This summer it seems like the algal bloom known as Red Tide just does not want to go away. It is hard to recall a time earlier in the year that it was not present in South Florida and its longevity is taking a toll on area marine animals.
This harmful algae bloom (HAB) occurs when microscopic algae multiply to a higher than normal concentration. More than 50 HABs occur in the Gulf of Mexico but the most well known is Karenia brevis, also called Red Tide. K. brevis produces toxins that are capable of killing birds, fish and a variety of marine animals.
Developing offshore winds and currents control the bloom’s movements. As it approaches the coastal shorelines it can be introduced to nutrients entering the water from land, which may promote the bloom expansion. A bloom can linger along the beaches for days, weeks and, this year, for months.
K. brevis produces brevetoxins, which are a group of neurotoxic compounds. Animals can be exposed to these organisms in several ways: dermal contact with toxic cells, ingesting them, inhalation, and consumption of toxic prey.
Fish seem to be affected the most. There are several notable signs of distress from these toxins in fish, including violent twisting, pectoral fin paralysis, loss of equilibrium, and convulsions. Ultimately, fish die because their gills stop functioning.
Invertebrates, including shellfish, crabs, shrimp, sponges and sea urchins, may also fall victim to Red Tide. Scientists seem to be unclear whether die-offs for these species come from exposure to the toxins or the low oxygen conditions in the water resulting from the blooms.
This year there have been many sea turtle strandings, as well as deaths. Fortunately, many have been successfully rehabilitated and released back into the wild. It has been noted that clearing the toxins from a turtle’s system could take six weeks or more. During prolonged blooms along Florida’s west coast in 2005 and 2006 more than 300 sea turtles were stranded, which represents a 400% increase from the previous 12-year average. The primary species affected was the loggerhead turtle, the most common nester on our area beaches.
Coastal shorebirds and seabirds can suffer mass die-offs since these species consume both fish and crustaceans already contaminated with toxins. The most common birds in our area to be affected are the cormorant and brown pelican. Birds exhibiting the ability to stand or walk, slumping of the head, reluctance to fly may possess toxins.
Some of our marine mammals can be victims by consuming contaminated fish or by inhaling the toxins. K. brevis blooms that span a large range and persist for a long period of time would be most devastating to our mammals.
A Red Tide spanning 150 miles of Florida shorelines between 1953 and 1955 resulted in a catastrophic loss of marine mammals.
The largest dolphin die-off caused by Red Tide resulted in more than 740 bottlenose dolphin strandings, many of which were already dead, from June 1987 to May 1988. Four other individual events have been recorded since then, with 93% of all dolphins testing positive for brevetoxins.
Deaths of manatees were first reported in 1965 as a result of Red Tide. These mammals are vegetarians, feeding from sea grasses. Researchers believe that the toxins accumulated in filter-feeding organisms attached to the grass blades were the cause. A large scale event occurred during a bloom in 1996 and the end of this die-off coordinated closely with the dissipation of the bloom. Additional deaths due to exposure to these toxins were recorded in Southwest Florida in 2002, 2003, 2005, 2007 and 2013.
As for 2018, there are numerous fish kills reported along the southwest coast beaches, with some areas being closed to the public. Manatees and dolphins are also current victims and even a whale shark washed ashore in an area of heavy Red Tide concentration.
Even when it is not noticeable the ingredients for a bloom are present. It’s just a matter of the proper mixture of temperature, wind and current to cause another event.
Bob is a Naturalist for a dolphin study team on board the Dolphin Explorer, departing from Rose Marina. He is the author of two books currently available at local stores. Bob loves his wife very much!