Is FWC feeding Lake Okeechobee algae blooms with Roundup and other glyphosate herbicides?
Is the effort to control invasive plants on Lake Okeechobee causing blue-green algae blooms that end up in the St. Lucie and Caloosahatchee rivers?
The answer depends on who you ask.
A Florida Gulf Coast University professor quoted in recent media reports says it does, but the effect is minimal, according to experts at Florida Audubon and the University of Florida, and Florida Fish and Wildlife Conservation Commission data TCPalm crunched.
More: TCPalm's complete coverage of the algae crisis
FWC sprays herbicides on Lake O mostly to control floating water hyacinth and water lettuce, according to agency spokeswoman Susan Neel.
That practice is feeding the lake's algae blooms in two ways, said James Douglass, a marine and ecological science professor at Florida Gulf Coast University in Fort Myers:
- When the sprayed plants die, they release the nutrients they're holding, particularly phosphorus, into the water.
- Glyphosate-based herbicides, such as the brand Roundup, contain phosphorus; and when the herbicides break down, they release it into the water.
Kill or let live?
Calculating the amount of phosphorus the dead sprayed plants release is almost impossible, however.
"You'd have to know the biomass of all the dead plants," Douglass said, "and you'd have to know the amount of phosphorus in those plants. I don't think the state accounts for all of that."
Letting the plants grow would be worse for the lake and the algae bloom, Neel countered.
"Actively growing invasive plants like water hyacinth and water lettuce continually shed old and damaged leaves," she said in an email.
The leaves, like the dead plants Douglass referred to, release phosphorus.
"Research has shown that keeping populations of these plants at low levels reduces the build-up of decaying plant material," she said. "Therefore, less nutrients are released into the system."
Matter of scale
Spraying herbicides on exotic plants is "clearly not" a major cause of the algae bloom on the lake, said Paul Gray, who's worked on the lake for decades as Audubon's Lake O science coordinator.
"I can see where the spraying could cause a small, temporary bloom back up in the marshes," Gray said, "but the amount of spraying they're doing now isn't causing the big bloom in the lake's open water."
Here's what the FWC sprayed on Lake O in 2017, according to its annual report:
- 20,688 pounds of pesticides, about the weight of a garbage truck, which included ...
- 12,263 pounds of glyphosate-based herbicides, about the weight of a school bus, which included ...
- 11,658 pounds, about the weight of a monster truck, of RoundUp.
Glyphosate-based pesticides typically are about 24 percent phosphorus, meaning the glyphosates used on Lake O in 2017 put about 2,943 pounds of phosphorus in the 730-square-mile lake. How much is that? Picture a compact car parked in Jacksonville.
"The phosphorus from the herbicides themselves is very small," Douglass said, "but it's a straw on the camel's back."
The amount is especially small when compared to the amount of phosphorus flowing into the lake. Slightly more than 1 million pounds of phosphorus flows into the lake each year, but nearly 2.3 million pounds — a record amount — entered the lake in 2017, a study released Tuesday by Stuart environmental engineer Gary Goforth estimates.
"It's a question of scale," said Ed Phlips, an algae expert at the University of Florida. "If you were to spray a huge area of the lake, that would be one thing."
But the spraying is focused in the marshy western third of the lake, "so it's not having a significant effect," Phlips said. "It's not causing major blooms."
Spray or not?
Gray and Douglass agreed the FWC should not stop spraying herbicides altogether.
The state banned the practice in the 1980s and it was a disaster. Water hyacinth on Lake O exploded from 200 acres to about 20,000 acres. Getting rid of it took two years and $2 million.
"The FWC has a very legitimate need to control those plants," Gray said. "I agree with a lot of people that the FWC sprays too much on the lake, but they can't quit spraying entirely or bad things will happen."
Douglass said finding the right balance is the key.
"I'm not saying don't spray," he said. "But there's an important balance between controlling invasive plants in the lake and considering the downstream effects of all those dead plants, and I don't think that's being considered adequately."
Lake O bloom
Lake O has had a large algae bloom for most of the summer.
In early July, satellite images showed the bloom covering nearly 90 percent of the lake's open water. The bloom's size has fluctuated since then, but most recently covered about half the lake.
Aerial photos show algae-laden water discharged from the lake flowing to the St. Lucie River estuary, where it tends to gather in backwaters, marinas and dead-end canals.
More: Martin County beaches closed
In early August, the blooms made their way out the St. Lucie Inlet and onto Martin County beaches.