The Rundown on Harmful Algal Blooms

News of the red tides washing around Florida shores prompt concern among all places where the sea is a necessary resource. Can this happen in Tobago? Dr Anjani Ganase discusses the red tide phenomenon.

On the back of a hot summer, reports and images of floating schools of dead fish, and dead dolphins, turtles, manatees and even one whale shark washing up along the shores of Florida’s west coast have been making their way into the news and social media. These are the devastating impacts of the harmful algal bloom, commonly known as the “Red Tide.” Although Red Tides tend to occur seasonally, most years, this latest harmful algal bloom, actually began in November 2017, 10 months ago, and the persistence and severity is exceptional, coming in second but slowly catching up to the length of the 2004-2006 Red Tide event that lasted seventeen months. 

Whale shark necropsy performed on Sanibel Island by Florida Fish and Wildlife Conservation Commission (FWC)  to determine cause of death which is suspected to be from the Harmful Algal Bloom. Photo and caption by FWC (CC BY-NC-ND 2.0)

The culprit of this algal bloom is the single-celled planktonic algae called Karenia brevis (K. brevis for short), which is no stranger to these shores. K. brevis is algae that produces a toxin called a brevetoxin that attacks the gastrointestinal and the neurological systems of marine animals. The toxin can also build up in filter feeding organisms, such as shellfish, which can then affect humans or other organisms that feed on the shellfish. At concentrations higher than ~ 1000 cells per litre of seawater, K. brevis is considered to be blooming. More than ~10,000 to 100,000 cells per litre of seawater, K. brevis numbers are high enough to cause fish kills; while concentrations in the millions are observable by the human eye, where the red coloration in the water is indicative of a persistent harmful algal bloom, which can cause mortality to larger marine life, and even become air-borne from the surface by winds and waves. On land it can impact humans and animals as a toxic irritant to respiratory systems (Steidinger 2009). Major die off events in the past - fish kills and other marine mortalities – associated with similar releases of this “toxic gas” from the ocean have been confirmed for at least 125 years. However, Spanish sailors have noted mass fish kills in the Gulf of Mexico during the 1500s and 1600s.

FWC's Harmful Algal Bloom scientist counting Karenia brevis (red tide) cells during water sample processing. Photo and caption provided by FWC (CC BY-NC-ND 2.0)

Scientists found that the algae prefer an optimal temperature of 20 – 28 degrees, typical of cooler months (winter and spring) along the west coast of Florida, but the algae can still occur in warmer temperatures up to 32 degrees. The K. brevis also prefers salty waters, and therefore they stay away from river mouths and estuaries. These specific temperature range and salty conditions usually keep the red tides from going too far north and into fresh water ecosystems with some exceptions. The algae is commonly found offshore in low numbers in oceanic waters throughout the Gulf of Mexico and is transported as part of the Gulf’s circulation system by winds that maintain the Gulf’s warm salty waters. Although the algae can occur in nutrient poor waters, nutrients from upwelling waters and other sources can boost productivity and result in an algal bloom. Blooms tend to begin offshore along the continental shelf around 12 - 50 m deep and first impact the fish life in these areas. Wind and wave may then push these blooms to coastal areas where the bloom may spread, sustained by nutrients and other factors. 

FWC biologist performing necropsy on goliath grouper. Photo and caption provided by FWC
(CC BY-NC-ND 2.0)

Even though the red tides of west Florida are no stranger to those shores, scientists have begun to observe patterns of increasing frequency and severity of blooms, and have some theories. 

Freshwater blooms – Apart from the Red Tide event occurring over the last ten months, another harmful algal bloom has occurred in the fresh waters of Lake Okeechobee over the last two months, and has spread to surrounding rivers. This is another type of harmful algal bloom produced by cyanobacteria called microcystis that produces a neurotoxin, which - in high enough concentrations - can also become deadly to aquatic life, and an irritant to neighbouring residents. This type of cyanobacteria bloom is typically associated with nutrient polluted waters (for example Lake Eerie, North America; Lake Victoria, Kenya and the Baltic Sea; Paerl and Huisman 2008). As the cyanobacteria are buoyant, on hot, calm summer days, they tend to outcompete other algae competitors by monopolizing the surface layer of the water, which results in a layer of green sludge. Often the blooms are broken as seasonal winds mix the surface of the lake allowing other algae to compete, but other countries have had to use drastic and expensive methods to actively mix up lake systems to prevent the cyanobacteria bloom. Some scientists believe that the outflow of the dead cyanobacteria into the surrounding coastal water may have been feeding the K. brevis during the summer months, causing the Red Tides to persist over longer periods.   

