A red tide washes over a Florida beach.
Harmful algal blooms, like this red tide washing over a Florida beach, are increasing in frequency and duration. Efforts at prevention, mitigation, and early warning are ramping up around the world. Credit: iStock.com/TriggerPhoto

With the annual emergence of summer heat, happy vacationers and weekenders are often eager to cool off at nearby lakes and beaches. More and more, however, they are having to cancel their plans, or at least stay out of the water, as reports come in from all over the United States and elsewhere warning of unwelcome summer visitors: algal blooms.

Among numerous recent examples, the Missouri Department of Natural Resources (MoDNR) notified the city of Sedalia on 13 June of a potentially hazardous algal bloom in nearby Springfork Lake. City officials had previously discontinued using water from the lake for drinking, and in response to MoDNR’s notice, they temporarily closed the lake to public recreation. The same day, the Florida Department of Health in Orange County cautioned Orlando area residents not to drink from or swim, wade, or boat in waters with visible algal blooms in two of the city’s lakes. Shortly before those instances, similar warnings were issued for Red Bud Isle near Austin, Texas, and for Minnesota’s Lake Hiawatha and Lake Harriet. And in mid-July, residents were advised about possible blooms in Lake Micmac near Halifax, N. S.

It is becoming increasingly crucial that we establish harmful algal bloom (HAB) early-warning systems and response strategies to help safeguard communities today as well as future generations.

Not all algae are hazardous—as a group, algae are, after all, nearly ubiquitous inhabitants of marine and freshwater environments. Yet some species produce compounds toxic to humans and wildlife, and at high enough concentrations, even species that are typically harmless can become harmful.

Harmful algal blooms (HABs) appear to have escalated globally in recent decades. In South Korea, for example, HABs typically lasted less than a week in the 1980s, but since 1995, they have often lasted more than a month [Park et al., 2013]. HABs are also becoming more frequent and are occurring over larger areas, as in the coastal waters of the northern Beibu Gulf, China, where HABs have expanded from covering tens of square kilometers in the late 20th century to hundreds more recently [Xu et al., 2019]. In south Texas, where I live and conduct environmental research, HABs also are emerging more often along the Texas coast.

With their growing frequency, duration, size, and impact over the past 30–40 years, doing nothing to control them is not an option; rather, it is becoming increasingly crucial that we establish HAB early-warning systems and response strategies to help safeguard communities today as well as future generations.

The ABCs of HABs

HABs come in different varieties—and colors—and occur in both marine and freshwater settings. Red and brown tides, aptly named for the hues they contribute to coastal waters, are caused by certain algae species—such as Karenia brevis and Aureoumbra lagunensis along the gulf coastlines of Florida and Texas. Meanwhile, blooms of blue-green algae (which are actually cyanobacteria rather than algae) commonly occur in freshwater and saltwater settings. The common thread is that they are toxic to aquatic life as well as to people.

Part of Corpus Christi Bay where the author and his colleagues have collected water samples is seen here in August 2015, 1 month prior to the development of a harmful algal bloom. Credit: J. E. Silliman

Respiratory and digestive symptoms and illnesses, sometimes severe, caused by airborne and waterborne toxins emitted during some HAB episodes are the most common impacts on human health. HABs have several negative effects on aquatic ecosystems. Microbial breakdown of large quantities of dead algae can cause hypoxia (low oxygen levels) or anoxia (depleted oxygen levels) in the surrounding water. And excessive algal growth can increase levels of toxic compounds and decrease water clarity. As a result, nearby fauna, including seabirds, fish, marine mammals, and sea turtles, can be poisoned or may alter their feeding behavior, which can sometimes prove fatal [Mateus et al., 2019].

Harmful algal blooms have afflicted coastal communities for centuries. Alaska first recorded issues with shellfish poisoning associated with algal blooms in the late 1700s [Anderson et al., 2019]. Not surprisingly, HABs are a major threat to today’s shellfish industry as well. In Europe, for example, persistent closures in shellfish-producing regions have resulted in annual costs exceeding $850 million [Mateus et al., 2019]. In addition to commercial fisheries, the aquaculture (fish and aquatic plant farming), tourism, restaurant, and lodging industries also have suffered income losses due to recurring HAB events.

