HABs are occurring around Florida, in the Gulf of Mexico, and the Atlantic waters of the southeastern United States with increasing frequency, and can have significant impacts on sea turtles.  While all species can be affected, HAB outbreaks in the Gulf of Mexico occur almost annually and primarily impact loggerhead, green, and Kemp’s ridley sea turtles.  A 2005-2006 outbreak of the single celled algae Karenia brevis (the organism that causes red tides) off the west coast of Florida led to 318 documented sea turtle strandings, with more than 90% of both live and dead stranded animals testing positive for the toxin produced by the algae.  While high levels of toxin in the stomach contents suggested that the turtles were consuming contaminated prey, turtles may also inhale the toxin.  When sea turtles surface, they usually take just 2-3 deep breaths before diving again, and so can inhale toxin stirred into the air by the action of wind and waves.  Aerosolized toxins cause irritation of the nasal passages and lungs, and affect beachgoers as well; people with asthma are particularly sensitive.  The toxin produced by Karenia brevis (brevetoxin) affects the nerves and muscles; in sea turtles this causes uncoordinated muscle movements, head bobbing, and swimming in circles, sometimes leading to coma and death.  Humans that ingest contaminated shellfish may get neurotoxic shellfish poisoning, though symptoms are usually relatively minor.  The all-seafood diet of marine animals like sea turtles can expose them to much higher, lethal levels of toxin.  Animals that are alive but unable to swim and dive properly often strand on the beach or are found floating in the water and are taken to rehabilitation facilities, where they receive supportive care.  Work supported by NOAA is currently underway to develop treatments to help sea turtles recover from toxin exposure.

Besides the direct effects of HAB toxins on sea turtles, there can be indirect impacts as well. The non-toxic HABs known as brown tides may be so extensive that they block sunlight and damage seagrass beds.  In the Indian River Lagoon, a large estuary on the east coast of Florida, brown tides in 2009-2012 caused the loss of more than 32,000 acres of seagrass, approximately 60% of the seagrass cover in the IRL.  Without seagrasses for foraging, the resident herbivorous green sea turtles eat more algae, which may be a less healthful diet and in the long term can make the turtles more susceptible to disease.  Massive loss of seagrass beds due to algal blooms has also been reported in Australia.  The loss of seagrass beds affects the whole food chain, with decreases in prawn and fish species reducing prey availability for other turtle and marine species as well as hurting the commercial and sport fishing industries.

Freshwater turtles and those that live in estuaries can also be affected by HABs.  In the spring of 2015, the deaths of hundreds of diamondback terrapins on Long Island (NY) and in Delaware were associated with a toxic algal bloom.  The toxin, produced by the algae Alexandrium catenella, was probably concentrated in shellfish eaten by the terrapins; people that consume similarly contaminated shellfish may suffer from paralytic shellfish poisoning. Microcystis aeruginosa is another cyanobacteria that can produce liver and neurotoxins that kill fish and turtles; it blooms in freshwater lakes overloaded with nutrients.   A cyanobacterial bloom in Lake Erie in 2014 was so large that city of Toledo residents were ordered not to drink or cook with the water for several days.  Similar blooms have killed turtles in lakes around the world, including in Algeria and China, and even in the moat enclosures of a zoo.