OCALA NATIONAL FOREST – A manatee and two calves sought refuge from the January morning chill in the warm, glassy waters of Silver Glen Springs. They drifted slowly, searching for a grassy breakfast.
“The manatees ate my samples!” joked Jason Evans from a canoe nearby.
The Stetson University environmental science professor was looking for water lettuce: a floating plant with ridged leaves long considered a nuisance in Florida waterways. The Florida Fish and Wildlife Conservation Commission calls it, “one of the worst weeds” and spends over $3 million annually to spray it – and water hyacinth – with herbicides.
Manatees, though, munch it up. And scientists study it with new fervor as they learn that at least some Florida water lettuce is native to the state.
“We have thought water lettuce was non-native for at least 50 years in Florida and that's the way it’s been managed,” Evans said. “But now we know, in at least some of the springs, that the water lettuce is in fact native and probably has been here for tens of thousands of years.”
As concerns grow about the impact of herbicides on manatees, new studies point to water lettuce’s roles in aquatic ecosystems. In each waterway, a choice emerges: spray it, pull it or leave it.
A prolific plant
Despite its name, water lettuce bears little semblance – genetically or visually – to iceberg and romaine. Its leaves are covered in tiny, soft hairs and made of a honeycomb structure that keeps the plant afloat.
Water lettuce can grow from dime to dinner plate-sized quickly, doubling in population size in three weeks. It’s pest-resistant, nutrient and heavy metal-tolerant and, “happy growing in nasty conditions,” Evans said, qualities that make it an attractive aquarium plant but a stubborn weed.
The Florida Department of Agriculture and Consumer Services lists water lettuce as a Class II Prohibited Aquatic Plant: “highly invasive and noxious in localized areas of the State of Florida.”
The vibrant green plant has been observed throughout the state, save for about a dozen counties in the Panhandle. Its reach extends globally, causing problems in slow-moving waterways, where the vegetation can form dense, interconnected mats.
These mats can block sunlight and oxygen from entering the water, creating a dark, low-oxygen environment that can harm underwater plants and other aquatic life. As the plant’s leaves die and decay, oxygen levels drop further.
Researchers in South Africa in 2020 found that in experimental pools, water totally covered with water lettuce had five times less dissolved oxygen than open water. Biodiversity was lower in covered waters, with more than 30 times fewer aquatic insects recovered compared to open water.
“You can really start changing the chemistry of the sediment and make it a very harsh environment,” said Jason Ferrell, Director of the UF/IFAS Center for Aquatic and Invasive Plants.
He pointed to boating obstacles, too. Large mats of water lettuce can block boat ramps or make waters impassable, a challenge to the state’s recreational boating industry, which had the highest economic impact of any state in 2023 at $31.3 billion.
“Even back to [William] Bartram, when he was here in the 1700s, he clearly describes very weedy characteristics in this plant,” Ferrell said. “So that's why, even if that plant is a native, we would call it a nuisance species.”
Though long suspected by some researchers, water lettuce’s nativeness was only recently established — and partially, at that.
Evans and his team found fossil evidence of water lettuce seeds, at least 12,000 years old, in sediments from Lake Annie in Highlands County. They published their findings last year.
Researchers at the USDA Invasive Plant Research Laboratory in Fort Lauderdale found genetic differences in water lettuce sampled throughout Florida two years earlier. The team concluded Florida has both native and non-native populations of the plant, though whether the latter arrived through dispersal by animals, transport on ships or other means still generates debate.
“Almost all of the research that's gone into water lettuce in Florida is just how to kill it more effectively,” said Evans. As the case for its nativeness grows, recent water lettuce studies focus on its ecological roles.
Water lettuce leaves are home, food or foraging ground to dozens of insects, snails and waterfowl including dragonfly nymphs and white ibis, according to retired aquatic biologist Ken Sulak. Its submerged roots are an underwater haven for crayfish, sunfish and mosquitofish.
The floating plant is spawning ground in Ichetucknee springs for longnose gar, their fishy snouts resembling a mechanical pencil with the lead overextended.
Sulak and colleagues Steve Walsh and Bill Hawthorne described the phenomenon for the first time in a study published last month.
Gar, virtually unchanged since prehistoric times, swim up North Florida rivers and into the springs in January or February. “All of the sudden, there are 50, 60 gar in one little area,” said Hawthorne, who dove for 12 hours over two days to observe and film the gar spawning.
A dozen males follow a female as she turns vertically, prodding floating vegetation. If she finds a spot she likes, she’ll shoot into the vegetation and release her eggs, males trailing close behind to release their sperm. Fertilized eggs stick to the roots of water lettuce. Developing gar larvae stay in the vegetative habitat for weeks.
Spray it
The Florida Fish and Wildlife Conservation Commission has spent $37 million on herbicides since 2013 to kill water lettuce and water hyacinth on 310,000 acres of public water bodies.
FWC contracts with cities, water management districts and herbicide consultants to spray the plants, often via airboat. Since the two plants are intermixed in floating vegetation mats, they are listed as a single “target plant” in FWC records and sprayed simultaneously.
Herbicide application can be faster and less expensive than other forms of removal for large mats of water lettuce, according to the FWC website. “One crew applying herbicide can cover approximately 10 acres a day, whereas a crew operating a harvester can clear only one-half acre a day,” reads the site.
