The following will help you understand the dangers of algal blooms, and the importance of having Black Berkey® Purification Elements to remove or reduce harmful algal toxins from your drinking water.

What are algal blooms?

Icky, smelly and potentially harmful to your health, algal blooms (or algae blooms) are overgrowths of algae or cyanobacteria in water. Algal blooms that contain toxins and can be harmful to human and animal health are more specifically called “harmful algae blooms”  or abbreviated to “HABs”.

Algal blooms can be made of algae or cyanobacteria. It is important to understand the differences and similarities of the two.


Algae are a diverse group of aquatic organisms that have the ability to turn sunlight into chemical energy and food through a process called photosynthesis. Algae thrive in freshwater environments, and can range from invisible single-celled microbes floating in water to the larger—and more familiar—kelp and seaweeds.1 Common green algae is multicellular, has chloroplasts that aid in photosynthesis, and ranges in size from 300 to 1,000 micrometers.

According to the Environmental Protection Agency (EPA), harmful algal blooms can have major impacts on everyday life, such as:

  • Producing dangerous toxins that can sicken or kill people and animals
  • Creating dead zones in the water
  • Increasing drinking water treatment costs
  • Hurting industries that depend on clean water2

Yet in spite of their unpleasant appearance, foul smell, and potential to harm the health of humans, animals and fish, the National Ocean Service reminds us that “not all algal blooms are harmful. Most blooms, in fact, are beneficial because the tiny plants are food for animals in the ocean. In fact, they are the major source of energy that fuels the ocean food web.”3


Cyanobacteria is another term associated with algal blooms. However, cyanobacteria is not a true algae at all. Since cyanobacteria are photosynthetic and aquatic, they are often called blue-green algae.4 Cyanobacteria cells range in size from 0.5 to 60 micrometers but when allowed to flourish, can produce impressive mat-like colonies in fresh and marine waters that look identical to true algal blooms. Although cyanobacteria cells lack flagella to freely swim about, they are able to change their depth in the water. Cyanobacteria-contaminated water does not always exhibit any taste or odor problems.5 Over 46 species of cyanobacteria are capable of producing toxins.6


Cyanotoxins are produced by cyanobacteria (also known blue-green algae), and are often released upon cell death or when the cell membrane ruptures. Some cyanobacteria can release some of their toxins without cell death.7

Types of cyanotoxins include:

    • Neurotoxins (nerves) – can cause paralysis of skeletal, respiratory muscles
    • Dermatotoxins (skin) – can produce rashes, skin reactions
    • Endotoxins (gastrointestinal epithelium) – can produce fever,  induce immune response
  • Microcystins are hepatotoxins that are a potential carcinogen of the liver and kidneys8

What causes algal blooms?

According to the National Ocean Service, “Harmful algal blooms (HABs) occur when colonies of algae—simple plants that live in the sea and freshwater—grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals, and birds.”9 Algal blooms appear most commonly during the summer, but can occur any time of the year.

How Algae Becomes a Harmful Bloom

As algae and bacteria begin to grow, they consume large amounts of dissolved oxygen and nutrients in the water. As a result, aquatic plants die off, allowing the algae and bacteria to grown in even greater numbers. Eventually the lack of dissolved oxygen in the water can cause massive fish kills, creating what are known as ‘dead zones’.

Growth factors for cyanobacterial algal blooms include:

  • Nutrients—promote and support growth
  • Temperature—grow best in temperatures above 77° during summer, and are less common during colder parts of the year
  • Light—optimal growth occurs with intermittently exposure to high light intensities, can adapt to variable light conditions
  • Stable conditions—most blue-green algae prefer light winds and minimal turbulence
  • Turbidity—low turbidity (the amount of particles and organic matter suspended in the water), such as in stagnant or slow-moving water, allows more light to penetrate the water. in turn helping bloom growth10

Nutrient Pollution

One of the biggest contributors to the problem of increased algal bloom activity is nutrient pollution. Nitrogen and phosphorus are vital nutrients that help support the natural growth of algae and aquatic plants, which provide food and habitat for fish and other organisms. Nitrogen is also a major component of the air we breathe. However, excess amounts of nitrogen and phosphorus in the water can fuel the development of unwanted algal blooms. Algal blooms can reduce or eliminate oxygen in the water, in turn harming or killing large numbers of fish.11

According to the EPA, excess nitrogen and phosphorus in the water can be attributed to four major sources:12

  • Fossil fuels: 250 million cars and trucks are responsible for 7 million tons of nitrogen oxide being added to the air in the United States each year
  • Agriculture: animal manure, excess fertilizer and soil erosion. From 1964 to 2008 agricultural fertilizer use has increased 25%
  • Urban sources: about 10% of nutrients flowing from the Gulf of Mexico come from urban stormwater and sewer/wastewater plants
  • Industrial: 592 industrial facilities released 100,000 tons of nitrate compounds in 2010

To see the effects of nutrient pollution, one has to look no further than the Gulf of Mexico. It is fed by the 2,300 mile-long Mississippi River, and therefore all the runoff nutrients that accumulate along the way to the Gulf.

