Surprise! Sewage Helps a Fishery Rebound

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."

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