“If it’s a big area, an herbicide treatment might be the most effective, the most rapid, solution to open that water back up and to start restoring flow or function,” Ferrell said. He noted that UF’s Center for Aquatic and Invasive Plants lists herbicide as an option within a larger “integrated plant management strategy” including biocontrol, mechanical harvesting and hand pulling.
FWC paused spraying in late January 2019 due to public criticism, but resumed a month later. In 2023, the agency applied 20,500 pounds of herbicide to floating vegetation, including 1,235 pounds of Roundup.
Roundup’s active ingredient, glyphosate, reduced immune cell activity by an average of 27.3% in manatee blood samples, according to a recent study by UF researchers. This finding comes four years after researchers detected glyphosate in the blood of more than half of manatees sampled in Central and South Florida.
As pollution drives up nutrient levels in many Florida waterways, water lettuce has more to feast on than ever before. If agencies stopped spraying entirely, Evans said, “we’d probably have more water lettuce in Florida than we had ever had within the paleoenvironment because there’s just so much more nutrient for it.”
“I'm not against the use of herbicides,” Evans said. “They can certainly be misused and they can be overused.”
In response to written questions from WUFT an FWC representative said the agency employs proactive management, “which is treating invasive plant populations as soon as possible before they grow into larger problems.” Although it still lists water lettuce as a weed, “the FWC is currently funding plant genetics research to advance our understanding of the species’ origins in Florida.”
Pull it
Since 2000, employees and volunteers at Ichetucknee Springs State Park have plunged their arms into clear waters to remove water lettuce by hand.
From 2000 to 2008, crews removed 4.2 million pounds of the plant, spanning about 45 football fields. After a pause, the “maintenance phase” began in 2014, with volunteers typically removing between 1,000 to 2,500 pounds of water lettuce per workday, hosted once per month.
A small group of high school students joined park service specialist Caitlin Gonsiorek to search for floating mats in January. They found a thick sea of water lettuce near Blue Hole Spring.
Scooped out and loaded in by the armful, the plants landed on the canoe floor with soft thuds. Crayfish, ramshorn snails and a tiny turtle, no bigger than a sand dollar, hid in feathery roots, carefully plucked out by volunteers.
As she pulled, Gonsiorek reminded volunteers not to grab billowy green eelgrass, “because this is what we're actually fighting to protect.”
The crew collected 28 five-gallon buckets of water lettuce during their four-hour expedition, dumping the plants on the shore, uphill of the floodline, to naturally decompose.
“ Even if it is a native plant, it's out of balance in our ecosystem,” Gonsiorek said, “and we still have to manage something if it's out of balance.”
Mechanical harvesters can be used on larger waterways where hand-pulling is impossible. A cross between a bulldozer and a barge, these machines lower belts into the water and pull water lettuce up, funneling it into a storage bin. After harvesting, the plants are dropped on the riverbank or offsite.
It’s an expensive process. In one example at Lake Kissimmee, mechanical harvesting was 10 to 20 times as costly per acre as spraying. When used on floating vegetation, up to 20% of the mat may scatter and drift downstream.
To Josh Greer, a fisherman and outreach coordinator for Captains For Clean Water, the extra cost compared to herbicides is worth it. “It's more money up front to do that, but if we don't, what is the cost to the environment? To the water quality and the animals and the native plants?” he asked.
“What are the chemicals costing us there?”
Leave it
Weighing water lettuce’s ecological impacts, some advocates and researchers would prefer to let it be.
Private boat charter captain Erika Ritter began pulling water lettuce from the Silver River to discourage sprayboats from using herbicide on the waterway.
“How can you put poisons on there over a God-given plant?” she asked.
The more she pulled, the more Ritter noticed the bugs, beetles, snail eggs, fish eggs and other wildlife entangled in the plants. “I realized that taking the plant out was just about as bad, if not worse,” she said. The boating charter captain resolved to stop pulling and leave water lettuce mats untouched.
“They're homes,” she said. “They’re platforms for birds. They’re full of food. They take up nutrients. They shade our shallows in the summer when it’s so hot and we need that shade.”
Sulak would prefer water lettuce not be removed from the Ichetucknee, especially given its role in gar spawning. “Setting makes a lot of difference,” he said, noting that the river’s fast-floating waters make it unlikely to obstruct navigation or reduce dissolved oxygen. “Water lettuce is good stuff in the right context.”
The plant slurps up dissolved nitrate and phosphorus, sequestering them away in leaves and roots. A stormwater treatment facility in Indian River County uses water lettuce to remove contaminants from ten million gallons of water per day. In addition to absorbing nutrients, the plants release algicidal chemicals, preventing harmful algae blooms.
Back at Silver Glen, Evans imagined the same could be done at the spring run’s mouth, where it meets Lake George.
“The lake is plagued by harmful algal blooms now,” he said. By planting native water lettuce in algal hotspots, “it would be very, very hard to get a start of an algal bloom there because you’ve got water lettuce taking up all the sunlight, all of the nutrients.”
It’s a distant possibility, and one that would require a better understanding of water lettuce genetics, Evans said, “but I'm optimistic that we might go in that direction.”
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