This story first appeared in the SERC quarterly Newsletter spring 2006

Turning the Tide on Harmful Algae


Dr. Mario Sengco running laboratory experiments on clays that may be helpful in mitigating the effects of Florida Red Tide.

Amnesia, paralysis, difficulty breathing—these are all possible outcomes of an uncontrolled harmful algal bloom. HABs, as they’re called, can wreak havoc on wildlife, human life and commerce. Stopping them is an elusive endeavor Mario Sengco has been working on for ten years.

HABs are increases in populations of algae that either produce a toxin or
occur in such densities that they have a physical impact on their environment. Some of the most insidious algal blooms do both.

The Florida red tide, caused by Karenia brevis, is just such a bloom. During a red tide, Karenia brevis becomes so dense that the water takes on the color of the organism, thus lending the name to the phenomenon. The neurotoxin it produces causes massive fish kills and accumulates in filter-feeders, forcing shellfish industries to close. Wave action can even aerosolize the toxin and cause respiratory illness in people living downwind of a bloom.

According to Sengco, the number and variety of HABs reported around the world has increased dramatically over the last 30 years. In the eastern United States alone, the New England shellfish industry shut down for months in 2005 as a toxic bloom lingered. A persistent red tide in southwestern Florida caused fish and manatee kills throughout the area.

Sengco is studying the potential for using clay to manage HABs and has been focusing much of his energy on the Florida red tide. "In these methods, a certain kind of clay is diluted into a slurry mixture and sprayed onto the water," Sengco explained. The clay sticks to the algal cells and causes them to clump and sink to the bottom. As the clumps sink, they capture more cells along the way. In East Asia, commercial fish farmers have been using clay for ten years, but according to Sengco, much more needs to be known about their impact, especially on the creatures living on the ocean floor.

It’s a fairly effective method under conditions of calm, low-flow water, but it’s just those conditions that may cause the clay to accumulate on the bottom. "I want to know what their impact is on the environment," Sengco said.

"One colleague from Korea has said they must have dumped the equivalent of a small mountain into the ocean after using clay to mitigate ten years’ worth of harmful blooms." Sengco is conducting lab experiments and small field studies to establish the most effective methods of using clays and to try to determine their impact on bottom communities.

For more information, or to reach Dr. Sengco, please contact SERC science writer Kristen Minogue.