Reversal of Fortune
Nitrogen pollution resurrects Nile fishery
Researchers have uncovered a strange twist in the otherwise sad story of agricultural runoff. Overloaded with nutrients from fertilizer, animal waste, and sewage, parts of the sea have become oxygen-deprived dead zones. But the same force appears to be driving at least one fishery’s dramatic recovery.
That’s the upshot of a study published in PNAS showing that, in the Nile River delta, fish are thriving on runoff from upstream cities and farms. Fish stocks around the Nile’s mouth crashed when the Aswan High Dam was built in the 1960s, cutting off the flow of nutrients downstream. But in the past 20 years, the fishery’s fortunes have more than reversed—today’s catches are nearly three times larger than those before the dam was built.
Autumn Oczkowski, a doctoral candidate at the University of Rhode Island, fingered nitrogen-rich fertilizer as the main, if unlikely, savior. Oczkowski and her colleagues analyzed nitrogen isotopes in more than 600 fish they bought throughout the delta region and along Egypt’s Mediterranean coast. The team found that about 80 percent of the fish offshore of the delta rely on nutrients from human sources. And neither pesticides nor heavy metals appear to be tainting the fish.
The Nile delta’s contradictory situation stems, in part, from the Mediterranean’s low nutrient levels; adding fertilizer gives the food chain a boost. It’s possible that, if runoff increases, nutrient overload might hinder the area’s ability to support fish. But stopping runoff would likely make the fishery go belly-up once more. ❧
Oczkowski, J.A. et al. 2009. Anthropogenic enhancement of Egypt’s Mediterranean fishery. Proceedings of the National Academy of Sciences DOI:10.1073/pnas.0812568106.
Photo by Autumn Oczkowski (University of Rhode Island)