long-term monitoring of the balance between the Black Sea's saline and fresh water can clarify the future of its fisheries, Murray says.