In the middle of the Ocala National Forest lies Silver Glen Springs, the one spring in Florida that has all the scientists scratching their heads.
Silver Glen is at least 30 miles from any suburban lawns, citrus groves or cattle ranches that could trickle pollution into its waters, the kind of pollution that has fueled toxic algae blooms elsewhere, such as Silver Springs and Rainbow Springs.
Yet Silver Glen — one of Central Florida's most popular spots for swimmers and boaters — has suffered from algae outbreaks just like the other springs. Thick mats of the brown gunk cover the sandy bottom of the run flowing out from the spring, and no one knows why.
"That's one of the big mysteries," said marine biologist Jenny Adler, who has spent the past year documenting conditions in 30 springs.
Many of the state's other springs suffer from overloads of nitrate caused by fertilizer, animal waste, septic tank leakage, sewer plant failures and auto exhausts. The pollution has been blamed for the algae blooms that have transformed crystal clear springs into waterways clogged with brown glop.
Silver Glen, on the other hand, has only "background levels" of nitrate pollution, state officials say. Its level is less than half that of Silver Springs and less than a third of what's in Rainbow Springs.
"The water is so darn clean it shouldn't be happening," said Mark Herrin, the U.S. Forest Service ranger in charge of the Ocala National Forest.
Despite the lack of pollution, a 2003 report by state officials described how widespread the algae had become: "In areas where it grew unhindered, it formed large mats that were several feet thick. In areas where it grew near structures or the water's edge, it formed 'towers' and grew upward in large, shapeless columns."
Wherever the algae grew, the report pointed out, "other vegetation did not grow. In fact, sulfurous bubbles emerged from the algal mats when stepped upon."
A 2009 study by a University of Florida doctoral candidate theorized that the algae mats in Silver Glen produce so much of their own nitrogen that it's sufficient for whatever the mat needs to keep growing.
Jim Stevenson, a state biologist who under former Gov. Jeb Bush launched a push to save Florida's springs, said he has heard two other theories. One blames the boaters who zoom around the spring run. They knocked out enough of the native wildlife, and left enough pollution, that it enabled the algae to spread, he said.
The other theory, he said, blames chemistry. The spring suffers from low oxygen and high levels of dissolved solids — particularly salt, which is increasing — and that's helping stimulate the algae growth.
Figuring out why the algae plagues Silver Glen Springs could lead to answers on how to clean up others. But the Silver Glen mystery is likely to remain a mystery for quite a while, Stevenson said.
During the Bush and Charlie Crist administrations, he said, the state spent millions on research on the springs. That ended abruptly last year.
"When we had the springs initiative, we had the money, and we could do research on things like this," he said. "But that money's disappeared."
Craig Pittman can be reached at firstname.lastname@example.org