Runoff Aids Some Egyptian Fisheries, Study Says
By Andrew C. RevkinJanuary 19, 2009; The New York Times
http://www.nytimes.com/2009/01/20/world/africa/20nile.html?_r=1&scp=2&sq=Nile&st=cse
In many coastal regions, runoff from farms and sewers has caused widespread deaths of marine life. But fisheries off Egypt’s Mediterranean coast appear to be thriving from a similar nutrient-laden brew, scientists reported Monday.
The fisheries had been stunted for decades after the Aswan Dam, built in the 1960s, blocked the flow of nutritious silt from annual flooding of the Nile. But an increasing flow of nitrogen-rich fertilizer and wastewater from an agricultural and urban boom around the Nile Delta has more than made up for the lost nutrients, scientists say.
By examining telltale traces of nitrogen in fish samples, American and Egyptian researchers found that fish that were caught where the Nile waters enter the sea were feeding on algae and plankton nourished by fertilizer and wastewater. In areas away from that flow, the nitrogen signature in sampled fish matched that of marine life in the central Mediterranean.
The situation stands in contrast to the so-called dead zones that large flows of human-generated nutrients have created in many other bodies of water. The zones are produced when excessive nitrogen nourishes blooms of toxic algae or plankton, or when it causes biological explosions in deep waters that can deplete oxygen, killing marine life.
The southeastern Mediterranean Sea is different because it is a “marine desert” in which nutrients are quickly exploited, said Autumn Oczkowski, the leader of the new study and a doctoral candidate at the Graduate School of Oceanography of the University of Rhode Island. “Add any fertilizer, and you’re going to see a big effect,” she said. The study will be published in the online edition of the Proceedings of the National Academy of Sciences.
Nancy N. Rabalais, a biologist focused on the human effects on fisheries, said the nitrogen-tracing showed the linkage between human effluent and fisheries in Egypt, but cautioned that conditions could still tip from providing benefits to creating problems.
The same transition happened along the Gulf Coast, said Dr. Rabalais, the executive director of the Louisiana Universities Marine Consortium. “The area near the Mississippi River Delta in the 1960s was known as the ‘fertile fisheries crescent,’ ” she said in an e-mail message. Now, she said, increased nutrients are “leading to a massive dead zone most springs and summers.”
There are lagoons within the Nile Delta where such dead zones have already appeared, Ms. Oczkowski said. But offshore, at least for now, the nutrients do not appear to have had harmful effects.