The Gulf of Mexico is responsible for an estimated $662 million in commercial fishery landings each year. It is also home to the second largest hypoxic zone in the world—over 5,800 square miles (about the size of Connecticut). Oxygen-deprived hypoxic zones can dramatically reduce the reproduction and growth of aquatic life.13

What is a red tide?

The term “red tide” refers to an algal bloom in which pigments in the phytoplankton give the water an orange, brown or red appearance. And like the algae blooms discussed earlier in this article, some red tides are harmful, while many others are not.14 According to the Florida Department of Health, “while people may swim in red tide, some individuals may experience skin irritation and burning eyes.”15

The Florida Department of Health notes that red tides are not a new phenomenon: “red tides were documented in the southern Gulf of Mexico as far back as the 1700s and along Florida’s Gulf coast in the 1840s. Fish kills near Tampa Bay were even mentioned in the records of Spanish explorers.”16

Florida’s red tide is caused by the dinoflagellate Karenia brevis (also known as K. brevis). Red tides can release toxins that kill millions of fish every year.17 Red tide is not caused by humans. It is a natural phenomenon that occurs when temperature, salinity and nutrients reach certain levels.18 The most well-known red tide occurs nearly every summer along in waters along the U.S. Gulf Coast from Texas to Florida.

Is it safe to eat seafood during a red tide?

This question is not easily resolved with a simple “yes” or “no” answer. Seafood susceptible to red tide toxins includes mussels, oysters, clams, scallops, crabs, lobsters, anchovies and sardines.19

The Florida Fish and Wildlife Conservation Commission offers this guidance regarding eating locally sold or harvested fish during a red tide outbreak:

Store-bought and restaurant-served shellfish are safe to eat during a bloom because the shellfish are monitored by the government for safety. Commercially available shellfish are often not locally harvested and, if harvested locally, are tested for red tide toxins before they are sold.20

Shellfish, including clams, oysters, and mussels can accumulate brevetoxins. Brevetoxins have no taste, smell, or color, and can’t be destroyed by cooking. If contaminated shellfish are eaten, people can become ill with Neurotoxic Shellfish Poisoning (NSP). Shellfish harvesting from regulated areas is banned during blooms of K. brevis. Fish are safe to eat as long as they are caught alive and only the muscle is eaten. The muscle of crustaceans, including crab, shrimp, and lobster, is not affected by red tide toxins and can be eaten.21

Also, recreational harvesting of shellfish is banned during a red tide. A complete list of red tide-related safety guidelines can be found on the Florida Fish and WIldlife Conservation Commission website.

What is the current state of our lakes?

In 2012, the EPA and its partners conducted the National Lakes Assessment. The survey evaluated a wide variety of waters, from small ponds to large reservoirs.

Key findings included:

  • Nutrient pollution: 35% of lakes have excess nitrogen, 40% have excess phosphorus
  • Microcystins: 39% of lakes had microcystin (a 9.5% increase since a 2007 study), but less than 1% contained “moderate” or “high” concern microcystin levels established by the World Health Organization
  • Cyanobacteria showed an 8.3% increase in the density of cyanobacterial cells since 2007
  • Atrazine: the herbicide was detected in 30% of lakes, but less than 1% had concentrations reaching the EPA level of concern for plants in freshwater

You can read more about the 2012 study at the EPA website.

Where do algal blooms occur?

Algae blooms are found in every U.S. state. They are not limited to the U.S., though. Algae blooms are a worldwide problem. Red tide is an annual occurrence along the U.S. Gulf Coast and many other coastal locations around the world.

Are algal blooms harmful to humans and pets?

Most algal blooms are harmless to humans and pets. In fact, less than one percent actually produce toxins.22 Blooms can deplete the water of oxygen, which in turn can damage fish populations—and the businesses that depend on them.  

Do cyanotoxins cause fatalities?

Toxins associated with algal blooms have caused fatalities in pets, livestock and fish. However, no human deaths in the United States have ever been reported due to cyanotoxins.23

The number of people that become ill each year due to exposure to harmful algal blooms is not tracked on a national level. The Centers for Disease Control’s (CDC) One Health Harmful Algal Bloom System (OHHABS) became operational in 2016 and is used by health agencies to report algal bloom-reported illnesses and exposures, and environmental data related to harmful algal blooms.

