Runoff from multiple sources led to a blue-green algal bloom that killed 2,976 fish in central Port St. Lucie in mid-late August, city officials said.
Residents of the approximately 450-acre Sawgrass Lakes gated community near Darwin and Tulip boulevards reported the dead fish as early as Aug. 13, city records show.
The event became the worst countywide fish kill in five years after large masses of carcasses washed up in the C-24 canal in unincorporated St. Lucie, according to the Florida Fish and Wildlife Conservation Commission.
The approximate 121 acres of water throughout the community incurred masses of fish carcasses that, officials and experts said, resulted from efforts to maintain lawns and rid invasive plant species over the past five years.
These topics shaped a tense town hall discussion held Aug. 27 at the Life Shift Church, 4311 SW Darwin Blvd. Nearly 120 residents turned out for a presentation moderated by City Council member Anthony Bonna; Public Works leaders Colt Schwerdt and Bret Kaiser; and Utilities project manager John Eason.
Tilapia – a freshwater fish species native to West Africa – made up a vast 98.4 percent majority of the 2,979 carcasses recovered from Sawgrass Lakes, according to the presentation. Personnel from Port St. Lucie Public Works and Aquatic Vegetation Control, Inc. (AVC) of Stuart gathered as many as 2,745 tilapia carcasses by Aug. 26.
Other fish carcasses included 15 bluegill; 12 catfish; 12 perch; six bass; and one bream. Personnel also found the carcasses of a frog and two turtles – one of which was found in a tire likely from a separate incident.
Tilapia, a non-native fish introduced to Florida in the 1960s, spread throughout St. Lucie via efforts to clean local canals and enrich fishing opportunities, according to deputy Public Works director John Dunton. He spoke from the audience near the panel at the town hall.
Tilapia could be found “throughout a lot of our waterways,” he said. “It’s an invasive, exotic fish that people like to eat. What you’ll find here a lot of the time is that they’re dumped in by residents who have no other use for them.”
The Tilapia population, he added, outgrew those of native freshwater fish species also used to filter contaminants in St. Lucie canals. “Because they’re not sterilized – unlike some other fish that we put in the waterways like the triploid grass carp – they will populate.”
Kaiser, project manager of the National Pollutant Discharge Elimination System (NPDES) through the Environmental Protection Agency, answered audience questions on water quality and contaminants that led to the fish kill.
Port St. Lucie staff, including Kaiser and Public Works spokesperson Beth Zsoka, sent four water samples to GreenWater Laboratories of Palatka, city records show.
According to an Aug. 27 GreenWater report, these samples contained up to approximately 41,600 colonies per milliliter of Microcystis, a genus of cyanobacteria that can thrive in nitrogen-rich conditions and precipitate blue-green algae blooms. These samples also contained up to 48,000 elongated cyanobacteria, or filaments, of the species Raphidiopsis raciborskii per milliliter.
Kaiser’s input also centered around an Aug. 25 report compiled by Terracon Consultants, Inc., a Port St. Lucie-based environmental firm the city consulted.
A series of 10 water samples Terracon collected registered a “relatively high” pH level between 8.6 and 9.0, the report said. The pH concentration was several times more basic than sea water and nearly on par with baking soda (aka calcium carbonate), according to the U.S. Geological Survey.
These samples also registered “elevated” dissolved oxygen levels that could otherwise sustain fish arising from “heavy rainfall prior to and during” collection, the Terracon report said.
Surveyors observed 5.47 inches of rainfall throughout July, 1.92 inches greater than the 3.55-inch average for St. Lucie County, the report said in reference to precipitation readings from the South Florida Water Management District.
The influx of rainfall, however, could not get rid of the cloudy, brown discoloration that characterized the waters of Sawgrass Lakes in the eyes of its residents.
Water turbidity over time there likely arose through a “lack of circulation” to neighboring waterways that led to the accumulation of large sedimentary particles, referred to as “flocculent,” Kaiser said. “That just means that something beyond the water quality parameters is causing the discoloration.”
The absence of an “upstream source” also led to “runoff from the surrounding properties” lingering in the community, he said. “The retention area relies solely on local runoff and rainfall to naturally flush the system underground with a limited downstream flow,” to the smaller A-16 and A-17 canals.
Downstream sources also proved lacking to filter contaminants with trace amounts of nitrogenous compounds – either ammonia, nitrates or nitrites – that fed into the algal bloom.
Cyanobacteria and associated toxins found in the community elicited input from Elroy Timmer, a retired U.S. Department of Agriculture aquatic herbicide researcher and 12-year Sawgrass Lakes resident.
Timmer, who city officials called a “resident expert” at the town hall, tapped into his “history of looking at dead tilapia” to explain the fish kill, the 82-year-old said. “This lake was healthy five years ago. It had to be treated about every three months.
“Usually, when you have one kind of fish dead, you have a specific problem,” he said. “You can stop the fish kill by killing the bacteria that they’re eating. When you do that, they can’t get a hold of the poisoned algae and the fish kill goes away. I’ve done that many a time on a golf course.”
The city stopped treating the waters of Sawgrass Lakes about “four years” before the fish kill, he said. Before then, landscapers fought stretches of invasive torpedograss that “just kept growing. You have to keep killing it because it grows and outcompetes everything else. It’s been going downhill all that time.”
Timmer’s own research on the Sawgrass fish kill, however, went no further than cursory “visual” confirmation of the carcasses washing up in the lake, he said. “I’m not capable of doing the scientific test on what kind of toxins they are.”
Sawgrass Lakes proved to be the second and largest of two high-profile fish kills in Port St. Lucie this summer.
Marcia Gillings is one of the first residents along the Elkcam Waterway. She moved here from Annapolis, Md., in 1981 after emigrating from the U.K. She contended with the earlier fish kill near her home, first reported in June.
Gillings also spoke about the Elkcam fish kill in front of that City Council at their June 23 regular meeting, she recalled. “This was the first time I’d seen fish dead. Nothing has replenished and come back. I believe it was to do with the spraying.”
AVC sprayed areas containing non-native vegetation near Elkcam with Diquat, an EPA-approved herbicide, in early June, city records show. The company proceeded with the work without first testing the waters, which violated agreements with Port St. Lucie.
Diquat sees near-ubiquitous agricultural and landscaping use throughout the Americas despite the European Union, U.K. and Switzerland banning it in 2019. The substance can cause cancer and organ damage to humans and other animals upon exposure, studies by the National Institutes of Health (NIH) since 1970 show.
However, AVC “has not sprayed” any herbicide in Sawgrass Lakes since Dec. 4, 2020, according to the town hall presentation.
The city continued working with AVC on the Sawgrass Lakes cleanup due to contract stipulations, Schwerdt said at the town hall. “We had emergency services in our contract for now where we tried to help out with cleaning up the fish.”
The aftermath of the Elkcam fish kill led to Vice Mayor Jolien Caraballo proposing meetings to discuss sustainable approaches to ridding invasive vegetation shortly after the incident, Gillings said. No meetings have been scheduled since.