by Adeline Muras, VWRRC Undergraduate Intern
Harmful algal blooms (HABs) occur when negative impacts to the environment or human health occur because of cyanobacteria producing toxins. Cyanobacteria (also known as “blue-green algae”) are a type of photosynthetic bacteria that, like algae, derive their energy from photosynthesis. Some cyanobacteria have been known to produce cyanotoxins such as microcystin, cylindrospermopsin, anatoxins, and more (EPA). These toxins cause of a range of health effects from minor to life-threatening. HABs can occur in a mix of different water environments including saltwater, freshwater, and brackish water (salt and freshwater mixed). Toxic blooms are influenced by nutrients, water temperature, stagnant water, and extreme weather. Some of these factors are being exacerbated by climate and land-use change. Ongoing scientific research is investigating the long-term effects of HABs and exploring more efficient methods for monitoring and predicting their impacts. By understanding the effects of HABs, we can better protect the environment, safeguard human health, preserve aquatic ecosystems, and enhance economic stability.
Figure 1. Examples of HABs. Photos courtesy of Smith Mountain Lake Association
Why do Algal Blooms Occur?
Why do these algal blooms happen, especially in the warmer summer months? Both algae and cyanobacteria can produce toxins under favorable conditions. Algal blooms are caused by chemical and physical factors. These can include, but are not limited to, nutrient loading of nitrogen and phosphorus, increased water temperature, light availability, changes in water flow, and variations in pH and salinity (EPA). A body of water with high nutrient levels, warm still water, and adequate sunlight is the ideal environment for algal blooms. Anthropogenic or human-caused factors are known to be increasing the frequency of HABs around the world. These factors include excess nutrients from sewage plants, farming, and other industrial industries. The fertilizers, chemicals, or nutrients that make their way into a water body can cause algae populations to increase, making HABs more likely. The U.S. Environmental Protection Agency (EPA) says that “warmer temperatures for longer periods of time caused by climate change favor HAB taxa and may be improving their competitive fitness over other non-harmful algal taxa” (EPA). While algal blooms are a natural process, human alteration to the environment increases the likelihood of them in the future.
What are the Impacts to Human Health?
As stated, when HABs occur, they have the potential to produce cyanobacteria and cyanotoxins that can cause illness in humans. The toxin microcystin-LR, is known to cause gastrointestinal issues, headaches, dry cough, pneumonia, and long-term liver damage. The impacts to human health can vary based on duration and level of exposure. Another toxin, cylindrospermopsin, can cause gastrointestinal issues, and potential damage to organs and have neurotoxic effects. The anatoxin-a group can cause tingling, burning, numbness, drowsiness, incoherent speech, salivation, and respiratory paralysis (EPA). Exposure to toxins is generally through swimming or drinking contaminated water, eating contaminated fish or shellfish, or possible inhalation of water droplets. Fish and shellfish can accumulate toxins over time, making it hazardous to consume aquatic life exposed to HABs. HABs can also have negative effects on animals such as pets and livestock. Encountering contaminated water can cause illness or possibly even death in livestock. Long-term health effects of HABs on humans and livestock are still under investigation, as chronic or long-term impacts remain unclear.
What are the Effects on the Environment?
In addition to affecting human health, HABs also have harmful effects on the environment. Algal blooms do not have to be toxic to have a negative effect on the environment. Negative effects can include reduced oxygen levels in the water, decreased light penetration, fish kills, and disruption of the food web. Low oxygen occurs when the bacteria and algae from a bloom start to decompose and use up the oxygen in the water. This reduction in oxygen availability for fish and other aquatic plants and animals and can lead to higher mortality rates for these species. Hypoxia (a deficiency in oxygen) and anoxia (the absence of oxygen) can be accompanied by an increase in acidity and even release metals and other pollutants from the sediments in the water. The decrease in light penetration negatively affects the growth of many plants that other organisms depend on. This has the potential to alter the flow of the food web as some producers may decrease in numbers. Toxins from HABs can inhibit the growth of other algae and other small or microscopic organisms in the water that may disrupt the food web.
What are the Detection Methods Used?
