Director Message

Putting Red Tide in Context

By Gil McRae, FWRI Director

“The Florida red tide was caused by the appearance in nearby coastal waters of extraordinary numbers of a microscopic sea creature.  Although individually so small as to be invisible to the human eye, the concentration of billions of Gymnodinium caused the sea water to take on a reddish or amber color…. Mass destruction of fish and certain other aquatic animals which was caused by a deadly toxin, the chemical composition of which is still unknown, which Gymnodinium liberated into the water…”

While this statement may seem to refer to the ongoing red tide that has impacted a large segment of Florida’s Coast this year, it was excerpted from a U.S. Fish and Wildlife Service report completed in December 1947.  The report on a study directed by the Service’s chief shellfish biologist, Dr. Paul S. Galtsoff, outlined the circumstances associated with a particularly severe red tide that began in November 1946 and persisted for 11 months causing massive fish kills and widespread respiratory irritation for beachgoers.  The report also took great pains to debunk a commonly held theory at the time that the red tide was caused by munitions dumped into coastal waters at the end of World War II.

While our understanding of red tides has advanced tremendously since 1946, challenges with predicting the formation, severity and duration of the blooms remain.  The red tide organism, first identified definitively in 1948 and now known by the scientific name Karenia brevis after the accomplished state of Florida scientist (and former Institute Director) Dr. Karen Steidinger, can produce a dozen or more types of toxins.  Cutting edge work done by our colleagues from the USF College of Marine Science and Mote Marine Laboratory using satellite monitoring, oceanographic modelling and autonomous underwater gliders have bolstered the theory that red tides begin offshore in the Gulf of Mexico.  This comports well with the observations of Dr. Galtsoff that the first indications of a red tide in 1946 were reported by fishermen who observed large fish kills 10-14 miles offshore in November of that year.

While the first scientifically documented red tide occurred in the Florida panhandle in 1844, they have undoubtedly been a feature of Florida’s coasts for centuries.  In the 16th century Spanish conquistadors documented oral histories from the Calusa native culture that speak of widespread fish kills and discolored water.  More recent observations indicate that some level of red tide occurs nearly every year off Southwest Florida.  The specific combination of circumstances that cause red tide remains elusive to scientists but the emerging consensus is that a combination of east winds and southwesterly currents in the Gulf of Mexico create upwelling conditions that provide nutrients and bring red tide cells from the bottom to the surface.  Once established, the red tide organism is tremendously versatile at using nutrients from a variety of sources, including those released by decaying fish killed by the bloom.  There is increasing evidence that another marine algal species, known as Trichodesmium, which is adept at turning atmospheric nitrogen into a nutrient form that Karenia brevis can use, plays a significant role in the maintenance and growth of red tides.  In turn, Trichodesmium can be nourished by iron which enters the Gulf of Mexico in dust storms from the Saharan desert which is approximately as large as the continental United States.  Each year over one hundred million tons of Saharan dust is blown across the Atlantic Ocean in spring, summer and fall.  In June of 2018, NASA satellites documented a massive cloud of dust from Africa moving westward across the Atlantic Ocean which was said to be largest observed in 15 years.

The offshore origin of red tides, and the likelihood that red tide blooms initiate hundreds of feet below the ocean surface, make it extremely difficult to detect red tide blooms in the early stages.  However, during the current red tide the Florida Fish and Wildlife Conservation Commission and the USF College of Marine Science have used data from autonomous gliders to target water sampling at depth which has confirmed the presence of the red tide organism.  Much like meteorologists have done for hurricanes, this work and related modelling and monitoring activities will lead to more accurate forecasting for future red tides.

Many of our FWRI colleagues, especially those in the harmful algal bloom, fish health, fisheries, marine turtles and manatee groups, have been working exceptionally hard responding to this event.  We have thrown everything we have at it.  Weary but committed staff continue to work in labs, on beaches, in trucks, boats and planes collecting important information that will improve our understanding of red tides and prepare us for future blooms.  I am extremely proud of the work they have done responding to this event – working long hours often in very difficult conditions.  They have handled contentious interactions with the public and local governments with professionalism and courtesy.  Most importantly, they have prioritized safety and the integrity of the data and information collected.

Red tide has been with us for centuries and will be with us in the future.  The red tide organism is particularly adept at using any nutrients that may be available.  In the current red tide we have documented highly concentrated blooms in many areas with little to no man-made nutrient pollution.  However, when red tides move inshore they can use nutrients that are more abundant in our estuaries, including those that may stem from agricultural, domestic, municipal or stormwater sources.  These man-made nutrients do not cause red tides but may contribute to their persistence inshore.  It is important to note that, even in the absence of a connection to red tide, there are numerous reasons to manage the input of excess nutrients into our coastal waters which can be detrimental to seagrass meadows and the fisheries they support.

The site and stench of millions of dead fish on our beaches is disturbing and disconcerting but it is an experience we share with Floridians of 1946, when the state’s population was about a tenth of what it is today, and the native cultures that occupied our region for thousands of years.  Our world-class fisheries have evolved and adapted to red tides and have shown tremendous resilience after previous severe events.  There are situations, such as residential canals and waters near aquaculture operations, where treatment and control of red tides may be feasible.  This is an active area of testing led by scientists at Mote Marine Laboratory and the Woods Hole Oceanographic Institute involving clays, ozone and other techniques.

While the factors influencing red tide formation and severity are complex and large scale, our science is at a tipping point aided by state of the art technology that will result in improved forecasting abilities to inform Floridians and visitors to our state.  Red tide blooms will persist, but we have an awesome team and we will be ready.


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