Too much `management'
can spoil a lake

By Research and Review
Special to the Florida State Times

FSU geologist Dr. Joe Donoghue has been studying a disappearing lake -- or rather a lake that used to disappear.

The lake is lake Miccosukee, in eastern Leon County. Left to itself, the lake would periodically dry up -- it's a large lake (about 6,300 acres) but averages only about six feet deep -- and then fill back up again.

But its vanishing act every decade or so didn't suit either fishermen or boaters, so in 1954, lake managers figured out a way to keep the lake full of water year around.

They built a dam at the lake's outlet on the southern end and a dike around a large sinkhole that had been acting as a drain at the northern end. Periodically, the area's hydrologic system would "pull the plug" on the lake, a common occurrence in many North Florida lakes.

Today, as a result of that 40-year-old engineering trick, the lake is plagued with aquatic plants so thick, Donoghue said, that boats can't traverse it unless the grass has been mechanically harvested. When the lake drained naturally, the aquatic plants died and decomposed in the air. But without that periodic draining, Donoghue said, dead plant matter accumulated on the lake's floor, depleting the water's oxygen and killing fish.

Officials have been obliged to "draw down," or partially drain, the lake by opening the dam twice since the structures were built -- once in 1977 and again in late 1988.

Then in 1993, the Florida Game and Fresh Water Fish Commission gave Donoghue a $28,000 grant to find out if tampering with nature was having any effect on the chemistry of the lake's sediment.

Donoghue, FSU geologist Dr. Paul Ragland and three graduate students spent a year analyzing sediment samples collected from the lake's bottom. The five checked for 30 types of metals -- including zinc, mercury, copper and lead --in 27 sediment cores. They also measured sedimentation rates in all of the cores, which helped establish a 500-year sedimentary history of the lake.

"We could see how the different levels of contaminants had changed over the years," Donoghue said. "We could see the concentrations increase at the top of the cores and see that the past few decades was the time that most of these metals were introduced ... most likely by people."

The FSU study will help the state decide how to manage the lake and others like it. At Lake Miccosukee, officials have several options, says Donoghue.

"They can continue to partially drain the lake every decade or so, or they can remove the two structures and let the lake disappear periodically as it has throughout its history. Or they can use a herbicide periodically to kill the plants or slow their growth."

If the lake had a vote in the decision, it would probably choose to rid itself of the two structures that restrict its natural proclivities, Donoghue said.

"The lake would be probably better off in a natural state without interference from manmade structures," Donoghue says. "Back in 1950, we didn't really contemplate the effect that such measures would have on the environment."