Is Red Tide Harmful to Humans? Florida's Toxic Algae Bloom Explained

Florida's coast has been in the grips of a toxic red tide over the past few months that has killed thousands of fish, decimated seagrasses, and wiped out large numbers of other marine animals like manatees.

In the week prior to March 31, the Florida Fish and Wildlife Conservation Commission (FWC) detected significant levels of red tide-causing algae Karenia brevis in 75 water samples across Florida's Gulf Coast, with 89 samples testing positive in mid-March.

Despite us living on land, red tides can still have a large impact on humans in the vicinity of these harmful algal blooms.

Red tides are algal blooms of the dinoflagellate algae species Karenia brevis, growing rapidly in the ocean water and causing a characteristic rust-red layer across the surface at high concentrations

"When excessive growth [of K. brevis], i.e., algal blooms, occurs they can result in toxic conditions for marine species to survive within," Lucas Griffin, a postdoctoral research associate in environmental conservation at the University of Massachusetts, Amherst, told Newsweek.

These algae produce a neurotoxin called brevetoxin, which is released into the water and air from the algal cells when they are disturbed.

"As this alga remains within an area, it can release neurotoxic brevetoxins that can result in loss of swimming ability, paralysis, and respiratory failure for marine life," Griffin said. "Ultimately, beyond fish mortality, red tide can affect the reproduction of fish and larvae survival resulting in large-scale population and food web shifts for coastal species. This exposure will occur from either direct exposure or through the consumption of prey that has already been affected."

In humans, the brevetoxins in the air can lead to eye and respiratory irritation, more severely so in those with conditions such as asthma.

"The toxin can be aerosolized at the water surface, making humans breathing air along the coast very ill or uncomfortable," Don Boesch, a professor of marine science at the University of Maryland Center for Environmental Science, told Newsweek.

Additionally, humans can be impacted by eating fish or shellfish that have consumed the algae, regardless of how long the food has been cooked or cleaned for. Consumption of the toxins may lead to neurotoxic shellfish poisoning, which can cause symptoms including numbness in the lips and extremities, dizziness, nausea, vomiting and diarrhea.

The red tide blooms can also impact other species without the use of their toxins by causing mass fish deaths by using up much of the dissolved oxygen in the water and killing critical seagrass populations by blocking the sunlight from reaching the marine plants on the seafloor.

"Once the dominant species, K. brevis takes over, and, by virtue of its toxins, it kills fish, thereby making its own nutrient supply from the decaying fish. That is why it is so insidious. Once it starts, it is hard to get rid of it because it nourishes itself," Robert H. Weisberg, a professor emeritus in physical oceanography at the University of South Florida's College of Marine Science, told Newsweek.

Red tides have been recorded to exist for centuries, but have been made more frequent and severe by the runoff of nutrient-rich fertilizers into the riverways and eventually, into the oceans.

"While these algal blooms are naturally occurring within the Gulf of Mexico, dating as far back as 1844, they can and have been fueled by human influence, specifically excess nutrients from agricultural and wastewater runoff (e.g., sewage discharge and septic system effluent)," Griffin said. "Specific to southwest Florida, which has experienced red tide blooms almost every year since 2000, nutrient-laden freshwater discharges from the Caloosahatchee River help drive the intensity and severity of red tide blooms in the area."

Fertilizer nutrients provide the algae with nitrogen and phosphorus, which kickstart the algae's reproduction, causing the blooms to rapidly grow. Over the past 50 years, there has been a 15-fold increase in the occurrence of red tides.

"Better regulation of these anthropogenic nutrient discharges will be essential in helping to reduce the frequency and intensity of red tide blooms and their negative impacts," Griffin said. "Beyond the tourist sector that comes to Florida to visit beaches, red tide has affected the seafood industry and recreational fishing, which is a major economic driver of Florida's economy."

Climate change may also impact the occurrence of red tides in the coming years, due to changes to the environment that will make the growth of the algae more favorable. Increased ocean temperatures make for an ideal growing environment for the algae, while rainfall is also expected to increase in certain areas, which may result in the increased run-off of nitrogen fertilizers into waterways. Climate change is also thought to be impacting the frequency and severity of tropical storms, which can help wash algal blooms toward the coast.

"Although red tide blooms have yet to be empirically linked with climate change effects, it is very likely that climate change will worsen the intensity, frequency, and geographic range of harmful algal blooms, such as red tide," Griffin said.

In the meantime, red tides are plaguing Florida's beaches now, with the National Oceanic and Atmospheric Administration (NOAA) issuing a warning for a high risk of respiratory irritation on beaches in Charlotte County on Tuesday.

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