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Editors note: A large portion of this content was authored by the late Florida Sea Grant director Karl Havens and has been republished here with permission from Florida Sea Grant. Dr. Havens’ area of research specialty was the response of aquatic ecosystems to natural and human-caused stressors, including hurricanes, drought, climate change, eutrophication, invasive species and toxic materials.  Read his original post at: Harmful Algal Blooms Affecting Florida’s Coast

What is it?  

In fresh water, the most common kind of algae that produce blooms are blue-green algae, or cyanobacteria. These single-celled organisms use sunlight to make their own food and are found naturally in fresh, brackish and salt water. In warm waters that are high in nitrogen and phosphorus, cyanobacteria can multiply quickly, creating blooms that spread across the water’s surface.  

There are several different genera that are notorious bloom-formers, including Microcystis, Anabaena and Cylindrospermopsis. 

Not all cyanobacteria produce toxins. And, some will produce toxins at some times and not others.

“We’re not very clear on what regulates the toxin production in cyanobacteria,” said Hans Paerl, Kenan professor of marine and environmental Sciences at the University of North Carolina-Chapel Hill. 

But, Paerl adds, he knows toxin production is not a defense against humans.

“They’re obviously not producing toxins to poison us. They love us,” Paerl said. “The more of us there are, the better their success.”

What does it look like?

When cyanobacteria grow to excessive levels in Florida’s freshwater bodies, the water can become stained bright green, sometimes forming thick scum on the water surface. This visibility of the bloom varies. Sometimes there may be a bloom that’s not quite visible.

What causes it?

Blue-green algae need a few specific ingredients and conditions to grow: 

  • Nutrients, particularly nitrogen and phosphorus
  •  Sunlight
  • Carbon dioxide
  • Relatively calm water 
  • Heavy rainfall followed by drought

Nutrients, in particular nitrogen and phosphorous, are critical to help build the algae’s cellular mass. In the watershed north of Lake Okeechobee, it is estimated that the soils and wetlands contain enough nutrients from past agricultural activities to fuel high nitrogen and phosphorus inputs into Lake Okeechobee for the next 50 years, even if farming stopped today. 

Sunlight and carbon dioxide from the water help fuel photosynthesis. And relatively calm conditions prevent algae from mixing too deeply into the water and being robbed of sunlight. But, in places with very shallow waters, calm conditions are not as critical because algae mixed to the bottom may still have enough light to fuel growth. 

Paerl said that in places further north like Europe, warmer temperatures can have an affect on the intensity of blooms. But in Florida, the climate is already warm, and precipitation plays a bigger role.

“The perfect storm for these blooms is a high rainfall event followed by a drought, plus high levels of nutrient inputs from runoff,” Paerl said. “And that’s Florida, right?”

Take for example last year’s massive bloom, which covered huge swaths of Lake Okeechobee. In 2017, Hurricane Irma pummeled a large portion of Florida’s peninsula, bringing heavy rains over the watersheds north of the lake and around the St. Lucie and Caloosahatchee estuaries. Each of these three watersheds contain sources of high nitrogen and phosphorus levels from past and present agricultural activities and leaking septic systems. One heavy rainfall can flush these bloom-fueling nutrients into the lake and estuaries. 

And, that’s exactly what happened. This rainfall, combined with extremely hot summer days and plenty of sunshine completed the recipe for those massive blooms.  

Water discharges by the U.S. Army Corps of Engineers from Lake Okeechobee are often blamed for blooms in the St. Lucie and Caloosahatchee estuaries. The Corps discharges water from the lake when the water levels get too high and threaten the integrity of the Herbert Hoover Dike. Through a set of canals, these water discharges eventually end up in the rivers and estuaries to the east and west. 

However, it is hard to pinpoint whether or not the lake discharges are the direct cause of the river and estuary blooms, because the watersheds around those rivers and estuaries also contain plenty of nutrients. Those watersheds also experienced heavy rainfall and nutrient runoff in some months before the 2018 blooms surfaced, for example. 

Interestingly, even during drought years, the Caloosahatchee River develops intense blue-green algae blooms, independent of discharges from Lake Okeechobee. The remedy is often a pulse of water from the lake to push the bloom out to sea where it dies because the algae cannot tolerate salt water 

How often to blooms occur?

