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The river runs through US

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They've also been causing other changes. Because mussels eat by filtering water, the thousands of invading zebras that scientists found on last year's River Run had made the water much clearer.

Though that sounds like a good thing, it causes problems for the Ohio. For one thing, the aggressive filtering stirs up old poisons like PCBs and dioxin locked in the sedimentary layers of the river bottom. These toxic chemicals work their way up the food chain until they're dangerously concentrated in the carp and drum fish that have adapted to feed on the zebras.

Also, cleaner water means more light penetration and algae growth. That could be healthy in moderation, but the explosive growth is now having disastrous consequences downstream in the Gulf of Mexico, where the huge algal blooms end up and eventually die. Bacteria rush to decompose them, sucking all the oxygen out of the water, and creating a "dead zone" in which nothing else can live.

Dave and Lisa find about 30 zebras clinging to the dock: not as many as last year, but more than the team found in the flood two years ago. After taking water and algae samples, the boat gets under way.

No. 2: It's not easy being green

At the back of the craft, aquatic ecologist Mike Miller is hand-pumping water through a series of filters, one to strain out larger algae, or phytoplankton, another to catch bacteria and river-bottom clay. These he'll send to the USGS and the EPA, which will use the data to decide how to regulate chemical dumping. But Dr. Miller, of the University of Cincinnati, is most concerned about nutrients: nitrogen- and phosphorus-rich fertilizers that wash off farmland and feed river algae instead.

"It's like someone turned on a huge spigot of corn syrup," he explains, squinting distractedly at a low-flying plane. "This river is the biggest contributor of nitrogen and phosphorus to the Gulf," and as the algae grow fat on these nutrients, so does the dead zone, an area thought to measure between 20,000 and 50,000 square miles at its summer peak.

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