Red Tides

From Beachapedia

Red Tides

Red tide is a phenomenon when a population of phytoplankton, a single-celled plant, grows very fast or “blooms” and accumulates into dense, visible patches near the water surface. Red tides are naturally occurring, but there is ample evidence that shows nutrient pollution and warming waters can fuel blooms, making them last longer and cover larger areas. It's important to note that red tides and blue-green algae blooms are similar, but different. Red tides mainly occur in marine/ saltwater bodies, while blue-green algae (cyanobacteria) mainly occur in freshwater bodies. Read about cyanobacteria in this separate article.

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Most species of algae, or phytoplankton, are not harmful, and serve as essential energy producers at the base of the food web, without which higher life on this planet would not exist. Phytoplankton are believed to generate as much as 80% of the world's oxygen supply. They absorb nutrients and carbon dioxide from the water and produce oxygen through photosynthesis.

Unfortunately, some phytoplankton are extremely harmful and toxic to humans and marine wildlife when they reach high concentrations. Certain types of phytoplankton, like the dinoflagellate Karenia brevis (K. brevis), that makes up red tides off the coast of Florida, can release harmful brevetoxins into the ocean and air, causing massive kills of fish, marine mammals, and sea turtles; and painful burning of the eyes and lungs for nearby beach goers. Additionally, large swaths of the phytoplankton in red tides and other algae blooms eventually die and sink to the bottom, where decomposition by bacteria reduces the dissolved oxygen (DO) content of the surrounding marine environment. If a substantial amount of DO is removed, that area of the ocean may be unable to support marine life, creating something known as a dead zone, causing additional fish kills and loss of potentially important habitat.

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Florida’s 2018 Red Tide

Florida has been experiencing one of the most widespread, harmful red tides in the last decade. Between August 2017 and August 2018, the 150-mile wide red tide has caused over 2,000 tons of dead wildlife to wash up on Florida beaches, and growing. The state has declared a state of emergency in seven counties, with Lee County experiencing the highest magnitude of dead wildlife (1,700 tons). Reports have documented deaths of 115 manatees, a juvenile whale shark, and 354 sea turtles including loggerheads and the critically-endangered Kemp’s Ridley. And this is in addition to massive fish kills being experienced along both coasts.

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Originally starting in the Gulf of Mexico, where there is substantial nutrient loading of uncontrolled runoff from land-based agricultural operations, the red tide has since wrapped around the tip of Florida, and is impacting Florida’s Atlantic coast including Palm Beach County. Water samples have shown extremely high concentrations of K. brevis in the prolific red tide, counting over one million cells (individual algae) per liter of water. The usual level of K. brevis in non-red tide conditions is less than 1,000 cells per liter. This extreme scenario in Florida is having a severe impact on coastal communities and local marine wildlife, with millions of dollars in clean up costs, human sickness, and significant losses in tourism revenue. As of August 2018, this red tide has already costed Florida $8 million in lost revenue from coastal businesses alone.

As Red Tides gain more and more media coverage, people across the nation have been wondering where do red tides come from, what are the impacts, what’s causing them, and what can we do to stop it.

Where Red Tides Come From

Phytoplankton exist naturally in our marine environment, and as mentioned, are essential to sustaining life on our planet. However, the usual level of phytoplankton, or the level in a “balanced-system”, is only at about 1,000 cells per liter of ocean water or less. During a red tide event, the amount of phytoplankton increases extensively to 1 million cells, even up to 20 million cells, per liter of ocean water, creating a “bloom”. So it’s not like this huge swath of phytoplankton exists for years and travels around our ocean impacting different coasts; it’s fairly local phytoplankton experiencing prolific growth in a short amount of time. What causes these prolific growths of phytoplankton? The general consensus is that red tides can and have happened naturally, but they are exacerbated by the addition of too many nutrients entering our marine waterways, paired with warming waters.

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In addition to sunlight, phytoplankton require nutrients like nitrogen and phosphorus to grow. Naturally, nitrogen and phosphorus are fairly limited in our marine environment, so phytoplankton populations remain under control, because there is just not enough nutrients to support bigger populations. However, when over-fertilized agriculture operations, or nutrient rich sewage from concentrated feeding operations or faulty septic systems and wastewater treatment facilities, runoff into our marine environments, it introduces additional nutrients into the waterways, allowing greater populations of phytoplankton to be supported, and massive blooms to occur. This process of nutrient-fueled algal growth and dead zones is called eutrophication. This nutrient rich water has also been associated with other forms of marine degradation, including coral reef bleaching, coral reef die offs, and more.