Factors Favoring Harmful Algal Blooms

Increasing populations, expanding agricultural activities, and industrial growth all have magnified nutrient inputs into coastal waters, further feeding HAB events.

Natural factors affect the development of HABs. The circulation of surface currents can promote algal growth by transporting nutrients (e.g., nitrogen, phosphorus, and iron) and algae to certain regions. Also, as phytoplankton and other plants near the surface die and settle through the water column, bacteria degrade them, releasing nutrients into the deep water. Coastal upwelling, the upward flow of deep, nutrient-rich water to the ocean surface, similarly can spur the formation of HABs in coastal areas like those in California, Portugal, and Angola, as can rivers that deliver nutrients from the land to the ocean.

Human activities contribute to HABs as well. Increasing populations, expanding agricultural activities, and industrial growth all have magnified nutrient inputs into coastal waters, further feeding HAB events [Bennett, 2017]. Climate change effects, such as ocean warming and marine heat wave episodes, also have enhanced HAB development [Gobler, 2020]. As if all that isn’t enough, some harmful algae can even make conditions more favorable for themselves by secreting chemicals to suppress the growth of other algae.

Green algae swirls around the Baltic Sea in this 20 July 2019 satellite image from the Copernicus Sentinel-2 mission. Credit: Modified Copernicus Sentinel data, processed by ESA, CC BY-SA 3.0 IGO

In a comprehensive survey of more than 5,700 scientific papers published from 1991 to 2020, Sha et al. [2021] tracked trends in HAB research and identified two main thrusts emerging in the past decade, which I summarize below. The first is understanding the effects of climate change on HAB development, and the second is developing methods for managing HABs and eutrophication (the enrichment of water bodies with nutrients).

Recent studies have predicted a variety of climate-induced changes relevant to HABs. Greenhouse atmospheric warming caused by rising carbon dioxide (CO2) concentrations is one of the main causes of ocean warming. Warmer shallow waters not only enhance HAB formation but also help establish a more stable or stratified water column that inhibits vertical mixing of water layers. Stratification encourages buoyant harmful algae to concentrate in surface waters.

Higher levels of atmospheric CO2 also increase CO2 levels in surface waters, which tends to support the growth of HAB-forming species more so than nonharmful algae [Gobler, 2020], although the reasons for this have not been established. In addition, climate change is expected to amplify rainfall totals and intensities in areas affected by extreme storms. Such increases in rainfall will swell rivers and augment delivery of nutrients from land to coastal waters, thereby aiding HAB development. Sinha et al. [2017] projected that the contiguous United States, India, China, and Southeast Asia all will experience significantly increased precipitation by the end of the century, resulting in elevated nitrogen loading in waterways that encourages eutrophication and HAB formation.

What Can Be Done About HABs?

With the problems posed by HABs worsening, the need for effective intervention and management strategies is growing more acute. As Sha et al. [2021] discussed, research over the past few decades has focused on evaluating various chemical, physical, and biological methods—each with its own pros and cons—to manage eutrophication of freshwater and marine ecosystems and thus to curb HAB formation.

Salts such as copper(II) sulfate and sodium hypochlorite (which, in solution, forms bleach) long have been used to destroy HABs, but management efforts with these materials increasingly are seen as unsustainable because of their toxicity to other aquatic organisms. Hydrogen peroxide, with the help of sunlight, has shown promise at selectively annihilating certain HAB species. However, each of these chemicals works by causing algae cells to rupture, which releases significant amounts of toxins into the surrounding water.

Newer algicidal chemicals, such as TD49 and DP92, have been designed to selectively inhibit harmful algal growth in seawater and to degrade readily rather than persist in the water column. These chemicals have shown promising initial results and thus may significantly improve our ability to prevent HAB development.

Physical methods used to deal with HAB formation include increasing feed efficiencies—by reducing feed consumption per mass of fish produced—and introducing nutrient recycling within aquaculture systems to lower nutrient discharges and limit HAB events. Such methods could be especially helpful in areas of heavy aquaculture activity, such as China, where nitrogen and phosphorus released from aquaculture have increased over the past 10–20 years and have enhanced eutrophication [Wang et al., 2020].

