Harmful Algal Blooms - Frequently Asked Questions


What is an algal bloom?

An algal bloom occurs when the numbers of algal cells increase rapidly to reach concentrations usually high enough to be visible to the naked eye. Many types of algae form blooms. Not all algal blooms are toxic. Some, such as the blooms of diatoms in the early spring, are very important to the health of the ecosystem.

What is a HAB?

HAB stands for Harmful Algal Bloom. There are many species of single-celled organisms living in the oceans, including algae and dinoflagellates. When certain conditions are present, such as high nutrient or light levels, these organisms can reproduce rapidly. This dense population of algae is called a bloom. Some of these blooms are harmless, but when the blooming organisms contain toxins, other noxious chemicals, or pathogens it is known as a harmful algal bloom, or HAB. HABs can cause the death of nearby fish and foul up nearby coastlines, and produce harmful conditions to marine life as well as humans.

What are Cyanobacteria (blue-green algae)?

Blue-green algae is the common name for several different types of algae. They are actually bacteria (Cyanobacteria) which are able to photosynthesize, hence the green color. Cyanobacteria are bacteria that grow in water and are photosynthetic (use sunlight to create food and support life). Cyanobacteria live in terrestrial, fresh, brackish, or marine water. They usually are too small to be seen, but sometimes can form visible colonies. Cyanobacteria have been found among the oldest fossils on earth and are one of the largest groups of bacteria. Cyanobacteria have been linked to human and animal illnesses around the world, including North and South America, Africa, Australia, Europe, Scandinavia, and China. Cyanobacteria are the most common, but not the only, group of algae to come from HABs.

What does a cyanobacterial bloom look like?

Some cyanobacterial blooms can look like foam, scum, or mats on the surface of fresh water lakes and ponds. The blooms can be blue, bright green, brown, or red and may look like paint floating on the water. Some blooms may not affect the appearance of the water. As algae in a cyanobacterial bloom die, the water may smell bad.

How do I know if water contains blue-green algae?

If you detect an earthy or musty smell, taste or see surface scums of green, yellow or blue-green the water may contain blue-green algae. Only examination of a water sample under the microscope will confirm the presence of blue-green algae.

How can I test for cyanobacterial toxins?

Most of the toxins require specialized testing that can weeks to perform. Some kits are available to test for microcystins on site.


What causes an algal bloom?

There is no single factor which cause an algal bloom. A combination of optimum factors such as the presence of good nutrients, warm temperatures and lots of light all encourage the natural increase in numbers of blue-green algae in our waterways. Nature mostly takes care of the temperature and light, but the increased presence of nutrients such as phosphorous is largely due to poor farming practices such as high use of fertilizers and presence of livestock near water supplies, as well as effluent and run-off from towns and cities near waterways. The ponding of water and reducing river flow rates tends to improve the light and sometimes the nutrient environment for algal growth making water turbulence a major factor in bloom development. Pesticides and other chemicals may affect the natural grazers which would otherwise control algal growth and their presence increases the risk of blooms.

How do cyanobacterial blooms form?

Cyanobacterial blooms occur when algae that are normally present grow exuberantly. Within a few days, a bloom can cause clear water to become cloudy. The blooms usually float to the surface and can be many inches thick, especially near the shoreline. Cyanobacterial blooms can form in warm, slow-moving waters that are rich in nutrients such as fertilizer runoff or septic tank overflows. Blooms can occur at any time, but most often occur in late summer or early fall. They can occur in marine, estuarine, and fresh waters, but the blooms of greatest concern are the ones that occur in fresh water, such as drinking water reservoirs or recreational waters.

I've heard zebra mussels are causing the blooms. How does that work?

Zebra mussels have been implicates as a factor promoting the formation of harmful algal blooms in the Great Lakes region, particularly for low-phosphorus inland lakes. By removing natural competitors (green algae) and/or altering the chemical composition of the water, zebra mussels may promote HABs. Zebra mussels have been shown to be capable of selecting which algae they consume -- spitting out presumably toxic forms such as Microcystis. By filtering the water, zebra mussels also increase the amount of light reaching the bottom of the lake, which promotes the growth of large benthic forms of algae such as Ciadophora which may break free during storms or due to wave action to form floating mats or wash up on the beaches.

Can you get a blue-green algal bloom in winter?

