The entire dispute over phosphorus in the Everglades comes down to an amount so small that it's roughly equal to 5 ½ grains of sand in a bucket filled with one billion of them.

"It's so small you wonder why we're even squabbling about it," said Philip Parsons, an attorney for the Florida Sugar Cane League.

Here's why: In the Everglades, even barely detectable amounts of the fertilizer ingredient have huge effects.

Largely lost in the fog of lobbying surrounding the controversial revision of Florida's plan to clean up the Everglades is the daunting problem that phosphorus poses: Despite significant strides by the state and the sugar industry in the last seven years, plumes already taint as much as 10 percent of the Everglades system and continue to creep across the sawgrass.

When pollution reaches unspoiled areas, algae that anchor the food chain die. If the level builds -- as it has in more than 61,000 acres of the north Everglades -- prairies morph into something distinctly non-Glades, cattail thickets largely devoid of any other life.

And, as yet anyway, no one knows exactly how to make farm and suburban runoff clean enough for the Everglades.

Environmentalists, the Miccosukee Tribe, many scientists, a federal judge and a bipartisan group of congressional leaders warn that a loophole-riddled measure passed by the Florida Legislature could roll back a strict phosphorus standard, delay a 2007 cleanup deadline by at least a decade, and threaten the grand $8 billion plan to restore the Everglades.

Gov. Jeb Bush, his aides and South Florida water managers dismiss the criticism as wild doom-saying. They acknowledge that it will take decades to clean up small heavily polluted pieces of the Everglades. But they insist that the measure, backed by $450 million to improve filtering marshes the state is relying on to knock down phosphorus levels, will ensure that cleanup and restoration stay on track.

"We can tell you today that by Dec. 31, 2006, we're probably going to be on the five-yard line," said David Struhs, secretary of the Florida Department of Environmental Protection. "We're not moving the goal post. We're not changing the deadline."

Bush plans to go to Capitol Hill this week to discuss the measure with congressional critics but has said he intends to sign it unless he hears convincing new concerns.

A POLLUTION STANDARD

Level of phosphorus in Everglades is at issue.

Water quality poses serious technical hurdles and potential delays for the state-federal effort to restore the natural flow to the River of Grass. If Florida doesn't meet a 10-parts-per-billion phosphorus level that most scientists endorse, some critics fear that the massive replumbing could slowly pollute the system.

Ron Jones, a Florida International University microbiologist and tribe consultant who pioneered much of the phosphorus research, likened it to a firing squad granting a victim the favor of smaller bullets.

"You can't make pollution look good just by making less pollution," Jones said.

The state dismisses that. "We're not going to introduce higher numbers into pristine areas," said Ernie Barnett, director of ecosystem projects for the Department of Environmental Protection.

Other than development, which has consumed half of the original system, phosphorus has been the Everglades' most persistent plague.

Phosphorus is an essential element of life, occurring naturally in soil, water, plants and human cells. Tap water from any faucet in Kendall has 30 times more than Everglades water, Coke 1,000 times more.

Pour phosphorus on lawns and sugar, sod and vegetable fields in the proper ratio to other nutrients, and it fuels green growth. But flushed from drainage canals into the Everglades, it begins to strangle indigenous plant life, even at barely detectable levels.

That's because in nature, phosphorus acts as a "limiting element," meaning that it controls just what sort of life emerges, said Joel Trexler, an associate professor of biology at FIU. "It's a very special thing that has formed in the Everglades," he said. "It was created because we have such low levels of phosphorus."

It's so low that no other marsh, lake or river anywhere approaches it. But levels benign elsewhere can act like slow-drip poison in the Everglades.

WHEN CHANGES BEGIN

Increase in phosphorus starts a chain reaction.

Just about everybody, including researchers supported by the sugar industry, agrees that change starts in at least part of the Everglades when phosphorus floats above 10 parts per billion. There is some dispute about what level really damages the system.

The first shifts can't be seen by the naked eye. Periphyton, algae that clump together to form the oozy mat that covers much of the sawgrass prairie, begins to vanish. Periphyton isn't sexy, resembling a scummy skin of curdled cream in a three-day-old cup of coffee. But it is critical, said Nick Aumen, an aquatic ecologist for Everglades National Park.

"That mat is the foundation of the Everglades food web," Aumen said.

As the algae vanish, tiny creatures go, too, with ripples continuing up the chain as concentrations build. Eventually, cattails -- long-leaf plants -- expand into dense stands, creating what FIU's Jones calls "markers on the grave of the Everglades."

When that happens, the Everglades no longer support the life that defines the system. Cattails, natural in clusters around the poop-enriched zones of alligator holes and bird rookeries, crowd out other native plants, then many animals.

"It's literally impenetrable," said Aumen, who leads a team of federal scientists studying water quality. "You have a very difficult time even getting an airboat into it, and it becomes very poor habitat."

Support for the 10-parts-per-billion standard isn't universal. A group of scientists who reviewed the South Florida Water Management District's 2003 water quality report agreed that it would protect the system but questioned whether it needed to be applied uniformly throughout the Everglades.

The sugar industry agrees.

Parsons, an attorney for the Sugar Cane League, said its own studies show that many areas outside the sawgrass prairies tolerate higher levels.

The industry, which has argued for a range of higher standards over the years, has now settled on 15.6 parts per billion. That number, proposed in early legislation, has been removed but isn't dead. The Department of Environmental Protection says it will stick to 10 parts per billion, but the industry continues to push it with a state panel that will set Florida's final phosphorus standard later this year.

At 15.6 parts per billion, Parsons argues, the few tiny changes don't matter much -- not enough anyway to tip the ecological scales.

"Our view is there is change but no imbalance," he said.

Other algae replace much of what dies, he said, and critical populations of tiny fish and animals remain steady. Nothing is lost "that is going to show up in the food chain," he said.

A FOCUS ON PROGRESS

State and industry argue that there is no crisis.

The industry and the state argue that critics are creating a crisis that doesn't exist and ignoring promising progress.

An analysis by the water district shows that some cattail marshes in the northern Glades doubled in size in just four years in the 1990s. Environmentalists call it evidence of how quickly "the nutrient front" can eat the Everglades.

But Barnett of the Department of Environment Protection said those studies provide only "snapshots" of the most affected areas. Soil samples show that the plume -- the result of 80 years of unregulated farming -- continues to spread but has "definitely slowed down," he said.

"We're not afraid to admit it's going to take several years, maybe decades, for Mother Nature to purge itself," he said.

Sugar farms also have cleaned up significantly. Irrigation and fertilizing changes have cut phosphorus flows by nearly three-quarters, besting state goals for seven years.

Sugar, in fact, is no longer even the major source of the pollutant, says Robert Coker, senior vice president of U.S. Sugar. More than half of it comes from suburbs, citrus groves and cattle ranges, many north of Lake Okeechobee. "If you look at what's happening on the ground, we're making real progress," Coker said.

But one big problem remains: No one is sure that runoff can ever be made clean enough to meet the standard of 10 parts per billion.

Still, by 2007, Barnett said, "we'll be knocking on the door of 10" and phosphorus loads will be a tenth of what they were only a decade earlier.

The sugar industry contends that should be close enough, given the staggering price to taxpayers and farmers of reducing the phosphorus content by a few more grains.

Others worry about the potentially immeasurable loss of getting only close enough.

"I guess," the park's Aumen said, "it becomes a political and societal question of how long do you want to wait and how much of the Everglades will be left when you're done."