VoteWater Deep Dive: Chemical treatments can kill toxic algae – but are they safe?

After several years of using an algaecide to kill blue-green algal blooms in Lake Okeechobee and the canals around it, the South Florida Water Management District now plans to test the chemical compound for possible damage to the environment.

At an Aug. 17 board meeting, Lawrence Glenn, the director of water resources, said assurances the algaecide is safe to use come “from the literature” about the product, but the district wants to “see for ourselves. We want to ensure ourselves we’re using the right product.”

Glenn and the district are right on both points:

  • Studies do show that hydrogen peroxide-based algaecides are safe and effective – if used in the right way and in the right dosage.
  • But the chemical compound needs to be tested in South Florida’s unique conditions, with the results evaluated by local scientists, before it’s put into possible large-scale use.

Glenn’s comments came in response to questions about whether the algaecides being used to treat blooms had been fully vetted.

“The increased use of such ‘treatments’ to douse blue-green algae blooms raises questions that have not been publicly answered,” said Gil Smart, Policy Director for Friends of the Everglades and Executive Director of VoteWater. “What are the long-term ramifications for the ecosystem? Not all algae is bad; is good algae being killed, too?”

Smart asked the SFWMD Governing Board to request a formal report on the effectiveness, impacts, cost and frequency of the chemicals used to treat blooms.

And subsequent commenters added that while using chemicals to douse blooms may be helpful, even necessary, it would be far better if the district focused on stopping pollution at the source, rather than merely treating the symptoms of that pollution later on.

‘Downstream effects?’

Ed Phlips, an algae expert at the University of Florida, supports the district officials’ plan to test hydrogen peroxide-based algaecides “because we definitely need innovative new ways of fighting harmful algal blooms,” and evidence of what exactly happens after treatment “is pretty nebulous.”

Toxic blue-green algae coagulates at Timer Powers Park in Indiantown earlier this month.

Phlips said the district’s research needs to focus on the algaecide’s effects on other plants and animals at the treatment site.

“Like the peroxide you buy at Publix, the (hydrogen-peroxide-based algaecide) kills bacteria but can also kill other cells,” Phlips said.

The district also needs to look for any possible “downstream effects,” Phlips said. After the blue-green algae is killed, the bloom’s cells – which can contain the nutrients nitrogen and phosphorus — remain in the water. Those nutrients can travel down the canals leading from Lake O.

“Other things can come in and take advantage of all those nutrients in the water,” Phlips said, potentially feeding other blue-green algae blooms in the St. Lucie River estuary or red tide in the Caloosahatchee River estuary.

“The district’s study is a good thing,” Phlips said. “I hope they come up with good answers.”

At least the district stopped using copper sulfate-based algaecides, which accumulates in the muck at the bottom of the water body and “bioaccumulates” in animals such as mollusks and fish.

(FYI:  Bioaccumulation is when a chemical or compound builds up in organisms over time, like mercury building up in fish and dolphins.)

Read the label

The Lake Guard Oxy packaging label (approved by the federal Environmental Protection Agency in March) states the granular form of the product is corrosive and hazardous to humans and domestic animals:

Dense blue-green algal mats clump up at the gates of Lake Okeechobee’s Port Mayaca on May 4, 2023, in Martin County.

“Causes irreversible eye damage and causes skin burns. May be fatal if swallowed. Harmful if absorbed through skin or inhaled. Do not get in eyes, on skin, or on clothing. Avoid breathing dust. Wear protective eyewear, such as goggles, face shield, or safety glasses. Wash thoroughly with soap and water after handling and before eating, drinking, chewing gum, using tobacco or using the toilet.”

The label also warns of environmental hazards: “This pesticide is toxic to birds. Do not apply this product or allow it to drift to blooming crops or weeds while pollinating insects are actively visiting the area.”

It’s only after the product mixes with water and turns into hydrogen peroxide that it becomes significantly less toxic.

Hydrogen peroxide-based algaecides “super-oxygenate the water,” Glenn said. “It’s that oxidative process that goes in and kills the blue-green algae.”

Glenn said Lake Guard Oxy is “very selective for blue-green algae” and doesn’t harm other types of “algae in the (eco)system that are good” or other plants and animals that aren’t affected by the “super-oxygenated” water.