How are humans and pets exposed to algal bloom toxins?

Humans can be exposed to cyanotoxins and microcystins by:

  • Eating contaminated shellfish24
  • Skin contact
  • Swallowing contaminated water
  • Inhaling airborne water droplets25

Similarly, water-loving pets can risk exposure by :

  • Swallowing water while swimming or retrieving toys
  • Licking contaminated fur or hair
  • Contact with skin26
  • Eating dried bloom crusts or mats27

What are the symptoms of harmful algal bloom exposure?

Symptoms of harmful algal bloom exposure may take hours or days to appear, but generally appear within a week of exposure. The Utah Department of Health offers a detailed list of human symptoms:


Via swallowing:

  • Vomiting
  • Diarrhea
  • Stomach pain
  • Weakness
  • Headache
  • Tingling sensation
  • Muscle cramps
  • Dizziness
  • Trouble breathing

Via skin contact with water:

  • Eye irritation
  • Rash
  • Hives
  • Blisters or sores

By breathing droplets of contaminated water:

  • Nose irritation
  • Sore throat
  • Coughing or wheezing
  • Asthma-like symptoms
  • Respiratory symptoms (including difficult, rapid or shallow breathing)28


Pets can be very vulnerable to bloom toxins due to their small size, so precautions should be taken to keep them from swallowing, swimming or playing in potentially contaminated water.

Harmful algal bloom exposure symptoms for pets can include:

  • Vomiting
  • Diarrhea
  • Rash
  • Difficulty breathing
  • General weakness
  • Liver failure
  • Seizures29

What treatment options are available?

There are no specific antidotes for cyanobacterial toxins. Generally the only treatment is supportive care to treat individual symptoms.30 Children, people with weakened immune systems and pets are most vulnerable to illness.

If you suspect that you have been exposed to a harmful algal bloom, immediately leave the water and thoroughly wash skin, hair and clothes. Wash your pet thoroughly so they do not risk further exposure while grooming their fur. Promptly consult your healthcare provider, local health department or veterinarian for treatment advice.

Can you tell if a bloom is toxic by looking at it?

It is impossible to tell if a bloom is toxic by merely looking at it. Toxin-contaminated water may or may not have an unusual taste or odor to indicate that it is unsafe to ingest.

What are the economic effects of algal blooms?

It is difficult to accurately assign costs to specific effects of algal bloom outbreaks, but the National Ocean Service estimates that the annual economic losses caused by algal blooms are about $82 million a year.31

Algal blooms can affect the economy via:  

  • Reducing tourism due to discolored or smelly water and unsightly fish kills
  • Healthcare costs related to treating humans and animals
  • Closing of beaches and recreational waters
  • Decreased fishing
  • Costs associated with algal bloom monitoring and management

For example, a single lake in Ohio lost as much as $34-47 million due to lost tourism over a two-year period.32 A 1997 outbreak in the tributaries of Chesapeake Bay caused approximately $43 million in economic losses for watermen, seafood dealers, and seafood restaurants.33

Can harmful algal blooms contaminate public water supplies?

Yes. Public water suppliers use a variety of processes to monitor and remove or reduce cyanotoxins, but it can be a difficult and expensive undertaking. Considering that toxins are often released after cells die (or their membranes are ruptured) and larger blooms can clog treatment plant filters, it’s easy to see why removing cyanotoxins from drinking water on a large scale is such a tremendous challenge.


Ohio residents experienced a bloom-related water crisis in August 2014 when a large harmful algal bloom lingered near a water intake on Lake Erie, Toledo’s primary water source. The bloom’s powerful toxins shut down Toledo’s drinking water supply for over two days, forcing the city to issue “Do Not Drink” and “Do Not Boil” advisories for its 400,000+ residents.34

To combat the dangers posed by the blooms, improvements to Toledo’s water system included:

  • Installing sensors that can measure factors such as water temperature, pH, and chlorophyll levels
  • Treating water with activated carbon prior to arriving at the treatment plant
  • Adding a new disinfecting facility to treat water before it gets to the finishing reservoir35

Even though there is still much to be learned about the causes and health effects of harmful algal blooms, a 2017 study by researchers at Bowling Green State University indicates that a virus might have played a part in Toledo’s 2014 water crisis. The BGSU team studied the 2014 Lake Erie algal bloom and found that:

…it was consistent with algal blooms from 2012 and 2013 except for one thing—the 2014 Microcystis cells had a viral infection. Typically, toxins from algal blooms are trapped within the cell until the cell dies. But virus infections can cause cells to break open, leaking the toxin into the water and subsequently into water facility intake pipes and treatment centers. The viruses analyzed in this study infect only bacteria and do not infect humans.”36


In June 2018, drinking water advisories were issued to more than 150,000 Salem, Oregon residents when a large algal bloom infected Detroit Lake, the city’s primary water supply located on the Santiam River.

