Summertime algae is so thick in the Gulf of Mexico's huge dead zone, a few miles off the Louisiana coast, that divers can't see their own hands at depths below about 10 feet. Around the world, there are more than 50 dead zones like this from the Chesapeake to Hong Kong, some of them covering thousands of square miles.
Moreover, while the impact on land of too much fertilizer can often be managed, its effects on water are frequently beyond human control: highly toxic blooms of algae known as red or brown tides, reductions in the water clarity that some fish need for hunting, overgrown corals and shaded-out forests of rooted underwater grasses that make up some of Earth's most productive habitats.
Around the globe, scientists believe, these lush, underwater grass meadows are disappearing even faster than tropical rain forests. In recent decades, for example, the Chesapeake Bay has lost about 90 percent of the grass beds that once carpeted a half-million acres or more of its bottom.
Chemical fertilizer is not the only source of too much nitrogen.
The burning of coal, gas and oil liberates nitrogen in quantities second only to fertilizer production. Worldwide, smokestacks and tailpipes are the source of about one-fourth of the "excess" nitrogen. In places downwind of heavy industry, such as the northeastern United States, fossil fuel burning can be the major source of nitrogen overload.
World demand for energy and cars means this will continue increasing. Some nations, like the United States, control nitrogen emissions from smokestacks and tailpipes, but that's mainly for smog prevention, and those regulations are not strict enough to protect water quality.
With 70 percent of humans worldwide living in coastal plains, human sewage is another large contributor. In most places, the preferred sewage disposal method remains piping it into coastal waters.
And then there's animal waste, such as the Dutch Levy Bureau monitors to limit the nitrogen polluting the Netherlands' air and water.
In the United States, animals produce 130 times more waste than people -- about 5 tons' worth of animal waste per person a year, says a 1997 U.S. Senate study. In the Netherlands, with far less land than the United States on which to spread manure, it's about 6 tons of manure per resident annually.
So the Dutch Levy Bureau tries to limit "surplus" manure -- animal waste that isn't recycled into growing crops and that can end up in the air or water-- with such vigilance that it tracks manure records with the same techniques used to catch drug money launderers. Taxes on the waste can run into thousands of dollars a year if a farmer isn't careful.
"But it is not like taxes," says Hendrick, "because our goal is that eventually, no one pays anything."
With its network of canals and dikes, Holland is one of the world's most altered landscapes, and human-made changes like these worsen the nitrogen problem. The draining of wetlands and cutting of forests have removed vital natural filters and buffers that could have sopped up nutrient-rich runoff before it ever reached rivers and the coasts.
Another factor that looms increasingly large is global warming, which appears to be heating up the world's oceans. Even slightly elevated water temperatures can increase the impact of nutrients like nitrogen on everything from coral reefs to cholera outbreaks.
So all of these factors -- fertilizer, fossil fuels, human sewage, animal waste, landscape changes and global warming -- combine for a bleak forecast, particularly for coastal waters.
"Our prediction is that, if there is no reversal in the next 10 years, we will see some amazing calamities in our coastal systems," says Robert Diaz, a marine ecologist at the Virginia Institute of Marine Science.
Eastern Europe's Black Sea is a case study in what can happen, Diaz says. Its depths have been lifeless for thousands of years, but 30 years ago the world's biggest dead zone formed in waters once rich with sea life along the northwestern coastline.
University of Maryland ecologist Donald Boesch wryly calls the Black Sea, which receives fertilizer-laden water from the Danube and other major rivers, "the poster child for anoxia," the absence of oxygen.
"In the 1950s you see the beginnings of [oxygen decline], worsening measurably by the '60s," Diaz says. "In the 1970s, you see people noticing, see popular press reports, and in the 1980s the system is going critical and collapsing.
"In the '90s, you see, of 24 prized fish species caught in the '60s, virtually all are gone."
A study of such oxygen-troubled waters around the world by Diaz and his colleague, Rutger Rosenberg of Sweden's Kristineberg Marine Laboratory, shows no large system -- including the Chesapeake -- has recovered once it suffered a persistent lack of oxygen.