Eventually, most the phytoplankton making up red tides die and sink to the bottom, where decomposition by bacteria reduces the dissolved oxygen (DO) content of the surrounding marine environment. If a substantial amount of DO is removed, that area of the ocean may be unable to support marine life, creating something known as a dead zone, causing additional fish kills and loss of potentially important habitat.


Excerpt on eutrophication from NOAA:

"Harmful algal blooms, dead zones, and fish kills are the results of a process called eutrophication—which begins with the increased load of nutrients to estuaries and coastal waters. Sixty-five percent of U.S. estuaries and coastal water bodies are moderately to severely degraded by excessive nutrient inputs, which lead to algal blooms and low-oxygen (hypoxic) waters that can kill fish and seagrass and reduce essential fish habitats. Many of these estuaries also support bivalve mollusk populations (e.g., oysters, clams, scallops), which naturally reduce nutrients through their filter-feeding activities. The primary culprits in eutrophication appear to be excess nitrogen and phosphorus—from sources including fertilizer runoff and septic system effluent to atmospheric fallout from burning fossil fuels—which enter waterbodies and fuel the overgrowth of algae, which, in turn, reduces water quality and degrades estuarine and coastal ecosystems. Eutrophication can also produce carbon dioxide, which lowers the PH of seawater (ocean acidification). This slows the growth of fish and shellfish, may prevent shell formation in bivalve mollusks, and reduces the catch of commercial and recreational fisheries, leading to smaller harvests and more expensive seafood."

Warming Waters

In addition to the overloading of nutrients into our marine waterways from agricultural operations, algal blooms (e.g. red tides and cyanobacteria) are exacerbated by warming waters. Unfortunately, toxic strains like K. brevis, seem to benefit more from warming waters than the harmless strains of phytoplankton. Warmer waters allow the algae to grow faster.

“Thus, if the ocean waters around Florida become warmer, it is reasonable to expect more frequent problems with the harmful microorganisms” - University of Florida.

In addition to longer-term average warming from climate change, the sea surface temperature varies significantly across seasons. This is why harmful algal blooms are generally experienced seasonally, but as we are seeing for 2018, can last up to a year if the water temperatures remain high enough.

Where Red Tides Occur

Harmful algal blooms and red tides occur all over the world, and have been recorded off the coast of every coastal state in the US. It’s hard to say when to expect red tides, since they have occurred throughout the year, but they are generally triggered when the water is warmer during the summer months. The map below shows the frequency of red tide-induced plant and animal mortalities between 1997 and 2006.

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Figure 1. Map by Woods Hole Oceanographic Institution showing location and frequency of red tide induced animal and plant mortalities.

In the Gulf - the Southeast portion of Texas and Southwest edge of Florida get almost annual red tides (learn more here). Between 2017 and 2018, red tides have occurred continually off the Gulf coast of Florida. On the West Coast, red tides seem to be most frequent in California, between Santa Barbara and San Diego, but again - they appear throughout coastal areas across the planet (see distribution maps here). If we associate red tides with areas of high nutrient loading, it makes sense that the Gulf (the release of the Mississippi River) would experience fairly frequent and large-scale algal blooms. Because red tides are coordinated with ocean temperatures and nutrient-loading, agencies are generally able to forecast when a red tide will occur. For the Gulf of Mexico region, check out this forecasting tool (for red tides and toxic cyanobacteria). For Lake Erie, use this harmful algal bloom forecasting tool (mainly for toxic Cyanobacteria).

The Impacts of Red Tides

The two main types of phytoplankton are diatoms and dinoflagellates. The main difference between the two is that dinoflagellates have the ability to propel themselves, while diatoms completely rely on ocean currents for movement. The harmful red tides experienced off US shores are usually fueled by dinoflagellates. The types of dinoflagellates vary by region, with the Southeast Coast and Gulf experiencing more instances of the harmful dinoflagellate Karenia brevis (K. brevis), the West Coast experiencing more instances of Gonyaulax, the Northeast experiencing Alexandrium, and the Caribbean experiencing Gambierdiscus toxicus.