What’s being done about harmful algae blooms?

In an effort to limit the growth and effects of harmful algae blooms, the CDC lists five measures are being taken by government agencies and algal bloom researchers across the country:

  • Monitoring for algal blooms and associated toxins in recreational water, drinking water, and fresh and marine water fish or shellfish
  • Reducing nutrient levels in water
  • Surveillance for harmful algal bloom-associated illnesses and events
  • Collaborating among agencies to better coordinate activities within and across states and nationally
  • Engaging with citizen scientists and other partners to monitor for harmful algae blooms37


Lake Erie has been affected by massive harmful algal blooms since the late 1990s.38Its largest algal blooms were recorded in 2011 and 2015.39The Great Lakes Observing System monitors a network of water sensors on Lake Erie, and you can explore a map with their latest data readings here. The National Weather Service’s Cleveland office has a special Lake Erie Harmful Algal Bloom page which features this year’s forecast, historical Lake Erie bloom severity data and links to other interesting algae bloom information. NOAA’s “Tides and Currents” website  provides cyanobacterial forecasts for Lake Erie from July through October.

NOAA’s Harmful Algal BloomS Observing System (HABSOS) provides a detailed map of algal bloom activity in the Gulf of Mexico. According to the HABSOS website, “HABSOS is a data collection and distribution system for harmful algal bloom (HAB) information in the Gulf of Mexico. The goal of HABSOS is to provide environmental managers, scientists, and the public with a data driven resource for HAB events.”40

The Center for Operational Oceanographic Products and Services (CO-OPS) provides forecasts for areas frequently affected by harmful algal blooms, including Florida, the Gulf of Mexico and Lake Erie.

The CDC’s One Health Harmful Algal Bloom System (OHHABS) is a voluntary reporting system available to public health departments that collects data on human and animal algal bloom exposure-related illnesses.41

What precautions can you take?

As stated earlier, there is no way to tell if an algal bloom is toxic by looking at it. To protect yourself and your pets, it is advisable to:

  • Check for beach warnings posted near the water or online
  • Avoid water that might be contaminated—look for the presence of mats/crusts, foul odor or color, or dead fish that have washed ashore
  • Follow guidance regarding eating fish and shellfish
  • Promptly seek medical care if you suspect you or your pet have been exposed to a harmful algal bloom

Remember, boiling water does not remove algal toxins—it can actually concentrate the amount of toxin in the water. Also, toxins may remain in water for some time after the bloom has gone away.

It’s also important to note that bloom toxins in drinking water are not subject to federal regulations. According to the CDC, “there are currently no federal regulations that determine the maximum number of cyanobacteria or the maximum concentration of cyanotoxins that are safe for public drinking water.”42

In 2015, the EPA issued health advisories for algae bloom toxins in drinking water:43

This is a table depicting EPA-issued health advisories for algae bloom toxins in drinking water. For more information, visit the link following this table.

Minnesota, Ohio, Oregon and Vermont have implemented their own guidelines for drinking water. In 1998, the World Health Organization (WHO) developed a provisional drinking water guideline for microcystin-LR of 1µg/L.44

Health advisories are, per the EPA, “non-regulatory values that serve as informal technical guidance to assist federal, state and local officials, and managers of public or community water systems to protect public health from contaminants.”45 The EPA also notes that health advisories are “not legally enforceable federal standards and are subject to change as new information becomes available.”46

To protect people enjoying our country’s recreational waters, more than 20 states have developed response guidelines for large blooms. Actions could include posting warnings or even closing beaches and waterways, depending on the severity of the bloom.

Take control of your drinking water: use a final barrier system every day

It’s critical to make sure you have a reliable potable water source every day. Harmful algal blooms are invisible and can contaminate even the cleanest-looking lakes or other bodies of recreational water.

Remember: boiling cyanotoxin-contaminated water can concentrate the toxins and make contaminated water even more harmful to drink. That’s why you need a reliable final barrier system that’s been independently tested to remove or dramatically reduce the levels of the toxic microcystin-LR and the common cyanobacteria microcystis in your drinking water.

Black Berkey® Purification Element Microcystin-LR and Microcystis Test Results

Drink with confidence—third-party lab tests show that Black Berkey® Purification Elements are extremely effective at removing microcystin toxins and microcystis algae.

