Does the residue from herbicides used to control invasive aquatic plants build up in the sediment on the bottom of the Big Lake?
During the July 28 virtual meeting of the Lake Okeechobee Aquatic Plant Management Interagency Task Force, Dr. Jason Ferrell at the Center for Aquatic and Invasive Plants, University of Florida, Institute of Food and Agricultural Services, reviewed a study undertaken to answer that question.
The case study focused on the loss of hydrilla in Lake Istokpoga.
Lake Istokpoga is a large, shallow lake, covering about 27,000 acres, with an average depth of 4 feet, he explained. Historically, the lake has included a lot of marsh area. And, historically this has been a very nutrient rich lake with a good fishery, he said. The lake was also heavily infested with hydrilla.
In the 1990s, Lake Istokpoga had about 20,000 acres of hydrilla, he said. “You could sit in a airboat almost anywhere in the lake and in any direction all you could see was topped-out hydrilla.”
The state began an aggressive program to treat the hydrilla, killing it with chemical herbicides.
Although a non-native invasive plant, hydrilla is very common in Florida lakes, however, he said.
“Most anglers liked having that plant there,” he said. “They love to fish it, and it supports some reasonable habitat when it is kept at low levels.”
Because of this, the state tries to control rather than eradicate the hydrilla in Lake Istokpoga. Endothall, Diquat, 2,4-D and glyphosate were used to control the aquatic plant.
In 2017, Hurricane Irma came across the state, Ferrell continued. The storm swept over the top of Lake Istokpoga, ripping the hydrilla out of the lake and churning the lake bottom. Waves 6 feet high were recorded in the shallow lake.
After that, the hydrilla hadn’t come back, he said.
“The question then became, is the reason there is no hydrilla in this lake because so much herbicide has been used for so long that the sediment is now contaminated?”
The Florida Fish and Wildlife Conservation Commission, University of Florida and the Lake Istokpoga Advisory Committee teamed up to explore that question.
In February 2019, the Center for Aquatic and Invasive Plants became involved, Ferrell continued. They collected sediment samples from the lake bottom at locations that historically supported hydrilla. Some samples went to labs to test for the presence of herbicides. Another set of samples was dried out and placed in pots for a bio-assay test to see if plants would grow in it.
A third study was done to collect sediment to search for hydrilla tubers in the sediment.
As controls, the study also sampled sediment from Lake Toho, where there is good hydrilla growth.
Initially the UF researchers proposed three sites in the lake for sampling but the advisory council wanted 30 sites. They agreed on nine sampling sites.
He said they used a mini dredge to take the sediment samples. One set was shipped to a laboratory in Georgia for testing. The lab analyzed these samples for nine herbicides used on Lake Istokpoga in the last decade. The tests detected no herbicides in the sediment.
For the bio-assay they collected sediment, took it back to the UF lab, dried it out, put it in pots and attempted to grow tomato plants in it. As control measures, they did the same for soil from Lake Toho, as well as sand and a mixture of sand and potting soil.
Ferrell said tomato seeds were chosen because they are very sensitive to herbicides.
The tests found the all of the samples supported plant growth, but in sediment from Lake Istokpoga, the seeds were slower to germinate. Once they germinated and took hold, they grew into healthy plants.
What was different about the sediment?
Lake Istokpoga has been a fairly rich algae system, Ferrell explained. “Algae cells are very different from plants that decompose. Algae cells are more hydrophobic,” he said. “They repel water.”
When the sediment that contained a lot of decomposed algae dried out, it was hard to rehydrate it.
“We had a lot of pockets of super wet and totally dry,” he continued. This made it more difficult for the seeds to germinate in this sediment.
The study also collected sediment to look for hydrilla tubers in the lake.
Historically, it was common to find 200-300 tubers per square meter.
The researchers took 20 samples in three areas of the lake, then ran water over the samples to look for tubers.
“When you do 20 samples at a location you should get a good sample count,” he said.
The researchers found no hydrilla tubers in the lake.
They went back and took sediment samples from Lake Istokpoga and Lake Toho and tried growing hydrilla in the lab.
All of the sediment samples supported good hydrilla growth. Hydrilla tubers planted in Lake Toho and Lake Istokpoga sediments grew equally, he said.
He said a similar thing have happened on Lake Weohyakapka. “There was a lot of hydrilla, then three hurricanes passed over, the hydrilla crashed and has not recovered,” he explained.
“A lot of other lakes had hurricanes pass over and it (the hydrilla) didn’t crash,” Ferrell said.
One theory is Irma ripped the hydrilla up, churned the bottom and buried the tubers deep.
The hurricane was like “a big wind bulldozer moving across the lake, burying a lot of tubers deep,” he said. “We think it oxygenated all of those tubers. A lot of them were buried deep and tried to germinate and died.”
Others germinated closer to the surface but because the churned water was muddy, they could not get enough sunlight and also died.
He said they theorize the hurricane killed all of the hydrilla.
“On Lake Istokpoga, we have lost pretty much all of the submerged aquatic vegetation (SAV). It’s probably not due to residual herbicides in the sediment,” said Dr. Paul Gray of Audubon Florida. “The question is why is stuff not growing back?”
Gray noted treatment is being done for invasive floating vegetation.
“One hypothesis: Maybe ongoing treatment of invaders of emergent marsh is impacting the emergent vegetation,” he continued. “Why is the marsh mostly gone right now? There are big open areas of water one foot deep and there is nothing growing there.”
Newton Cook of United Waterfowlers Florida suggested they designate an area of the lake and stop using herbicides in that area.
“Just let the hyacinths go,” he suggested. “It can’t be any worse than it is now.
“Just take an area and let it go. Do not do any spraying in that area and see what happens,” he suggested.