Table 1. List of Harmful Phytoplankton Species, Impacts & Region of Occurrence (Not Comprehensive). Source: Scripps Institute of Oceanography & University of California, Santa Cruz

Phytoplankton Species Toxin & Impact Region of Common Occurrence
Karenia brevis Brevetoxin - direct respiratory irritation, shellfish poisoning from consumption of exposed shellfish, direct poisoning of marine wildlife (e.g. fish, marine mammals & sea turtles) Gulf of Mexico & Florida
Dinophysis Okadaic acid and dinophysistoxins - diarrhetic shellfish poisoning West Coast
Gonyaulax Saxitoxin - paralytic shellfish poisoning from consumption of exposed shellfish West Coast
Alexandrium Saxitoxin - paralytic shellfish poisoning from consumption of exposed shellfish Northeast
Gambierdiscus toxicus Ciguatera - seafood poisoning from consumption of exposed fish Caribbean

As noted, not all phytoplankton are harmful, in fact most are harmless, but the species listed above represent a handful of the harmful phytoplankton able to produce toxins that can either become airborne (K. brevis), or can poison marine wildlife and humans that consume them or contaminated seafood.

Human Health

The human health impacts associated with red tides depend on the phytoplankton species, as Table 1 shows. Some red tides are relatively harmless, with people able to swim or surf during an event. Some toxic phytoplankton, such as K. brevis in Florida, release airborne toxins that can be directly inhaled by people on the beach, causing irritation of the eyes, skin, throat and lungs, and potentially severe impacts to those with lung conditions like asthma. Some estimate that brevetoxins from K. brevis can travel up to 1 mile inland pending wind speed and direction. Lastly, other toxic phytoplankton only pose a risk to human health by the consumption of contaminated seafood. This is why it’s important to know what type of diatom or dinoflagellate makes up red tides in your area. See the resources section below for more information on regional threats and occurrences, and this CDC website for specific information on human health impacts.

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Pet Health

Toxic red tides can also pose a risk to your pets, causing neurological impacts including seizures and paralysis if your pet consumes contaminated seafood or the phytoplankton directly. The River Landings Animal Clinic recommends: “If your animal is acting differently, experiences a seizure, is clumsy, has diarrhea, is shaking, loses his balance, appears confused or seems to not be able to see, seek veterinary care immediately.”

Marine Wildlife

Red tides can harm marine wildlife by creating temporary dead zones and reduced DO; poisoning wildlife directly; poisoning food sources; and physically covering wildlife, reducing their ability to move, hunt, and feed. Much of the following information is sourced from the Florida Fish and Wildlife Conservation Commission.

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As mentioned, when red tides eventually die off, they sink to the ocean floor and get consumed by marine bacteria. During this process, bacteria also consume much of the dissolved oxygen in the surrounding marine water, reducing the amount of oxygen available for other marine wildlife that depend on that oxygen to breathe and survive. Pending on the magnitude of the red tide, this can create a temporary “deadzone”, unable to support marine life. Fortunately, lots of marine wildlife are able to simply swim away from these hypoxic areas, but not all, as invertebrates like mussels, sponges, and sea urchins are unable to move far distances. Additionally, when hypoxic events happen in important nurseries, breeding areas, and feeding areas, the impacts on the local ecosystem and nearby populations can be severe.

Direct poisoning of marine wildlife is expected to be the main cause of the massive fish kills experienced in Florida during the 2018 red tide event. Exposure to K. brevis causes fish to experience paralysis, convulsions, “violent twisting and corkscrew swimming”, and the eventual inability to properly use and control gills, resulting in suffocation and death. This can occur even in relatively low to medium concentrations of K. brevis, at 250,000 cells per liter of ocean water. Sea turtles can also be directly poisoned, and display similar effects of loss of coordination, uncontrolled body movements, and lethargy.

Just like humans, consuming contaminated seafood can have severe consequences on marine wildlife. For instance, in Monterey, California, over 400 sea lions died after being poisoned by consuming anchovies contaminated with domoic acid during a local red tide of Pseudonitzschia australis. In 1988, a horrific die off of over 740 bottlenosed dolphins occurred off the US Atlantic coast. Scientists suspect that it was a mix of direct inhalation of toxins, and consumption of contaminated seafood. Three other dolphin die-offs have occurred since, mainly in the Florida panhandle, with 93% of tested deceased dolphins containing brevetoxins in their system. Even manatees have been poisoned from consuming contaminated seagrass during K. brevis blooms.

