algae bloom

A satellite image of Lake Okeechobee during summer 2016 when a large blue-green algae bloom occurred on the surface of the lake. Source: NASA

This page represents the latest information on harmful algae blooms in Florida and has been compiled by an editorial team consisting of:
Karl Havens, Director of Florida Sea Grant
Lisa Krimsky, Sea Grant Regional Specialized Agent for Water Resources with UF/IFAS Extension
Becca Burton, Communications Coordinator, Florida Sea Grant
Dorothy Zimmerman, Communications Director, Florida Sea Grant

What are algae blooms?

When naturally occurring microscopic algae, or phytoplankton, grow to excessive levels in the ocean, estuaries, rivers, lakes and ponds, the water can become stained bright green, red, orange and other colors. This phenomenon is called an algae bloom. Algae blooms are nearly always harmful in some manner, whether it be to people or to the ecosystems in which they occur. 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, skin irritation. 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 lakes, ponds and rivers, the most common kind of algae that produce blooms are blue-green algae, or cyanobacteria. There are several different genera that are notorious bloom-formers, including Microcystis, Anabaena and Cylindrospermopsis. In the ocean and nearshore coastal waters, blooms are most often  caused by a kind of algae called a dinoflagellate. One of these, Karenia brevis, causes the well-known red tides that are common along the Florida Gulf Coast. Fish kills are often associated with that particular algae. Other dinoflagellates cause harmful algae blooms in our Atlantic coastal waters.

What causes algae blooms?

Algae blooms prefer high concentrations of nutrients in the water. Nutrients and light are the fuel that algae need to rapidly grow. Researchers once thought that blooms were driven by phosphorus, but now it is recognized that both nitrogen and phosphorus play a key role in bloom formation. Those nutrients come into the water from the runoff of farm land where fertilizer is applied or where manure collects from animals, from urban stormwater runoff and from septic tank leakage.

Recent algae blooms in Florida waters

In recent summers, massive blooms of different kinds of algae have impacted the Indian River Lagoon, the St. Lucie Estuary, Lake Okeechobee and the Caloosahatchee Estuary. Blooms of algae called diatoms have also impacted the northern part of Biscayne Bay, which generally is considered to be the most pristine estuary in Florida. In the Florida Panhandle, a naturally-occurring bloom of Pseudo-nitzschia caused Gulf County’s recreational scallop season to be postponed in 2017.

Why are blooms more intense in certain years?

In some lakes, rivers and estuaries, blooms occur every year. In others, they wax and wane from year to year. In the ocean, red tides vary in extremity. 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. An example of this is Microcystis, the species that caused massive blooms in Lake Okeechobee in summer 2016 and 2018.

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.

Hurricane Irma, which hit the Florida peninsula with heavy rainfall in 2017, delivered a large quantity of water and nutrients to Lake Okeechobee. When the lake warmed and experienced intense sunshine in summer 2018, conditions were perfect for a massive bloom. The bloom started along the western shoreline and expanded to eventually cover the surface of the lake.

How will climate change affect algae 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.

What is the solution?

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.

Relevant blog posts

Additional Resources

Krimsky, L., Havens, K.E., Phlips, E. 2018. A Response to Frequently Asked Questions about the 2018 Algae Blooms in Lake Okeechobee, the Caloosahatchee, and St. Lucie Estuaries. ED-2, Florida Sea Grant College Program, University of Florida.

Havens, K.E. 2018. Managing High Water Levels in Florida’s Largest Lake: Lake Okeechobee. TP-232, Florida Sea Grant College Program, University of Florida.

Havens, K.E. 2018. The Future of Harmful Algal Blooms in Florida Inland and Coastal Waters. TP- 231, Florida Sea Grant College Program, University of Florida.

Florida Fish and Wildlife Commission. Red Tides.

Hauxwell, J., Jacoby, C., Frazer, T., and Stevely, J. 2001. Nutrients and Florida’s Coastal Waters. SGEB-55, Florida Sea Grant College Program, University of Florida.

Havens, K.E. 2012. Effects of climate change on the eutrophication of lakes and estuaries. SGEF-189, Florida Sea Grant College Program, University of Florida.

Havens, K.E. 2015. Climate change and the occurrence of harmful microorganisms in Florida’s ocean and coastal waters. Florida Sea Grant College Program, University of Florida.

Havens, K.E. and T. Frazer. 2012. Rethinking the role of nitrogen and phosphorus in the eutrophication of aquatic ecosystems. SGEF-190, Florida Sea Grant College Program, University of Florida.

Havens, K.E., M.V. Hoyer and E.J. Phlips. 2016. Natural climate variability can influence cyanobacteria blooms in Florida Lakes and Reservoirs. SGEF-234, Florida Sea Grant College Program, University of Florida.

Havens, K.E. and H.W. Paerl. 2015. Climate change at a crossroad for control of harmful algal blooms. Environmental Science and Technology, DOI 10.1021/acs.est.5b03990.

Paerl, H.W., W.S. Gardner, K.E. Havens, A.R. Joyner, M.J. McCarthy, S.E. Newell, B. Qin and J.T. Scott. 2015. Mitigating cyanobacterial harmful algal blooms in aquatic ecosystems impacted by climate change and anthropogenic nutrients. Invited Review, Harmful Algae, Online:

Paerl, H.W., T. Scott, M.J. McCarthy, S. Newell, W.S. Gardner, K.E. Havens, W. Wurtsbaugh and S.W. Wilhelm. 2016. It takes two to tango: why dual nutrient (N&P) reductions are now the rule rather than the exception for eutrophication and harmful algal bloom control along the freshwater to marine continuum. Environmental Science and Technology, DOI 10.1021/acs.est.6b02575.