Microcystin LR Toxin Filtration

Microcystin-LR is the most common microcystin variant found in bloom samples.47 Black Berkey® Purification Elements remove microcystin-LR to greater than 99.7% (>log 2.5), reaching the test’s reporting limit with no toxin detected.

For more information, visit the Berkey® Knowledge Base.

Microcystis Algae Filtration

Planktonic cyanobacteria can produce harmful cyanotoxins. Microcystis is the most common bloom-forming genus, and is almost always toxic.48 Black Berkey® Purification Elements remove 99.97% of microcystis algae (log 3.5).

For more information, visit the Berkey® Knowledge Base.


Make sure you’re prepared for your next outdoor adventure. Berkey® gravity-fed water purification systems equipped with Black Berkey® Purification Elements are capable of treating raw, questionable sources during emergencies or extreme off-grid scenarios— yet are also enjoyed every day. Black Berkey® Purification Elements have been proven to remove greater than 99.999% of viruses and greater than 99.9999% of pathogenic bacteria, while also removing or dramatically reducing trihalomethanes, inorganic minerals, heavy metals, pharmaceuticals, pesticides, VOCs, petroleum products, perfluorinated chemicals, rust, silt, sediment and even radiologicals. You can enjoy clean, delicious purified water that retains the healthy minerals your body needs for less than 2 cents per gallon of purified water. A pair of Black Berkey® Purification Elements lasts up to 6,000 gallons, or approximately five years with typical use.


1What are Aquatic Plants and Algae Nutrient Pollution – Harmful Algal Blooms
3National Ocean Service: What Is a Red Tide?
4Introduction to the Cyanobacteria Cyanobacteria and Cyanotoxins: Information for Drinking Water Systems
6Guidelines for Canadian Recreational Water Quality Learn about Cyanobacteria and Cyanotoxins
8Cyanotoxin Fact Page
9National Ocean Service: Why Do Harmful Algal Blooms Occur?
10What Causes Algal Blooms? Nutrient Pollution – The Issue
12EPA Infographic – Nutrient Pollution Explained The Challenge of Tracking Nutrient Pollution 2,300 Miles
14Red Tide FAQ
15Florida Dept. of Health in Lee County – Red Tide Information
16Florida Fish and Wildlife Conservation Commission – Red Tide FAQ
17Microcystins – a Brief Overview of Their Toxicity and Effects, With Special Reference to Fish, Wildlife and Livestock
18Texas Parks & Wildlife – Red Tide FAQ
19Red Tide FAQ
20Florida Fish and Wildlife Conservation Commission Red Tide FAQ
21Florida Fish and Wildlife Conservation Commission Karenia Brevis Fact Sheet
22National Ocean Service: Are All Algal Blooms Harmful? Harmful Algal Bloom (HAB)-Associated Illness
24National Ocean Service: Why Do Harmful Algal Blooms Occur?
25Minnesota Department of Health – Causes and Symptoms of Harmful Algal Bloom-Related Illness
26Minnesota Department of Health – Harmful Algal Bloom-Related Illness in Animals
27Microcystins – a Brief Overview of Their Toxicity and Effects, With Special Reference to Fish, Wildlife and Livestock
28Utah Department of Health – Harmful Algal Blooms Health Information and Symptoms
29Minnesota Department of Health – Harmful Algal Bloom-Related Illness in Animals Physician Reference
31National Ocean Service: Why Do Harmful Algal Blooms Occur? Harmful Algal Bloom (HAB)-Associated Illness
33Harmful Algae – Economic Impacts
34Harmful Cyanobacteria and Algae Blooms: Human Dimensions
35WTOL – Toledo’s Water Plant Continues Fight Against Harmful Algae Blooms
36BGSU News: Study Shows Toledo Water Crisis May Be Linked to Virus Infection Harmful Algal Bloom (HAB)-Associated Illness
38National Science Fountation: Lake Erie’s Toxic Algae Blooms: Why is the Water Turning Green?
39Cleveland, OH National Weather Service Office: Lake Erie Harmful Algal Bloom
40NOAA Harmful Algal BloomS Observing System (HABSOS) One Health Harmful Algal Bloom System (OHHABS) Harmful Algal Bloom (HAB)-Associated Illness
43EPA: Drinking Water Health Advisories for Cyanotoxins
44Microcystin and Other Algal Toxin Guidelines
45EPA: 2015 Drinking Water Health Advisories for Two Cyanobacterial Toxins
46EPA: 2015 Drinking Water Health Advisories for Two Cyanobacterial Toxins
47Guidelines for Canadian Recreational Water Quality
48World Health Organization