Lastly, the physical swath of red tides can cause massive marine wildlife die offs. In 2007, hundreds of seabirds were killed from their feathers being covered in the dinoflagellate Akashiwo sanguinea during a red tide event off of Monterey, CA. The slimy coating of phytoplankton caused the bird feathers to lose their water repellency, which is essential for birds to remain their necessary body warmth. Without water repellency, birds become too wet and their body temperatures fatally drop.


Economic costs of toxic red tides can be extensive, and include government funded clean ups (mainly of dead wildlife), loss of ocean tourism and recreation revenue, increased hospital visits and sick days, and loss of revenue from commercial and recreational fisheries. A 2000 study found that the annual impact of HAB’s on commercial fisheries range from $13 million to $25 million. County-level clean up costs in Florida have been calculated up to $250,000 for a single county during a red tide event. And as of August 2018, the 2017-2018 red tide has already costed Florida $8 million in lost revenue from coastal businesses alone.

While the complete economic impacts of red tides and HAB's are difficult to quantify, the following slide provided by the National Water Sentinels Harmful Algae Blooms (HABs) and You February 26, 2019 webinar provides a nice summary:

Slide from a recent Harmful Algae Blooms (HABs) and You webinar by National Water Sentinels.

How to Prevent Red Tides

While this is generally a natural occurring phenomenon, the truth of the matter is the severity of these events is worsening due to climate change and the large amounts of nutrient pollution discharged into coastal waters. Agriculture and landscaping fertilizers leach into ground and surface waterways. Wastewater infrastructure such as septic systems and sewers, break, overflow and ultimately fail to treat our waste. Sadly, during all of this crisis, Congress allowed the Harmful Algal Bloom and Hypoxia Research and Control Amendments Act of 2017 (S.1057) to expire in September 2018, which would have required an assessment of harmful algal blooms resulting from Lake Okeechobee discharges; and established a process to allow algal blooms and hypoxic events to be declared an “Event of National Significance”. There has been a failure at all levels of government to take action to protect clean water for safe recreation, wildlife and prosperous beach tourism economies.

Our elected officials need to pay attention to this public health and economic crisis and take action to control the flow of pollution into the waterways and beaches that drive our coastal tourism economy. See below for actions local community members, cities, and states can do to stop future severe algae blooms:

Actions we can all take:

  • Scoop the Poop. Pick up your pet’s waste
  • Wash your car over grass or gravel, not on the street. Better yet take it to a commercial car wash.
  • Make your yard more Ocean Friendly
    • Go organic. Stop using chemical fertilizers and pesticides
    • Apply mulch and compost to build healthy living soil instead
    • Plant native and climate-appropriate plants
    • Direct rain gutters and downspouts into your landscaping to slow down and sponge up rain. Learn more here.
  • Show up at City Council and demand the actions below

Local jurisdictional actions could include:

  • Conduct mandatory septic tank inspections
  • Post beach closures and health advisories quickly and publicly
  • Pass strong local fertilizer restrictions (see Manatee County for a good example)
  • Implement proactive programs to control stormwater pollution, such as Ocean Friendly Gardens
  • Inspect and maintain sewage infrastructure

Statewide actions could include:

  • Funding the Florida Healthy Beaches program to test recreational beaches for bacterial indicators of impaired water
  • Set and enforce stronger water quality standards and regulations
  • Help local communities transition away from septic systems to sewers
  • Provide funding to local municipalities to inspect and maintain their sewage infrastructure
  • Regulate and restrict fertilizer inputs from agriculture into freshwater areas
  • Restore natural flow of water to the coast

How to Stay Safe During a Red Tide

Red tide poses numerous serious health threats, please respect the recommendations by local authorities and any beach closings, warnings and guidelines. Check with local authorities to see what areas are being affected (look at the resources section below to see if there’s information for your state). Report illnesses or symptoms, by contacting your local Poison Control Center. You can also take the following steps to stay safe:

  • Learn about the types of red tides that frequent your area
  • Check with your local Department of Agriculture to learn about risks of consuming local seafood during a red tide event
  • If local red tides contain harmful species of phytoplankton (like K. brevis), avoid the beach & stay out of the water
  • If you have known breathing sensitivities, stay inside or get away from the coast
  • Review the resources below for more information
  • Take action now to prevent future red tides

Important Resources

Understanding Algae Blooms

Florida & Texas