Scientists do not have a complete understanding of the factors that trigger blooms to occur in particular years, or to become more severe or toxic in certain years. However, that information is rapidly developing.

It is known, for example, that in rivers and lakes, stagnant conditions favor blooms. This is because as long as there is an ample supply of nutrients, they are able to grow unchecked by flushing of the water. Some species need to stay right at the water surface when they grow. These species prefer calm periods when the water is not disrupted by wind mixing.

Natural variations in climate are known to affect the development of algae blooms. In Florida, some years are wetter than others. This can be linked to ocean warming patterns such as the El Niño/La Niña cycle, which affects the amount of rainfall we get in the dry winter and early spring in Florida. In some cases, high rainfall in El Niño years can result in a large input of nutrients into lakes that later can stimulate algae blooms. Tropical storms and hurricanes also can bring heavy rainfall that carries nutrients into lakes.

Where else do blue-green algae blooms occur? 

Blue-green algae blooms are a worldwide problem in lakes heavily polluted with nutrients. Current research shows that if climate change causes further warming of lakes and estuaries, blooms will become more intense and more toxic. It will be easier to control blooms by curtailing nutrient inputs now than it will be in a warmer future. 

When was the last bloom? How long did it last? 

Lake Okeechobee and the Caloosahatchee and St. Lucie Estuaries experienced intense blooms in both 2016 and 2018 that lasted throughout the summer into the fall. There is currently a bloom in Lake Okeechobee, but it is not as large or intense as the 2018 bloom. To see the latest satellite imagery of the bloom, visit: NOAA National Centers for Coastal Ocean Science Harmful Algal Bloom Monitoring System. 

What are the impacts?

Environmental & public health

During an algae bloom, millions of phytoplankton cells lurk in every milliliter of water, and sometimes they produce toxins. Some of these toxins can cause fish kills, respiratory distress and skin irritation. Blue-green algae blooms have been known to kill household pets, such as dogs and farm animals like cattle. Prolonged or high-level exposure to the blooms can also lead to liver disease and neurological problems. Just the presence of a dense algae bloom can harm other aquatic life as well.  For example, dense blooms can prevent light from reaching the aquatic plants growing on the bottom of estuaries and lakes, thereby suppressing their growth.

In some Midwestern states and in California, blooms are appearing in drinking water reservoirs, limiting the drinking water supply.


When blue-green algae blooms spread to areas where people recreate or plan vacations, the sight and smell of the blooms can lead to canceled trips, which can impact local economies and businesses, particularly those related to fishing and tourism.

How do we monitor and forecast blue-green algae blooms? 

Richard Stumpf, an oceanographer for NOAA’s National Centers for Coastal Ocean Science, uses a variety of tools that are helping scientists better forecast future blooms. His team has developed a web page showing satellite imagery of red tide blooms that will go live either in September, or at the onset of the next bloom.  

When asked if enough monitoring is taking place in Florida, Stumpf responded with, “I think the correct question is what needs to be monitored. Because there could be a lot of monitoring but it may not be addressing what needs to be done.”

According to Stumpf, scientists could better forecast future blooms by having access to:

  • Systematic monitoring of phosphorus and nitrogen ( the frequency of monitoring will depend on the size, flow and amount of precipitation to a water body.)
  • Measures of toxins

Stumpf said he and his team are now processing satellite imagery from blooms in Florida since 2002 so that they can have a better idea of the environmental triggers that cause these blooms for more accurate forecasts. He added that if the U.S. Army Corps of Engineers has access to more accurate forecasts, they may be able to refrain from releasing water from lake Okeechobee at a time when there is a high risk for a bloom developing.

“If we can say with confidence that the bloom might start early this year or maybe it’ll start later,  then they [the Corps] could plan a little better with their response,” Stumpf said.

To see the latest satellite imagery of the bloom, visit: NOAA National Centers for Coastal Ocean Science Harmful Algal Bloom Monitoring System. 

How do we contribute to the problem? 

Intense algae blooms reflect decades of human intervention on the land – adding fertilizer, growing crops, raising cows and cattle, fertilizing lawns and installing septic tanks – around these once-pristine water bodies. 

