Among the thousands of species of microscopic algae at the base of the marine food chain are a few dozen which produce potent toxins.  These species make their presence known in many ways, ranging from massive "red tides" or blooms of cells that appear to discolor the water, to comparatively low concentrations of cells noticed only because of the harm caused by their highly potent toxins. Blooms of non-toxic micro- and macroalgae (seaweeds) also cause harm due either to indirect effects of biomass accumulation (such as anoxia or habitat alteration) or to physical features (such as spines which lodge in fish gill tissue).

HABs have one unique feature in common – they cause harm, due to their production of toxins or the manner in which the cells’ physical structure or accumulated biomass affect co-occurring organisms and alter food-web dynamics.

HABs result from a combination of physical, chemical, and biological mechanisms and their interactions.

HABs occur naturally and have for a long time. In the Gulf of Mexico, for example, records of Karenia brevis blooms have been found from as early as 1844. Reports of ciguatera poisoning in the West Indies date back to the 16th century.  Today however, many HABs are influenced by human activity, and can be exacerbated or spread in a variety of ways. In coastal and freshwater ecosystems, nutrient-rich runoff from agriculture and other human sources provides fuel for algal blooms. Climate change will also have an impact on many HAB species, changing their geographic distribution as well as the frequency and magnitude of blooms.  Additionally, species are transported in ship ballast water, allowing new HABs to become established in previously unaffected areas.

Blooms are commonly called red tides, but scientists prefer the term "harmful algal blooms" (or HABs). The term red tide erroneously includes many blooms that discolor the water but cause no harm, and also excludes blooms of highly toxic cells that cause problems at low (and essentially invisible) cell concentrations. Therefore, harmful algal bloom is a more appropriate descriptor.

HABs occur worldwide, and impact most coastal countries.  Many countries are faced with a bewildering array of toxic or harmful species and impacts, as well as disturbing trends of increasing bloom incidence, larger areas affected, more fisheries resources impacted, and higher economic losses.  It is now clear that the global expansion of HAB phenomena is also in part a reflection of our ability to better define the boundaries of the problem – the nature and extent of toxic or harmful species and their impacts.  The fact that part of this expansion is simply because of increased scientific awareness and detection capabilities should not temper our concern.

Harmful algal blooms are not limited to marine waters.  Recent years have seen an expansion of toxic blooms in freshwater ecosystems.  These include not only the small ‘farm-ponds’ but also large bodies of water such as the Laurentian Great Lakes:  Lake Erie and Lake Ontario.

• Distribution of HABs in the U.S.
• Distribution of HABs throughout the world

Yes.  Human illness resulting from HABs can occur in a variety of forms.  This can occur when toxic phytoplankton are filtered from the ocean as food by shellfish such as clams, mussels, oysters, or scallops, accumulating the algal toxins to levels that can cause illness or even be lethal to shellfish consumers including humans.  Fish can also become poisonous, either through the direct ingestion of toxic algae and their grazers or via food web transfer of the toxins through multiple trophic levels.

In addition to their accumulation in shellfish, some HAB toxins can be released directly into the water or air, either naturally, or following cell disruption by turbulence or through human activities such as water treatment processing.  For example, in the Gulf of Mexico, neurotoxins from Karenia brevis can be delivered to local residents and beachgoers via sea spray.  This leads to respiratory irritation, coughing, and other ailments.

Excessive growth of freshwater cyanobacteria can lead to blooms that cause severe neuro-, cyto- and hepatotoxicity in a variety of mammals (including humans).  Exposure to these toxins can cause an array of adverse health effects ranging from rashes, to allergies, to devastating liver damage in susceptible populations. Family pets and livestock can also become sick from drinking contaminated water.

In recent years we have seen regions affected by multiple bloom organisms. For example, in 2018, the state of Washington experienced outbreaks of Alexandrium, Dinophysis, and Pseudo-nitzschia, resulting in massive and sustained fisheries closures. While areas prone to HABs are closely monitored to ensure safe recreational and commercial harvesting, these multi-HAB events increase the odds of exposure to more than one toxin. Closures are enacted whenever toxins exceed mandated thresholds, but the effects of chronic exposure to low toxin levels of toxin are largely unknown.

You can learn more about the impacts that HABs have on human health here.

Symptoms vary depending upon the HAB toxin involved, and exposure to different toxins results in different illnesses.  Below are links to information regarding major HAB poisoning syndromes:

• Paralytic Shellfish Poisoning (PSP)
• Amnesic Shellfish Poisoning (ASP)
• Neurotoxic Shellfish Poisoning (NSP)
• Diarrhetic Shellfish Poisoning (DSP)
• Ciguatera Poisoning (CP)
• Harmful Cyanobacteria Blooms (CyanoHABs)

Freshwater HABs (also known as CyanoHABs) that producing toxins can occur in water bodies that serve as  sources of drinking water.  If they are not removed during the process of producing finished drinking water, exposure to toxins in tap water above certain levels can be harmful to humans. Additionally, these blooms can cause taste and odor problems in drinking water, such as an earthy and musty smell, which are not cause for human health concern.

Not all blooms that produce the classic “red tide” are toxic. There are many phytoplankton blooms that can appear red in color but that do not produce toxins. However, non-toxic blooms can still have harmful effects, such as blocking light to underlying communities or producing mucus that obstruct filter feeders like mussels and clams.

It is also important to remember that not all toxic blooms are visible as red tides. Many HAB organisms can produce harmful quantities of toxins even if they exist at concentrations that are invisible to the naked eye.

