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.
What Is a 'Dead Zone'?
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."
So, when it came time for her to begin work on her doctoral dissertation in oceanography, she turned to the Nile delta.
The huge delta is the home to millions of Egyptians, and the delta's farms and factories sustain much of the population. But that brings tons of waste, including human sewage, into play.
Even the city of Cairo, with almost 20 million residents, releases sewage that flows into canals that eventually feed into the Mediterranean. And the use of fertilizer has quadrupled in recent years.
Some of the Nile waters flow into a series of inshore lagoons, feeding a large fishery of tilapia. The lagoons provided a laboratory setting for the researchers, because they could compare fish in lagoons fed by enriched waters with those in lagoons where the water was not enriched.
The scientists studied 45 different genera from the lagoons and from offshore catches and found enhanced levels of nitrogen and phosphorus in the muscle tissue of fish that had access to the enriched waters.
But ... Too Much of A Good Thing Can Lead to Collapse
Interestingly, in one lagoon, just inshore of the city of Alexandria, Oczkowski said the fish rebounded up until about 1980, because of increased nutrients from fertilizer, but when sewage was added to the water, the fishery collapsed.
"If you add nutrients to a system, you see a positive response, but you get to a point where it becomes too much of a good thing and you start to see the fishery collapse," she said.
So, at this point, it appears that a measured release of what would otherwise be considered contaminants may be good for an ecosystem, at least in this one specific case.
But systems vary widely from one area to another, depending partly on currents and whether a system continually flushes itself, as is the case in the Mediterranean.
In some cases, some species have been shown to thrive, even in a dead zone, while others die out. Scientists at Brown University reported last year that quahog clams did quite well in the hypoxic waters, where oxygen was depleted, in Naragansett Bay, one of the largest estuaries on the U.S. East Coast.
The clams could survive without much oxygen, but their predators couldn't, so the population of quahogs grew unmolested.
But was that a good thing? Not necessarily. As has happened in numerous cases, the number of species declined, leaving the estuary vulnerable.
"You'd be hard pressed to say dead zones are good," Andrew Altieri of Brown said in releasing that study. "But with this study you just can't say that dead zones are simply doom and gloom. Ultimately, it's a silver lining on a very dark cloud."