In 1900, Lake Okeechobee was at the center of a large inter-connected Everglades wetland. Water flowed into the lake from the northeast and exited to the west, south, and southeast, either through meandering creeks or as a broad sheet flow. The swamp forest of pond apple trees and much of the sawgrass plains at the southern end have now been drained and converted to agriculture. To learn more about how the current plumbing of Lake Okeechobee differs from the state’s historic water flow, visit:

Additionally, towns, cities and the Corps have installed a massive system of canals that act as a fast route for water from the land to the lake and the estuaries. Historically, rainfall slowly made its way to those waters through wetlands and meandering creeks that naturally filtered excess nutrients out of the water. Today, it quickly shoots downstream in straight drainage canals. 

Much work has been done to reduce the amount of nutrients running off agricultural lands. The Corps and the South Florida Water Management District are currently working on projects to capture water after large storms and filter out legacy nutrients before the nutrients reach lakes and estuaries. These projects are decades from completion in most cases, and we will likely continue to see times of heavy rainfall followed by periods of sunshine and hot days, which will lead to more severe algae blooms on our lakes and estuaries. 

What are researchers working on now? 

In January, Gov. Ron DeSantis announced the creation of the state’s first Blue-Green Algae Task Force, which is composed of scientists and researchers focused on reducing blue-green algae outbreaks predominantly found in freshwater systems.

Algae scientists are also investigating the role of climate change in these blooms. We know that temperature can have a synergistic effect with nutrients in stimulating blooms – and that the relationship is complicated. When nutrient levels are low, rising water temperature leads to just a small increase in the amount of algae. But when nutrients levels are high,  increases in temperature of just a few degrees can result in exponential increases in the amount of algae. This presents a challenge for those who aim to control blooms. In the future, it is clear that water temperatures will be higher due to climate change. In fact they have already increased over the last 100 years. It will be much more challenging to control algae blooms in a warm future than it is today. It is more important than ever to reduce levels of harmful nutrients now.

Paerl and other researchers are also investigating the recent proliferation of blooms into estuarine and coastal waters.

“We’re seeing more of this happening. We’ve seen a replacement of a freshwater cyanobacteria with one that can tolerate reasonable amounts of salinity,” Paerl said.  “So that’s a big issue to being able to understand and predict what the likely expansion of these blooms is going to be into to the saltwater environment.”

What can we do? 

At a large scale, reducing the emission of greenhouse gases in order to slow the warming of the earth’s atmosphere and the water in the ocean, lakes and rivers is an important part of the solution to prevent more severe algae blooms in the future. However, it is becoming apparent that warming will occur by at least 2 degrees Celsius over the next 75 years. We know that this will lead to more extreme blooms where high nutrient levels already allow them to occur.

The most direct solution is to reduce nutrient inputs to estuaries, the coastal ocean, lakes and rivers before that happens.

Remediation measures include:

  • Switching old neighborhoods from septic to central sewage
  • Reducing the use of fertilizer to recommended levels on crops or capturing and cleaning the runoff water from those lands
  • Controlling the export of nutrients in manure from animal agriculture
  • Smart development solutions that integrate better stormwater management options
  • Changes in land use, including in residential neighborhoods, to species of plants that do not require addition of nutrients because they are adapted to grow in our native soil.

The overall costs are great, because there are places in Florida where past nutrient pollution has left a legacy of nutrients in wetlands, soils and on the bottom of rivers and canals – that all drain into lakes, estuaries and the ocean after heavy rainfall. Controlling that legacy pollution may take massive public works projects to capture water and treat it. Yet, inaction has the potential to allow massive highly toxic blooms to impair the use of our inland and coastal waters in the future and increase the risks to human health. It is certain that if we wait until the waters are warmer, the costs will be much higher and the outcomes less certain to be successful.

Where can I find up-to-date information?

To see the latest satellite imagery of algae blooms, in Lake Okeechobee and elsewhere, visit: NOAA National Centers for Coastal Ocean Science Harmful Algal Bloom Monitoring System. 

Learn more:

Visit Florida Sea Grant’s page, Harmful Algal Blooms in Florida, to learn more.