Using clay mixtures to remove HABs from coastal waters has shown success in some places, including around South Korea, for example [Park et al., 2013]. In this approach, a slurry of finely ground clay and water is sprayed onto surface waters. Repulsive surface charges between the clay particles are neutralized in seawater, causing the particles to flocculate, or stick together, to form larger and denser aggregates. As these aggregates sink and settle rapidly through the water column, algae cells become entrapped within them and are removed from the surface waters to the bottom of the bay or ocean. Other substances, such as sediments, aluminum salts, and iron salts, have been used in similar manners in both freshwater and marine settings [Sha et al., 2021].

Ultrasonic treatments also have been investigated and have been found to be effective at destroying harmful algae. However, as with some chemical treatments, their use causes algae cells to rupture and release toxins into the water. Using mechanical pumps to increase the mixing of different water levels can disrupt HAB formation in surface waters, although this approach is expensive and is likely appropriate only for smaller water bodies.

Biological techniques also have been implemented to prevent HAB development [Pal et al., 2020]. Such approaches include encouraging the activity of aquatic organisms like bacteria, viruses, fungi, fish, and zooplankton (which feed on phytoplanktonic algae) that target HAB-forming species through infection, predation, or algicidal inhibition.

Nutrient pollution caused this 2007 duckweed outbreak along the shore of Lake Maracaibo in Venezuela. The shade provided by duckweed, a type of flowering plant, can reduce the growth of certain types of algae that rely on photosynthesis. However, duckweed mats also can create hypoxic conditions in the water below. Credit: Wilfredor/Wikimedia, Public Domain

To combat harmful algal blooms, it is often more effective to implement multiple measures to inhibit HAB development while keeping nutrient inputs to a minimum as well.

Biological treatments can be more economically viable and typically do not produce unwanted and potentially hazardous by-products for other aquatic life. However, they often have only short-term efficacy. In one study, for example, reducing the number of zooplankton-eating fish in lakes, which in turn promoted zooplankton consumption of HAB algae, showed promising initial results. However, in high-nutrient waters, HABs returned shortly after treatment. Similarly, because most bacteria and viruses tend to associate with only a single species each, their introduction to water bodies has had limited success in combating HABs over long periods. In addition, applying biological techniques may alter the food web in a water body and thus have wider consequences than intended.

Because of limitations in the success and safety of the various chemical, physical, and biological prevention and control methods investigated, it is often more effective to implement multiple measures to inhibit HAB development while keeping nutrient inputs to a minimum as well [Sha et al., 2021]. It is also important to understand as much as we can about local HAB formation and about the potential ramifications of different treatments before taking action.

Mitigation Through Early Detection

Because so many factors affect their formation, HABs are difficult to control or eliminate entirely, even with the best available management options. But detecting and treating them as soon as possible help contain them and prevent their worst health and economic impacts. Thus, routine monitoring of coastal waters and developing early-warning systems (EWSs) for HABs have gained popularity. Several coastal regions in the United States have implemented their own EWSs, which the U.S. Integrated Ocean Observing System is organizing into a national system. NOAA and other federal agencies in the United States are involved in HAB research and EWS efforts as well. On an international level, GlobalHAB and PRIMROSE (Predicting Risk and Impact of Harmful Events on the Aquaculture Sector) are organizing HAB research and developing EWSs.

An EWS provides a short window for producers (e.g., commercial fisheries) and regulating entities to take preventive measures against impending threats, thus protecting human health and financial revenue [Mateus et al., 2019]. Preventive measures could include implementing biological, chemical, or physical methods to inhibit HAB formation or closing beaches.

Because water bodies can vary so much, however, there is no one-size-fits-all early-warning system.

Most early-warning systems are complex and require information from various sources, including manually or remotely collected in situ data detailing water, sediment, plant, and animal conditions from the area of interest; satellite imagery and data indicating sea surface temperatures, chlorophyll levels, and ocean color; and mathematical modeling simulating physical and ecological processes in oceans and lakes. Once this information is gathered, it must be interpreted and run through predictive models to generate forecasts, which then can be disseminated to interested stakeholders [Mateus et al., 2019]. Because water bodies can vary so much, however, there is no one-size-fits-all EWS [Anderson et al., 2019].

