Stay in the know about wildlife, water quality, and ecosystems on Sanibel and Captiva Islands and in Southwest Florida

Wildlife Impacts Reveal Red Tide’s Persistence

May 13, 2024
shorebirds stand next to dead fish on sanibel

Shorebirds next to dead fish that washed ashore on Sanibel during a red tide event in 2022

Over the past two weeks, SCCF’s partners at the Clinic for the Rehabilitation of Wildlife (CROW) treated an adult laughing gull and a ruddy turnstone, both admitted for suspected red tide/toxicosis. They died within 24 hours. An adult great egret and two juvenile double-crested cormorants were also admitted and are still in care. 

These wildlife impacts from brevetoxicosis are currently being reported without the local presence of a red tide bloom.  

One of the more confusing aspects of red tide — which is caused by high concentrations of a single-celled algae named Karenia brevis — is that it’s sometimes found at ‘background’ or very low levels.

“It can seem counterintuitive that animals are experiencing symptoms when K. brevis cell counts read as ‘not present’ or at ‘background concentrations,’ but multiple explanations for this exist,” said SCCF Policy Associate Allie Pecenka.

The first is the process of bioaccumulation, or the amount of toxins from K. brevis increasing in organisms as they move up the food chain. These toxins accumulate in the tissues of small fish and invertebrates like oysters and crabs during the feeding process and are ingested by predators at higher trophic levels. The toxin can also settle on plants, where it can remain long after a red tide event has passed and be further ingested by marine life.  

“The largest predators tend to hold the highest levels of toxins, which build up faster than they can be broken down. This process explains why we may see the impacts of brevetoxicosis on wildlife later in the winter after animals have been ingesting the toxin for months on end,” said Pecenka.

Another reason we can see impacted wildlife in our communities when we are not experiencing an active bloom is due to the nature of animals, they tend to move around.  Seabirds cover vast distances, so an animal could be feeding far offshore in the gulf where a bloom is present, and then return to shore where it begins to exhibit symptoms.  Since blooms require wind and currents to travel, an impacted bird is able to travel to areas where a bloom cannot.  This means that we may see impacts from blooms that never interact with our coastal communities.

Each week, SCCF reports the number of wildlife with potential brevetoxicosis — primarily birds — in its Water Conditions Tracker

When and How Red Tide Blooms

Red tide in Florida is a phenomenon caused by high concentrations of Karenia brevis, a dinoflagellate that occurs naturally in the Gulf of Mexico. When present at trace levels, the organism has minimal impacts on the environment. However, when counts of K. brevis cells begin to rise and form a bloom, the harmful effects intensify.

The Florida Fish and Wildlife Conservation Commission (FWC) considers red tide to be at a “low-background” level if there are 0-1,000 K. breviscells per liter, with “very low” falling between 1,000 and 10,000 cells.

These background reports of K. brevisrepresent the naturally occurring concentrations of the organism present in the Gulf year-round, posing no threat to the coastal environment. While organisms may begin to be impacted from “very low” concentrations, once K.brevis cells per liter surpass 10,000 (moving into low, medium, or high levels) harmful effects to the environment become increasingly likely. 

red tide map fwd
Red tide map from early May 2024 (Source: FWC)

“It’s important to understand that FWC’s red tide reporting system is excellent at identifying the presence of K. brevis close to our coastal communities, but it’s limited to reporting at specific sampling sites,” said Pecenka. “The associated data should not be regarded as a complete snapshot of existing red tide blooms in the Gulf nor their potential for impacts to our region.”

Red tide blooms typically initiate off Southwest Florida’s coast between late summer and early fall. In general, harmful algal blooms are fed by warm water temperatures and high levels of nutrient-rich runoff from increased rainfall. These conditions tend to compound between August and November, providing optimal conditions for K. brevis growth. 

SCCF monitors red tide and reports data in its weekly Caloosahatchee Conditions Report and Water Conditions Tracker, even when conditions remain stable.

History and Intensification

While anecdotal accounts of red tide date as far back as the 16th century, the extent, severity, and duration of blooms are now far more intense than they were historically. 

“Research has shown that red tides are exacerbated by human activity; namely increased nutrient loading into waterways from agriculture and human development,” said Pecenka. “High levels of nitrogen and phosphorous from these sources feed the growth of an algal bloom, leading to the longer, stronger red tides we face today.” 

When conditions conducive to growth are met, preexisting K. brevis cells begin to multiply 10-45 miles offshore to form a bloom. After initiating, blooms grow larger and more intense and may travel throughout the Gulf based on wind and current patterns. 

When a bloom moves inland and is intensified by human-caused nutrient loading, the risk for a large-scale, long-lasting harmful algal bloom with devastating consequences becomes tangible. 

“The negative impacts to our communities and environment from a large-scale red tide event are undeniable,” said Pecenka. “The lasting effects from a single event can have massive ramifications on the health of our ecosystems, economies, and individuals within affected communities.”

ECONOMIC REPORT: Another bad harmful algal bloom could cost Southwest Florida $5.2 billion >>

“As we improve local water quality, the overall health of the Caloosahatchee system will reduce the potential for large-scale harmful algal blooms that harm our local ecosystems, communities, and economies,” she added.

K. brevis cells release a toxin that attacks the nervous system of animals. These neurotoxins, called brevetoxins, are often fatal to fish, birds, sea turtles and marine mammals and can have adverse impacts in humans. Report wildlife behaving erratically (unable to stand or fly, displaying tremors, apparent weakness or confusion) to Sanibel’s Clinic for the Rehabilitation of Wildlife (CROW), where treatment can be administered by calling the CROW Wildlife Hospital at 239-472-3644 ext. #222.

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