There are many different methods used in the detection of HABs. The initial detection is usually visual, often by people living near the waterbody. This can include surface water discoloration resembling spilled paint or scum on the surface of the waterbody. However, this method is not effective in detecting toxins released from the algal bloom. The water must be analyzed to determine if toxins were released and if they reached the threshold to issue a swim advisory. Another helpful method in looking for HABs is using satellite imagery to determine when blooms occurred and how long they lasted. This method can help identify trends in the causes of HABs and inform strategies for their mitigation. The EPA has several resources that show satellite and real-time monitoring data on HABs that can be found here. After determining that there is an algal bloom, samples of the water will be collected and tested in the lab for toxins such as microcystin-LR, cylindrospermopsin, and the anatoxin-a group. This will determine if the algal bloom is considered harmful or not and is used when issuing swim advisories or warnings for the affected waterbody.
How Can We Prevent and Manage Algal Blooms?
What can be done to prevent and manage algal blooms when they do occur? One corrective action in a watershed can be to reduce nutrient overloading into the waterbody. This can be achieved through Best Management Practices (BMPs), such as fences to keep cattle out of streams, cover crops, and riparian buffers in agricultural areas, and using water efficiently, picking up pet waste, avoiding the use of certain chemicals and fertilizers on lawns, and properly treating wastewater in urban areas. Maintaining septic systems is another key factor in managing nutrient inputs to a waterbody. When septic systems are not cleaned or inspected regularly, they may develop leaks or fail to function properly. Nutrients from these septic systems can get into groundwater or surface water and result in greater inputs to surface water bodies. Individuals can help prevent nutrient pollution by picking up after pets, using commercial car washes, applying fertilizer only when necessary, using phosphate-free detergents and soaps, and conserving water and energy.
There are also interventions during algal blooms that can limit the spread and suppress the blooms. The most common measure taken to control HABs is called flocculation. Flocculation involves adding specific types of clay to the water, which bind with particles and cause them to aggregate and sink to the bottom (WHOI). This can either rupture the cell membranes or bury the algae in sediments on the floor of the water body. This practice is more widely used in South Korea and China and has yet to be implemented on a large scale in the United States. Further research is needed to understand the potential effects of clay on phytoplankton and the aquatic ecosystem as a whole. Chemical methods such as dispersing blooms with hydrogen peroxide are also being investigated to inhibit or kill HABs. Copper sulfate was used in the 1950s to control HABs but had negative effects on other marine organisms (WHOI). While there are practices being tested to manage HABs when they have already occurred, the best practice is to reduce the factors that are the root cause of these blooms.
What Can You Do If You Suspect a Bloom?
What can be done if you suspect there is an algal bloom in a waterbody by you? If you see what you think is a HAB in a waterbody near you, it is best to stay out of the water and report the suspected bloom to your local health agency. In Virginia, Department of Health (VHD) has an online form that you can submit if you suspect a harmful algal bloom. Click here to pull up VDH’s HAB form. This form includes fields such as waterbody name, the date and time the HAB was observed, information on the waterbody location, and space for comments and contact information if VDH needs more information about the HAB.
Figure 2 shows VDH’s Algal Bloom Surveillance map found on their website located here. This map is updated regularly from May to October and can be useful in identifying algal blooms that are being investigated or blooms that have occurred in the past.
Conclusion
HABs are a significant environmental problem that is becoming increasingly relevant worldwide. As the frequency and intensity of these blooms increase, the need for monitoring and management strategies become more critical. Focusing on preventative measures, such as reducing nutrient runoff and implementing BMPs, is one way to combat HABs. Additionally, increased research into managing algal blooms and understanding their long-term effects is essential. Through continued research, public awareness, and environmental management, the threats posed by HABs can be mitigated.
References:
National Institute of Environmental Health Sciences. (n.d.). Algal blooms. Retrieved April 26, 2025, from https://www.niehs.nih.gov/health/topics/agents/algal-blooms
University of Central Florida. (2020, June 30). UCF scientists study method to combat Florida red tide. Retrieved April 26, 2025, from https://www.ucf.edu/news/ucf-scientists-study-method-to-combat-florida-red-tide/
U.S. Environmental Protection Agency. (n.d.). What are the effects of HABs. Retrieved April 26, 2025, from https://www.epa.gov/habs/what-are-effects-habs
Virginia Department of Health. (n.d.). Algal bloom surveillance map. Retrieved April 26, 2025, from https://www.vdh.virginia.gov/waterborne-hazards-control/algal-bloom-surveillance-map/
Woods Hole Oceanographic Institution. (n.d.). Control and treatment. Retrieved April 26, 2025, from https://hab.whoi.edu/response/control-and-treatment/
