Excess nitrogen combined with placid summer weather results in an oxygen-poor bottom layer of water. The process works this way: In the top layer, the nitrogen and sun feed phytoplankton, which grow rapidly, then die and fall to the bottom. As they decay, they consume oxygen. Called eutrophication, the cycle depletes oxygen in isolated bottom waters. In 2002, one of the worst years since it was first documented in the 1970s, the northern Gulf's hypoxic zone reached more than 7,700 square miles. Despite its size, the problem is largely hidden from view, except to the trained eye.
"I see massive schools of stingrays, bottom dwellers, moving on the surface. Even shrimp come up 20 feet or so off the bottom trying to get to oxygen," says Nancy Rabalais, a marine biologist at the Louisiana Universities Marine Consortium in Chauvin, La. Only because they are desperate to breathe do such bottom-dwelling creatures flee upward, risking becoming easy prey.
Such scenes will become more common worldwide, scientists predict. As populations grow, nitrogen and phosphorous-caused eutrophication will more than double in coastal areas by 2050, predicts a 2001 study published in Science magazine.
"There's been a big increase in these hypoxic zones that correlates strongly with increased use of nitrogen fertilizers, particularly in the '60s and 1970s," says Robert Howarth, a coauthor of the Science study and professor of environmental biology at Cornell University in Ithaca, N.Y. "About half of the nitrogen fertilizer used on Earth in all of history has been used in the last 15 years."
One positive trend: Total global fertilizer use seems to be growing more slowly than in the past few decades. It plateaued in 1990 then declined after the collapse of the Soviet Union. In the mid-'90s, global growth resumed, but much more slowly. For the decade, nitrogen fertilizer rose only slightly from 79 million to 82 million tons.