Watch the dosage

A research paper titled “An environmentally friendly approach for mitigating cyanobacterial bloom and their toxins” in the May 2018 edition of the journal “Science of the Total Environment” seems to back up Glenn’s assertion — to a point.

The study’s authors came to the conclusion that hydrogen peroxide-based algaecides (The study focused on PAK 27) “left no footprint … in water; hence, … is an eco-friendly compound.”

According to Lake Guard Oxy’s EPA-approved label information: “Water bodies, such as impounded water, raw-water for drinking, lakes, ponds, reservoirs, waste water, irrigation, drainage, aquaculture, conveyance ditches, pipes, canals, laterals, estuaries, bayous, lagoons, and brackish and salt, sea and ocean water contaminated with algae/cyanobacteria can be treated with Lake Guard Oxy.”

The “Science of the Total Environment” paper recommended, however, that doses of the algaecide be limited to 2.5 milligrams per liter because higher doses “exerted negative impacts on … phytoplankton species and zooplankton biota” (aka tiny plants and animals, respectively, at the bottom of the food chain).

Instructions for using Lake Guard Oxy say it’s most effective “at early bloom stages, when harmful algal/cyanobacteria numbers are at 5,000 to 20,000 cells” per milliliter.

But such a small concentration can’t be readily seen. Instructions say that for “heavy blooms, when cyanobacterial scum or aggregates are visible to the naked eye” (more than 100,000 cells per milliliter), treat with doses between 30 and 98 pounds per acre.

Maximum single application rate allowed should not exceed 294 pounds per acre, according to label instructions.

It does work

District officials indicated they would consider the request on a report to the SFWMD board on the use of the chemicals — including how much has been and is being used. 

Nonetheless, so long as the product is safe as advertised and application rates remain within the guidelines, it appears highly effective.

Lake Guard Oxy, Glenn said, “can kill and reduce (blue-green algae) biomass by 50% within 48 hours, and we’ve seen it work even faster.”

The algaecide also has been shown in some studies to kill the toxin, microcystin, produced by blue-green algae.

A blue-green algae bloom reported at the Timer Powers Park boat ramp on the C-44 Canal in Indiantown had a microcystin level of 800 parts per billion, 100 times the level the federal Environmental Protection Agency considers safe to touch, when it was reported in early August.

Within 24 to 48 hours of treatment with Lake Guard Oxy, Glenn said, the toxin level was 5 parts per billion.

The district has used hydrogen peroxide-based algaecides before:

  • At a June 21, 2021, meeting of Florida’s Blue-Green Algae Task Force, district officials reported using Lake Guard Oxy to treat a 20.8-mile stretch of the Caloosahatchee River/C-43 Canal (81 treatments at 42 sites) the previous May.
  • The district also reported using PAK 27 to treat a 21-mile section of the West Palm Beach Canal, a.k.a. the L-10/L-12 borrow canal. According to the report, canal water contained the toxin microcystin at a level of 24 parts per billion, three times the amount considered safe to touch by the EPA, on May 23, 2021, before treatments started. Three days later, the microcystin level was 1.8 parts per billion – safe enough to drink.
  • In October 2020, the state paid Bluegreen Water Technologies $945,000 to deploy a team at the Port Mayaca Lock and Dam, where discharged Lake Okeechobee water was entering the C-44 Canal toward the St. Lucie River estuary, to look for blue-green algal blooms and treat them with Lake Guard Oxy.

Glenn said at the Aug. 17 board meeting that the district plans to use a grant from the Army Corps of Engineers research branch to test the algaecides.

But as Mark Perry, executive director of the Florida Oceanographic Society in Stuart, said at the Aug. 17 water district board meeting, treating algal blooms once they pollute our waterways isn’t a solution to the problem.

“We’ve got to go to the source of the problem,” Perry said, “and that’s upstream” in farms and ranches north of Lake Okeechobee that send polluted water to the lake.

A video of the South Florida Water Management District’s August board meeting can be seen here. Lawrence Glenn’s presentation begins at the 2:33.08 mark; he starts talking about algaecide use at the 2:53.24 mark. Public comment by Gil Smart, Policy Director for Friends of the Everglades and Executive Director of VoteWater begins at the 38:50 mark.