Though bordered by just six countries — Ukraine, Russia, Georgia, Turkey, Bulgaria, and Romania — the Black Sea is impacted by at least ten more nations through the five major rivers that flow through its watershed and empty into its waters. With the population of these 16 countries exceeding 160 million, it is no wonder that human activities have contributed in major ways to the Black Sea’s plight. Pollution of every variety, invading plant and animal species introduced through human activities (such as shipping), overfishing, and wetlands loss have all taken a toll on this once-healthy sea and its tributaries.

This true-color image of the Black Sea was taken by the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), flying aboard the OrbView-2 satellite, on May 4, 2002. The turquoise swirls are large blooms of microscopic plants (phytoplankton). Courtesy of the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE.

The second problem became obvious after scientists began to analyze how chemical and physical properties change throughout the Black Sea’s “water column” — an imaginary cylinder that represents all the water contained in that section from the surface to the bottom. What they discovered was that water density changes so sharply throughout the column (as it reflects changing salt concentrations), that again, mixing from top to bottom is inhibited.

Besides the Black Sea having a substantial “anoxic” zone (no oxygen, high levels of sulfides), there exists in this basin a very unusual region known as the “suboxic” zone, which is found between the oxygenated surface and the anoxic depths. Because of how chemicals in saltwater typically react with one another, as well as the usual relationship of oxygen to sulfides (whenever one increases, the other decreases), the suboxic zone of minimal oxygen and minimal sulfide levels probably should not exist. But it does, with remarkable stability, for a vertical distance ranging between 20 and 50 meters. That is wide enough for some serious research to take place. And of course, scientists want to know more about how all this curious chemistry occurs!

From April 14 to May 16, 2003, an international team funded by the National Science Foundation (NSF) is undertaking an expedition on the 279-foot U.S. research vessel Knorr to learn more about some of the important chemical, physical, and biological processes occurring within the suboxic zone of the Black Sea.

The chief scientists representing the United States on the expedition are George W. Luther III, of the University of Delaware College of Marine Studies; James W. Murray of the University of Washington School of Oceanography; Bradley M. Tebo, Scripps Institution of Oceanography; and Stuart Wakeham, Skidaway Institute of Oceanography. Also participating in the cruise are scientists from the Ukrainian and Russian Academy of Sciences and Turkey, plus some additional American researchers and students.

Specifically, the scientists are studying the cycling of nitrogen, manganese, iron, and sulfur into different forms as chemical reactions take place under changing conditions (such as variations in depth, temperature, salinity, water density, and oxygen levels).

Another aspect of this research is to investigate the role bacteria play in cycling these chemicals. Previous studies have shown that specific bacteria will generally be found in identifiable water densities to take advantage of the energy released by certain chemical reactions common at those levels. It is also known that different species of bacteria in suboxic and anoxic zones release waste products that may cause other chemicals in the area to be “oxidized” (combined with oxygen), “nitrified” (changed into a nitrate or other form of nitrogen), or “denitrified” (have nitrogen removed). Examining how all of these variables impact one another could give a clue as to how the Black Sea’s unusually stable suboxic zone can even exist.

The current cruise is building on research already begun in the Black Sea under a North Atlantic Treaty Organization (NATO) program designed to provide a more detailed picture of the Black Sea ecosystem and its regional variability. Such baseline data will be extremely useful for predicting and understanding changes in this system, as well as in suboxic zones elsewhere.

Research Cruise

The month-long cruise is taking place in three parts, with quick turn-around stops in Istanbul, Turkey, between each leg of the expedition. The first leg started near Istanbul, where the Bosporus River empties into the Black Sea. As the research vessel draws a number of lines (known as “transects”) across each area to be studied, a total picture of the chemical processes under examination can be acquired by taking readings and samples at chosen stations along each transect. For example, in the first part of the expedition, readings at three different depths (300, 500, and 1750 meters) will be taken along seven transects, each divided into three sampling stations.

Leg 1 (April 15 – April 25) focuses on the southwest Black Sea.
The primary goal of this research leg is to do 3-D mapping of the Bosporus plume with detailed chemistry data. The figure above shows seven transects with three stations each at the following water depths: 300 meters, 500 meters, 1750 meters. Microbiological incubation experiments and suspended sediment analyses also will be performed.

Leg 2 (April 25 – May 10) focuses on the suboxic zone in the eastern basin. A detailed study of the suboxic zone will be made. A short trip to Sevastopol, Ukraine, also is planned to meet with scientists from the Marine Hydrophysical Institute and Moscow State University. Box cores, which can provide a good, undisturbed look at the geological and biological structure of the seafloor sediment, will be taken near the Crimea. Detailed physical measurements of the far southeastern region of the Black Sea will be obtained to enable one of the researchers to test his hypotheses on the eddies in that region.

Leg 3 (May 10 –15) focuses on the southwestern Black Sea.
This short leg will provide more time for sediment sampling and other types of sampling in the southwestern Black Sea that could not be accommodated in the first two legs. The scientists will be joined by two Delaware eighth-grade teachers — Lynn Scanlan and Hepsi Zsoldos from Talley Middle School in Wilmington — who will be reporting the team's findings back to their classrooms.