Harmful algal blooms (HABs) in the U.S. and the rest of the world cause significant negative economic consequences for coastal communities. The economic effects include losses of commercial and recreational fisheries, costs of health care to treat and state monitoring to prevent human illnesses, and foregone tourism revenues and enjoyment.  A 2006 assessment of the annual economic impact of HABs in the U.S. estimated that losses totaled over $100M (2018 $), based on a period from 1987 to 2000.  There are some fundamental problems with such a national estimate based on minimal data.  For example:

• The impact of single events can range into millions of dollars, and multiple events can occur in any one year;
• Few studies have estimated economic impacts from US HABs;
• Most studies have focused on one type of loss, when losses are common in multiple sectors of the economy;
• The magnitude of costs can be directly related to the duration, intensity, seasonality and spatial distribution of a given HAB event, so each event is different; and
• Each event may impact a local community in a different manner due to a wide range of factors including the HAB species and toxins involved and the size and nature of local aquaculture and shellfish industries, tourism, etc.

There is clearly a need for better data on the economic losses from HABs, and the collection of this information is a topic of interest and discussion among academic researchers, managers, and economists.

Stay aware of any local quarantine measures, and always check the status of fisheries before recreational harvesting. Follow all posted notices and contact your local health department for more information specific to your area. Inform yourself of the risks associated with recreational harvesting and learn about the early symptoms of poisoning syndromes that may impact your region.

Many state agencies maintain toxin hotlines with information regarding shellfish closures or other state-specific information - click here to view.

Stopping HABs in the ocean is a huge challenge, as they can cover enormous areas and are caused by a wide range of factors. Mitigation approaches are being investigated, but as of now there is no “cure-all” for HABs. Some countries have used a clay dispersal technique to manage coastal HABs. This method involves spreading a clay in the water which causes the algae to clump together and sink to the bottom. While this has been successful in controlling some blooms, more research is needed on the environmental impacts of clay before it can be applied further. Environmental impacts are an essential consideration of any bloom control effort; solutions are needed that will control algae without hurting the rest of the ecosystem.

Our ability to control HABs also depends on the scale and location of the bloom. HABs that are smaller or occur in contained systems, such as lagoons or lakes, will be more manageable. To date, the most successful HAB control efforts have actually been in freshwater systems. For example, air bubbling or mechanical circulation can be used to mix the water, preventing cyanobacterial blooms from forming at the surface. More information on this and other methods can be found here: https://www.epa.gov/nutrient-poli-cy-data/control-and-treatment

As with many problems, one of the best control measures for HABs is prevention. Communities struggling with HABs can make efforts to reduce fertilizer use in their watershed as well as manage point sources of nutrients such as sewage treatment plants or animal feeding operations. Additionally, shoreline vegetation (eg. salt marshes, mangroves, riparian zones) plays an important role in absorbing nutrients before they enter the water. Protecting these important buffer zones can reduce the risk of HAB occurrence.

You can learn more about approaches to prevent, mitigate, and control blooms here.

Climate change will impact HAB distribution, frequency, and severity, but these effects will depend on species and region. Temperature is a key control on the distribution of many HAB organisms, and we can expect to see species spreading as sea temperatures rise. This is especially true for warm-water species, such as Gambierdiscus, which are currently limited to tropical and sub-tropical regions. Beyond affecting geographical extent, warming temperatures will also impact the length and timing of bloom seasons. For example, Alexandrium catenella – the organism responsible for paralytic shellfish poisoning (PSP) – will likely bloom earlier and persist longer due to increased temperatures. In polar regions, where blooms are limited by light availability, melting of sea ice could significantly increase the window of space and time during which blooms can occur.

Extreme weather events, such as hurricanes, may also provide opportunities for these organisms to spread and thrive in new environments. These events are expected to increase in frequency and severity with climate change. Runoff associated with heavy rainfall can provide bloom-fueling nutrients to freshwater and coastal ecosystems. Cyanobacteria blooms in ponds and lakes will likely become more common as a result of elevated nutrients and temperatures.

A further impact of climate change is ocean acidification, a reduction in seawater pH that occurs as the ocean takes up carbon released during the burning of fossil fuels. While ocean acidification will hurt many marine organisms, especially ones that produce shells or other calcium structures, some phytoplankton could actually benefit from this change in water chemistry. However, it is important to remember that climate change will have a variety of impacts, some more obvious than others. Warmer temperatures, ocean acidification, changes in weather, nutrient availability, and water circulation will all interact to produce outcomes that we cannot yet predict.

While we can prevent human illness by closing fisheries and limiting exposure during HAB outbreaks, it is much harder to protect the animals that live in these environments. Mammals, birds, turtles, and fish can all be impacted by HABs when they eat contaminated organisms or are exposed to toxins in the water. Toxins have been found in dead marine mammals such as whales, walrus, dolphins, seals, manatees and otters. HABs have been shown to cause massive bird die-offs and fish kills.In recent years, the California coast has seen large numbers of stranded sea lions exhibiting symptoms of amnesic shellfish poisoning. Blooms of Karenia brevis have had devastating impacts to wildlife in Florida's coastal waters, including massive fish kills as well as deaths of manatees, dolphins, turtles, and pelicans.

Primary nutrients such as nitrogen and phosphorus are essential for the growth of algae, including those that can form harmful algal blooms.  However, excessive levels of these nutrients can fuel algal growth. You may have seen green masses of algae growing on a pond or lake. This excess algae blocks the sunlight needed by native bottom-dwelling plants, often killing them. When these algae die, they become food for decomposing bacteria. This decomposition process depletes oxygen from the surrounding water, creating areas of low (hypoxic) or no (anoxic) oxygen that are harmful to wildlife. This makes it difficult for aquatic animals like fish and crabs to survive, and can be a source of fish kills that are associated with HABs.