A team with researchers from Woods Hole Oceanographic Institution in Massachusetts, Mote Marine Laboratory, Florida Fish and Wildlife Conservation Commission, the University of Central Florida, and the University of South Florida has the funding they need to field test a potential new treatment that has been very successful in China.
Finding a way to control red tide is important for Tampa Bay – and Florida’s west coast – because events can cause massive environmental and economic losses. Just in 2021, more than 3.9 million pounds of dead sealife were collected during a red tide event by Pinellas, Sarasota, Manatee and Hillsborough counties.
The technology being studied now is called clay flocculation. Clay particles are applied to waters infested with Karenia brevis, the algae which causes red tide. The cells become enmeshed in the clay and fall to the sea floor. Initial results from a small field test in July 2021 showed that it killed about 75% of the red tide cells in two hours. Despite the high kill rate, the levels of the K. brevis toxins decreased only slightly.
“We would argue that high cell removal and moderate toxin removal is still a very positive result because you’ve stopped that bloom and it will not continue to grow and damage the environment over weeks and months,” said Don Anderson, senior scientist at Woods Hole and the principal investigator for the NOAA-funded project. “But we would still like to do better and we’re looking at ways to modify the clay so that it either destroys or captures the toxins.”
Different types of clay clearly have different results, he said. In Korea, where clay flocculation is routinely used to prevent harmful algal blooms near the country’s fish farms, a natural yellow clay is used but it requires up to 400 tons of clay per square kilometer, or about 250 acres.
In China, the clay has been modified with a polymer that changes the surface of the clay molecule so it’s more attractive to the red tide cells, while also building a chain that works like a net. “You only need about one-hundredth of the Chinese clay so that drastically reduces impacts and costs,” Anderson said.
One of the biggest concerns with clay flocculation is its impact on benthic ecosystems, where the clay could potentially smother bottom-dwelling creatures. That hasn’t been an issue in Korea or China, where multiple reports show minimal impact on benthic communities, he said. “After many, many studies – in Korea, they’ve been assessing impacts since 1998 – they do not see any significant differences in the species composition, biomass, or the health of all these different benthic organisms between treated areas and control areas.”
In the U.S., an EPA group studied the impact of clay on benthic organisms and found that the clay flocculation didn’t impact benthic communities any more – or any less – than red tide toxins.
Tests in Florida are underway at Mote, including laboratory and mesocosms – or raceway tanks designed specifically to provide a preview of possible environmental impacts. All showed that the abundance of K. brevis cells dropped rapidly, reaching mortality of up to 94% in 24 hours, with toxins decreasing by 40% to 50%. A few mortalities occurred, including 4 of 54 urchins and 6 of 36 crabs but no clams died, and several of the crabs had been purchased with black gill disease. There was no statistically significant difference between benthos in treated tanks and those in control tanks, Anderson said.
Planning for Pilot Studies
Mesocosm tank experiments will continue to test new clay formulations, but Anderson’s focus has moved to identifying locations along the west coast of Florida where two small-scale pilot studies have already been undertaken in canals with positive results. A list of 12 potential sites has been identified but Anderson is still searching for a total of about 20 sites that could be added to his list.
The perfect site is the embayment at Admiral Farragut Academy in St. Petersburg, which can be easily closed with a boom, is accessible by both land and water, typically has high levels of red tide, and surrounding landowners support the experiment, he said. Other sites under consideration include embayments at Jungle Prada and Fort De Soto Park.
Since the clay treatment has already been successful in canals, the issue is scalability. “I can see a cottage industry springing up someday where people would come out and treat a canal the same way they might mow lawns for a neighborhood association and be able to keep that a livable environment for people who want to use their backyards or their decks. I don’t see any problems doing that as long as we can get the permits.”
And large-scale, open-water projects are certainly feasible. “It’s being done elsewhere, but it’s going to require multiple vessels like you see in China, or maybe use the same approach you do for spraying mosquitoes. In this case, you could spray the clay and cover large areas potentially much more cheaply and effectively than you can with the vessels.”
A Long and Winding Road
While efforts to control red tide and other harmful algal blooms are finally back in the spotlight, the delay may be attributed to previous efforts that were so horrific.
The first – and last – large-scale treatment of red tide occurred in 1957 during a major event just north of Tampa Bay. More than 100 tons of copper sulfate were spread over 16 square miles using boats and crop-dusting planes. It was extremely effective – red tide cell concentrations dropped to zero – but it killed nearly every creature at all levels in the water and was never tried again.
Another effective – also one-time shot – treatment was using phosphogypsum, the radioactive waste material generated when fertilizer is produced from phosphate rock. “It was incredibly effective at removing the cells so we, sort of naively not being from down there, worked on it quite a bit,” Anderson said. “We did a small study down in Sarasota Bay releasing this clay and looking at its dispersion, and it was all very promising. But it came to a screeching halt, in large part because we had very strong opposition from environmental groups.”
Other chemical controls have been considered but no “magic bullet” that would control K. brevis without killing other life has been found. “I use the analogy where you can go out and put a material on your lawn that kills crabgrass, or another that stops dandelions from growing, but the grass will still grow. There are clearly chemicals that can target specific pathways that are unique to the pest that you’re trying to get rid of, but we don’t have that in this field.”
Biological controls – like parasites, viruses or bacterial pathogens – can be very species-specific but there has been very little progress in treating red tide, particularly in scales of hundreds of square miles. “And then imagine you’re someone who says ‘I’m going to drop this virus into the waters of Tampa Bay.’ You can imagine the kind of concerns people would have about trying to prove that it’s not going to affect organisms other than the one it’s targeting.”
Despite the setbacks, Anderson is optimistic that red tide can be controlled. “The problems we face with harmful algal blooms are similar to those encountered in agriculture or medicine – fields in which control of pests and disease is a practical reality,” he wrote in 1997. “The marine environment is admittedly different, but our hesitancy to pursue bloom control strategies reflects a de facto acceptance that the problems are insurmountable. I believe they are not.”