Toxins released during certain red tide events can overexcite sharks, sometimes leading to death.
Scientists have documted how certain algal bloom toxins affect a free-ranging marine species.
Humans may be partly to blame, since agricultural run-off can lead to the formation and growth of red tides.
Toxins produced by red tide events can alter shark brains, resulting in "hyperexcitability" and even death, according to a new study that will appear in the September issue of the journal Aquatic Toxicology.
The study is the first to document how brevetoxins, which are brain-changing compounds synthesized by some harmful algal blooms, affect a free-ranging marine species. In this case, researchers focused on lemon sharks, but they believe many other types of sharks could fall victim to the toxins.
"Sharks are exposed via consumption of brevetoxin-contaminated water and food, such as shellfish," co-author Niladri Basu explained to Discovery News, mentioning that the toxins can easily cross the shark's blood-brain barrier that otherwise protects the brain.
"Once inside the brain, brevetoxins bind very strongly to a protein that controls sodium flow," added Basu, an assistant professor of environmental health sciences at the University of Michigan School of Public Health. "By disrupting sodium flow in the brain, nerve cells will over-fire and cause hyperexcitability and ultimately result in death."
Humans may be at least partially to blame. Basu said that "agricultural run-off can influence the formation and growth of blooms," which is when certain sea algae rapidly increase in number. Since the blooms can make water turn red, these events are sometimes known as "red tides."
For the study, Basu and his colleagues analyzed the brains, other organs and tissues of 30 juvenile lemon sharks from a site directly off Cape Canaveral in east-central Florida.
"The area we studied represents a recently discovered nursery habitat for lemon sharks, and it may serve as one of the most valuable lemon shark nurseries in U.S. waters," co-author Douglas Adams of the Florida Fish & Wildlife Conservation Commission told Discovery News.
Some of the sampled sharks were exposed to Florida red tide events, while others were not. At least one of the sharks perished as a result of a toxic algal bloom. The exposed sharks showed high levels of the poisonous substances in their brain, liver and gills, with the brain exposure causing significant brain chemistry changes.
The findings raise concern over the impact of the red tides, which happen almost annually in eastern Gulf of Mexico and southwest Florida waters, according to the scientists. Blooms also intermittently occur along the U.S. Atlantic coast.
Between Sept. 2007 and Jan. 2008, a single bloom spread over approximately 124 miles in Florida Atlantic coastal waters, "representing the longest duration and most extensive red tide recorded for the Atlantic coast of Florida," the scientists wrote. People in the region suffered respiratory problems as a result of this bloom.
Prior studies have linked such events to multiple fish die-offs. Invertebrates, turtles, birds and mammals can also die as a result of the algal bloom toxins.
In a separate study, Leanne Flewelling of the Florida Fish and Wildlife Conservation Commission, and her team linked one red tide event in October 2000 to mass mortality of blacktip sharks and Atlantic sharpnose sharks in northwest Florida.
Even if a shark survives such a harmful algal bloom, the long-term impact to its population remains unknown, since "the presence of brevetoxins in shark embryos raises questions about the effects these toxins may have on the reproductive success of sharks," concluded Flewelling and her colleagues.