Yes, however, this is less likely than in summer. Algal blooms can occur at any time of year as long as conditions such as temperature and nutrients are right for growing.


What are the dangers of Harmful Algal Blooms?

  • They spoil water quality when present in large numbers by producing odors or thick scums.
  • They can make drinking water smell and taste bad.
  • They can make recreational areas unpleasant.
  • Dense blooms can block sunlight killing other plants and animals.
  • When algae decompose they may use up oxygen in the water and cause fish kills.
  • Some cyanobacteria can produce toxins that are among the most powerful natural poisons know. These toxins have no know antidotes. The toxins are poisonous to humans and may be deadly to livestock and pets.
  • CyanoHABs can make people, their pets, and other animals sick. Often, the first sign that a HAB exists is a sick dog that has been swimming in a algae-filled pond. Children are at higher risk than adults for illness from CyanoHABs because they weigh less and can get a relatively larger dose of toxins.

Are all blue-green algae poisonous?

No. There are many species of blue-green algae. Some are not known to have any toxins, others have one or more different types of toxin. Species known to be toxic may only be toxic at certain times and places within the bloom. Blue-green algae are a natural part of all waterways. Under certain conditions some blue-green algae multiply to bloom levels and may produce toxins which are dangerous to livestock and humans.

What species of cyanobacteria form harmful algal blooms in fresh water?

The most common HABs in the Great Lakes region are:
  • Microcystis aeruginosa
  • Anabaena circinalis
  • Anabaena flos-aquae
  • Aphanizomenon flos-aquae
  • Cylindrospermopsis raciborskii

What types of illnesses can people and animals get from exposure to HABs?

  • Getting it on the skin may give people a rash, hives, or skin blisters (especially on the lips and under swimsuits).
  • Inhaling water droplets from irrigation or water-related recreational activities can cause runny eyes and nose, a sore throat, asthma-like symptoms, or allergic reactions.
  • Swallowing water that has toxins in it can cause:
    • Acute, severe gastroenteritis (including diarrhea and vomiting).
    • Liver toxicity (i.e., increased serum levels of liver enzymes). Symptoms of liver poisoning may take hours or days to show up in people or animals. Symptoms include abdominal pain, diarrhea and vomiting.
    • Neurotoxicity. These symptoms can appear within minutes after exposure. In dogs, the neurotoxins can cause salivation and other neurological symptoms, including weakness, staggering, difficulty breathing, convulsions, and death. People may have numb lips, tingling fingers and toes, or they may feel dizzy.
    • Paralytic shellfish poisoning (PSP). PSP is caused by consumption of shellfish (e.g., mussels and clams) which bioaccumulate a toxin produced by dinoflagellates (red tide). Dinoflagellates similar to those responsible for PSP are occasionally found in the Great Lakes, but dangerous levels of PSP toxin have not been observed there.

How could you be exposed to HABs and toxins?

  • Drinking water that comes from a lake or reservoir with a HAB.
  • Drinking untreated water.
  • Engaging in recreational activities in waters with HABs.
  • Inhaling aerosols from water-related activities such as jet skiing or boating.
  • Inhaling aerosols when watering lawns, irrigating golf courses, etc., with pond water.
  • Using cyanobacteria-based dietary supplements that are contaminated with microcystins.
  • Consuming contaminated fish or shellfish (see safety precautions below).

Health and Safety

How can you protect yourself, your family, and your pets from exposure to HABs?

  • Don't swim, water ski, or boat in areas where the water is discolored or where you see foam, scum, or mats of algae on the water.
  • If you do swim in water that might have a HAB, rinse off with fresh water as soon as possible.
  • Don't let pets or livestock swim in or drink from areas where the water is discolored or where you can see foam, scum, or mats of algae in the water.
  • If pets (especially dogs) swim in scummy water, rinse them off immediately - do not let them lick the algae (and toxins) off their fur.
  • Don't irrigate lawns or golf courses with pond water that looks scummy or smells bad.
  • Report any musty smell or taste in your drinking water to your local water utility.
  • Respect any water body closures announced by local public health authorities.

Is it safe for livestock to drink water with blue-green algae in it?

No. An alternative safe drinking supply must be found until the water supply is declared safe. Most livestock will prefer not to drink the water if an alternative supply is available.

Are all animals affected the same?