The EWS used in Portugal, for example, relies on weekly monitoring of water samples for harmful algae and of shellfish specimens for the presence of biotoxins in addition to ocean current data obtained from an operational oceanographic modeling system [Silva et al., 2016]. Meanwhile, in Scotland, where many blooms originate offshore and are driven to coastal regions by wind and surface currents, an EWS in use is based on a combination of mathematical modeling (providing wind, rain, and temperature predictions), satellite ocean color data, and cell counts of harmful algae obtained from weekly sampling of coastal waters [Davidson et al., 2021]. Both these EWSs provide weekly forecasts, and each has reduced the overall impact of HABs in the country where it’s used.

Two C’s Are Key

Every body of water is different, each with its own conditions, climate, ecosystems, and more. Thus, communication and collaboration among scientists, planners, regulators, and industry in affected regions are crucial for effectively developing strategies to manage HAB events in different settings.

Successful communication and collaboration involve timely sharing of scientific and industrial data (i.e., environmental and monitoring information) with planning and regulatory bodies, proactive responses by planners and regulators to local residents regarding HAB warnings, and implementation of reasonable solutions to minimize HAB impacts. In other words, it takes a village to improve a community.

At the same time, research into the factors that favor HAB formation and development will continue to be important—the more we learn, the better prepared we will be to forecast and mitigate the disruptions and detrimental effects they have.

In the meantime, individuals can help themselves stay out of harm’s way by paying attention to warnings and advice regarding HABs. However, if you do happen to encounter an unpleasant odor coming from the water on your next stroll along the beach—compliments of a no-good, bad, and ugly algal bloom—just keep on walking.


Anderson, C. R., et al. (2019), Scaling up from regional case studies to a global harmful algal bloom observing system, Front. Mar. Sci., 6, 250, https://doi.org/10.3389/fmars.2019.00250.

Bennett, L. (2017), Algae, cyanobacteria blooms, and climate change, Clim. Inst., Washington, D.C., climate.org/wp-content/uploads/2017/05/bennett_algalblooms-1.pdf.

Davidson, K., et al. (2021), HABreports: Online early warning of harmful algal and biotoxin risk for the Scottish shellfish and finfish aquaculture industries, Front. Mar. Sci., 8, 631732, https://doi.org/10.3389/fmars.2021.631732.

Gobler, C. J. (2020), Climate change and harmful algal blooms: Insights and perspective, Harmful Algae, 91, 101731, https://doi.org/10.1016/j.hal.2019.101731.

Mateus, M., et al. (2019), Early warning systems for shellfish safety: The pivotal role of computational science, in Computational Science—ICCS 2019, Lect. Notes Comput. Sci., vol. 11539, pp. 361–375, Springer, Cham, Switzerland, https://doi.org/10.1007/978-3-030-22747-0_28.

Pal, M., et al. (2020), Biotic control of harmful algal blooms (HABs): A brief review, J. Environ. Manage., 268, 110687, https://doi.org/10.1016/j.jenvman.2020.110687.

Park, T. G., et al. (2013), Economic impact, management and mitigation of red tides in Korea, Harmful Algae, 30S, S131–S143, https://doi.org/10.1016/j.hal.2013.10.012.

Sha, J., et al. (2021), Harmful algal blooms and their eco-environmental indication, Chemosphere, 274, 129912, https://doi.org/10.1016/j.chemosphere.2021.129912.

Silva, A., et al. (2016), A HAB warning system for shellfish harvesting in Portugal, Harmful Algae, 53, 33–39, https://doi.org/10.1016/j.hal.2015.11.017.

Sinha, E., et al. (2017), Eutrophication will increase during the 21st century as a result of precipitation changes, Science, 357, 405–408, https://doi.org/10.1126/science.aan2409.

Wang, J., et al. (2020), Aquaculture production is a large, spatially concentrated source of nutrients in Chinese freshwater and coastal seas, Environ. Sci. Technol., 54, 1,464–1,474, https://doi.org/10.1021/acs.est.9b03340.

Xu, Y., et al. (2019), Historical occurrence of algal blooms in the northern Beibu Gulf of China and implications for further trends, Front. Microbiol., 10, 451, https://doi.org/10.3389/fmicb.2019.00451.

Author Information

James E. Silliman (james.silliman@tamucc.edu), Texas A&M University–Corpus Christi

Citation: Silliman, J. E. (2022), Harmful algal blooms: No good, just the bad and the ugly, Eos, 103, https://doi.org/10.1029/2022EO220440. Published on 9 September 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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