SILVER SPRINGS, Fla. - Of Florida's 700 artesian springs, Silver Springs shimmered the brightest. Its fresh water was so translucent that the white sand and tiny shells at the bottom glistened, giving the river and springs a beautiful blue tint from above.

Glass-bottomed boats grew famous here as did underwater photography. Even Tarzan was lured to the springs; six of the movies in the 1930s and '40s were filmed here. Tourists arrived in droves to these springs, just outside Ocala.

The riverscape - with anhingas drying their wings in the sun, alligators lolling near the banks and native hibiscus in bloom - is beautiful. But its fragility is plain to see. Except for a few patches, the bottom of Silver Springs and Silver River are no longer visible, covered by invasive weeds coated with algae.

The springs scarcely bubble up. Its flow rate has dropped by a third. The current moves as slowly as the red-bellied turtles that sun themselves on logs, allowing toxic nitrates to choke the water.

The culprits, environmental experts say, are a recent drought in north-central Florida and decades of pumping groundwater out of the aquifer to meet the demands of Florida's population boom, its sprinklers and its agricultural industry. To what degree the overconsumption of groundwater is to blame for the changes is being batted back and forth between environmentalists and the state's water keepers. But, for the first time, a state with so much rain - the vast majority of it uncaptured - is beginning to seriously fret about water.

"It's a very dramatic drop-off in flow; it raises the hair on the back of your neck if you are concerned about springs," said Robert L. Knight, the director of the Howard T. Odum Florida Springs Institute who has spent decades studying Florida springs. "Springs are a very good canary in a coal mine because they pull water off the top of the aquifer."

But Silver Springs is not alone in its distress. In the last 10 years, many of the famous freshwater springs and rivers in the central and northern parts of the state have seen a sharp drop-off in flows and a steady rise in algae. Nearby Rainbow Springs and River are also suffering, although not as much. The declines have accelerated rapidly in the past five years, so much so that they have galvanized Florida environmentalists to launch a broad campaign to bring attention to the problem and spur Gov. Rick Scott to act.

"Florida is a state that has historically had an abundance of water," said Bob Graham, a former Democratic governor and United States senator who assembled the Florida Conservation Coalition last year to help safeguard the state's water. "We have learned that we can degrade our water supplies to the point that water becomes a limitation on the quality of life in Florida. We don't think that is necessary. But we think it is possible, if not probable, unless there are strong policies and enforcement at the state and local level for sound water practices."

In a letter last week, the coalition called on Mr. Scott to direct a state agency to assess the decline in Silver and Rainbow Springs and Rivers and come up with a plan to help them. The plan could then benefit the state's other ailing springs.

Lane Wright, a spokesman for Mr. Scott, said the governor understands how important water is to Florida. "The adequate supply of water resources is obviously something that is vital if we want to have people living here in our state," Mr. Wright said.

The sudden attention on Silver Springs is the result of an application for a permit from the St. Johns River Water Management District to use 13 million gallons of water a day, about the same amount used by the city of Ocala. The permit is being sought by Frank Stronach, a Canadian auto parts magnate and horse breeder who is building Adena Springs Ranch nearby, a 25,000-acre cattle ranch and slaughterhouse that will produce organic grass-fed beef.

Mr. Stronach's ranch is expected to provide about 150 jobs in the slaughterhouse and perhaps more as the operation grows. According to its Web site, the ranch plans to carefully monitor fertilizer use, which can dump nitrate into the springs and rivers, and will only use the amount of water necessary.

Scientists commissioned by Adena Springs Ranch to study the issue have concluded that the property's water use will have an "immeasurable impact" on the surrounding area.

"The experts we have hired say that the impact on the springs and river will be insignificant," said Ed de la Parte, a lawyer who is representing Adena Springs Ranch in its permit application.

But other experts, including Dr. Knight, disagree, saying the freshwater springs are in such a precarious state that they will be adversely affected even if the flows drop a tiny amount.

"If they get a permit for that amount it adds insult to injury because we already know it's not sustainable," Dr. Knight said. "It certainly has become a lighting rod for public attention."

The application for so much water - Mr. Stronach initially wanted 25 million gallons - has brought the battle between water conservation and economic development in Florida into sharp relief.

Just a few years ago, a request for 13 million gallons would not have turned many heads.

But water experts and environmentalists say the effects are cumulative. Although water use has recently decreased, the amounts over all have been set too high for too long and the consequences are only now becoming obvious, they say.

Florida's population boom led to an increase in the number of people and businesses demanding sprinklers (more water is used outside the home than inside). All of it is groundwater from the Florida Aquifer. The decrease in rainfall in central and northern Florida has worsened the situation.

"We are either in or headed for a water crisis," said Estus Whitfield, a former principal environmental adviser to five Florida governors.

Ann Shortelle, the former director of water policy for the Department of Environmental Protection and now the director of the Suwannee River Water Management District, one of five districts to oversee water quality and quantity, said it is fair to say that both drought and water use permits affect the state's groundwater supply.

She said the state and two water management districts are conducting a joint review of the data to see why the Silver Springs flow has dropped and what is causing it. The state also has launched projects to reuse water and capture rainwater, although the water management districts saw their budgets decrease sharply this year.

The five districts are also working more collaboratively since groundwater does not adhere to boundaries.

"We do not want to lose our springs," Dr. Shortelle said.

Leaning into the still, murky Silver River, Karen Ahlers, a local environmentalist, grabbed a clutch of slimy hydrilla that is now clogging the waters.

"It's scary how fast this is happening," she said. "It seems as if we have reached some type of a tipping point."