Coastal "dead zones" that are so toxic that nearly everything within them dies have expanded exponentially in recent years around the world, but an international team of scientists has found that in at least one case, sewage and fertilizers flowing into the sea have been dramatically beneficial for fish.
The Nile delta on the northern coast of Egypt sustained a robust fishery in the years preceding the completion of the Aswan High Dam on the Nile River in 1965, but the fishery collapsed when nutrients were trapped behind the new dam and no longer flowed across the delta and into the Mediterranean Sea.
But researchers have found that the fishery has rebounded vigorously in recent years, apparently because of the release of agricultural fertilizers and human wastes into the waters below the dam.
Researchers from the University of Rhode Island, the U.S. Environmental Protection Agency and the University of Alexandria in Egypt collected 600 fish from merchants and fishermen in and near the delta in 2006 and 2007 and found high concentrations of nitrogen and phosphorous that came from sewage and fertilizers added to the outflow from the dam.
Those nutrients, they concluded, account for a tripling of the fish landings today compared to the landings prior to the construction of the dam.
"The fishery is very vigorous now," Autumn J. Oczkowski, lead author of a study published in the Proceedings of the National Academy of Sciences, said in a telephone interview.
Although the study seems to dispute conventional wisdom about the disastrous effect of coastal pollution on marine systems, Oczkowski said the study should not undermine concerns about so called "dead zones" that have wiped out fisheries around the globe in recent years.
Instead, she said, the study shows just how little we really understand about human impact on the environment.
"This is really a story about how people unintentionally impact ecosystems," she said.
Another study, published last year in the journal Science, found that the number of dead zones in coastal waters worldwide has grown to at least 405, covering 95,000 square miles. The largest in the United States is at the mouth of the Mississippi River.
Dead zones are the most extreme example of the impact of coastal runoff, and they occur when excess nutrients, primarily nitrogen and phosphorus, enter waters and fertilize blooms of algae.
The microscopic plants dine too briskly on the nutrients, then die and sink to the bottom where they are consumed by bacteria which, in turn, decompose and dissolve oxygen. That leaves the water depleted of oxygen, which is normally deadly to marine systems, but several recent studies have found exceptions to that.
Prior to construction of the Aswan dam, the Nile flooded regularly across the delta, adding nutrients to the soil and nourishing the marine environment. That sustained a large fishery, mostly of sardines.
A few years ago Scott Nixon, Oczkowski's faculty adviser, started looking into the situation in Egypt in preparation for a talk on how humans can unintentionally impact the environment. Surprisingly, he found that the fishery has been rebounding since the 1980s.
"He did some back of the envelope calculations and estimated that there was enough sewage from people alone to completely account for the recovery of the fisheries," Oczkowski said. "I thought that was cool."