Red tides follow periods of intense storms – Scientists have correlated exceptionally persistent Red Tides in the years that followed an intense hurricane season. 2004 had an active hurricane season where Florida was hit by Hurricanes Charley (Category 4), Jeanne and Frances (both Category 3), which passed through central Florida and doubled the amount of rainfall experienced for that time of year. In the following year, the Red Tide lasted a record breaking 17 months. It is suggested that the ground water carried with it a flush of nutrients that stimulated the growth of the algae offshore causing its persistence along the coast. In September 2017, Hurricane Irma (Category 5) battered the west coast of Florida and may have influenced the current algal bloom, which began in November 2017 (NASA).

Direct pollution – Apart from the episodic flushing events of nutrients from heavy rainfall, over the past fifty years the land areas surrounding Lake Okeechobee and along the coast in Florida have become developed for agriculture, as well as housing. The increase in the nutrient input of the lakes and rivers from agriculture also results in the regular transport of nutrients to the marine environment that could result in harmful algal blooms. The frequency and the timing of the harmful algal bloom off shore has shown to be greater and even persisting in other seasons today, compared to fifty years ago (Brand and Compton 2007). 

FWC biologist collecting water sample during red tide event. Photo and caption provided by FWC (CC BY-NC-ND 2.0)

Warming ocean and climate change - Seasonal changes and warming temperatures can expand the geographical range of the harmful algal blooms to higher latitudes, farther up Florida’s west coast. The warmer temperatures would also extend the algae blooming period well into other seasons (Paerl and Huisman, 2008). This does not only apply to Florida as this persistence has been observed in other lakes and coastal ecosystems in the US and Asia. Along with the theory of more storm activity, it appears that environmental conditions will continue to favour more harmful algal blooms.

It is likely that a combination of many factors have resulted in this blooming event. But more importantly are the steps to mitigate. Last month, Governor of Florida declared a state of emergency and the long-standing debate about whether enough is being done to ensure good water quality continues. In the meantime, monitoring agencies – NASA and NOAA – are using satellite data to set up an alert system to warn about future Red Tide outbreaks, and these will be available to the public. 

FWC manatee rescue field staff respond to a manatee with signs of red tide. The manatee was rescued and transported to SeaWorld. Photo courtesy of Susan Smart (CC BY-NC-ND 2.0)

Although K. brevis has been found in some seawater samples from our marine waters, Tobago does not have a history of harmful algal blooms or Red Tides (Steidinger 2009). This is fortunate, especially when we consider our long history of nutrient pollution along certain coastal areas and coral reefs. Nevertheless, we need to be vigilant in mitigating the impact of our nutrient pollution against any potential algal blooms that may occur in the water column or on our reefs. One scurrent threat we get from algae is the decline in water quality and degrading coastal ecosystems as a result of the sargassum decaying in our bays and on our beaches because of more frequent inundation by Sargassum from offshore in recent years. Although the high sargassum abundance may not have originated from a bloom, the effects of the sargassum inundation may still be harmful to the near shore ecosystems and coastal communities.

All images were sourced and used under the creative commons licence (CC BY-NC-ND 2.0) from the Florida, Fish and Wildlife Conservation Committee who tracks the status of the 2018 Red Tide event along Florida's west coast


Brand LE, Compton A. Long-term increase in Karenia brevis abundance along the Southwest Florida Coast. Harmful Algae. 2007 Feb 1;6(2):232-52.

Paerl HW, Huisman J. Blooms like it hot. Science. 2008 Apr 4;320(5872):57-8.

Steidinger KA. Historical perspective on Karenia brevis red tide research in the Gulf of Mexico. Harmful Algae. 2009 Mar 1;8(4):549-61.


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