Most important is the amount of exposure to the toxins. Some animals are particularly sensitive. Dogs have sensitive noses and lick their fur to clean themselves, possibly taking in concentrated algae. Fish and water birds appear to be little affected. Most livestock will avoid contaminated water, if possible, but where they are forced to drink through scum (for example where a fence forces them to the leeward side of a dam) they may die.

Can livestock pick up toxins from irrigated pasture?

Yes. Some of the blue-green algal toxins will remain toxic in dry form. Continued application of heavily affected waters (say from dairy waste recycling dams) can lead to significant toxin build up on foliage. Although this is a rare occurrence requiring special circumstances, this residue can affect livestock.

Is blue-green algae affected water safe to drink after it has been boiled or filtered?

NO. The water needs to be filtered through activated carbon to remove any toxins. Toxins will not be removed by boiling, normal water filter systems or adding household disinfectant.

Can I cook with water with blue-green algae in it?

NO. Remember boiling does not remove toxins from the water.

Can I wash clothes and dishes in water with blue-green algae in it?

Where possible use alternative water supplies. If you are unable to find another water source, take the following precautions:
  • Use rubber gloves when handling wet washing or dishes.
  • Rinse dishes with uncontaminated water.
  • Remove surplus water with a dish towel.
  • If possible, give the laundry a final rinse with uncontaminated water. Sun dry the clothes and air them for a few days.

Can I water fruit and vegetables with contaminated water?

Yes. These do not appear to take up the toxins. However, avoid fruit and vegetables coming in contact with the contaminated water. Make sure you wash the fruit and vegetables in clean water before eating.

Can I eat fish or shellfish caught in water with blue-green algae in it?

You should not eat shellfish as they can concentrate toxins. The liver and gut of fish are also likely to be toxic. Other parts of the fish may be eaten but they must be well cleaned. Further studies need to be done on the build-up of toxins in fish.

Is it safe to swim in water with blue-green algae in it?

Generally no. This will depend on the numbers and type(s) of algae present. At even low levels of some blue-green algae people and animals with sensitive skin may show some allergic reactions to the toxins present in the water. Not all people are sensitive to blue-green algae allergens and for those who are the effect increases with increased exposure. The more concentrated the algae and the longer people remain in the water the more severe the symptoms.

Is it safe to boat or canoe in water where there is an algal bloom?

Safety will depend on the level of the blue-green algae. Always avoid skin contact with the water. Not all people are sensitive to blue-green allergens and for those who are, the effect increases with increased exposure. The more concentrated the algae and the longer people remain in the water the more severe the symptoms.

Will wearing a wetsuit protect me from algal toxins?

No. In fact wetsuits may concentrate algae at the collar and cuff areas and rub cells against the skin. This may cause a particularly strong skin reaction at the points where water enters the suit. Be careful to rinse algae off the suit with fresh water if you have used it where algae concentrations are high.

Will blue-green algae affect my irrigated crops?

Not directly in relation to toxins. The crops are not known to take up the toxins. However, blue-green algae are fast growing and can shade and foul rice crops if they occur in bays before the crop established a cover above water level. This can cause significant yield loss

When blue-green algae is present should I pump water from a greater depth?

Water in deep dams may form layers of different temperature. If there is no alternative water supply using water form areas not covered by scums or from deeper (notably cooler) layers in dams MAY reduce algal exposure and risks.

Is the water safe once it appears to no longer have a bloom on it?

No. Blue-green algal cells lave gas bubbles in them which affect how they float. It is common for blooms to rise to the surface in calm light conditions and sink down at other times becoming much less evident from the bank. When cells die and break up it can take days for nerve toxins to disintegrate and weeks for liver toxins to disappear. It is best to wait a couple of weeks after a scum forming bloom has gone before using the water

How to treat people or animals that have been exposed to cyanobacterial toxins?

Get medical treatment right away if you think you, your pet, or your livestock might have been poisoned by cyanobacterial toxins. Remove people from exposure and give them supportive treatment.


How can I prevent an algal bloom?

Algae need three things for optimal growth: light, nutrients and high temperatures. Lowering the nutrients, light and temperature available to the blue-green algae in the water supply will help reduce algal growth. The speed at which water is flowing and mixing is important in controlling light and nutrient availability to algal cells. Keeping livestock away from the farm dam or water supply; avoiding run-off into water supply from fertilizers and pesticides; taking some water treatment measures BEFORE bloom starts; and if practical - changing mixing patterns or covering the dam or water supply to screen out light may help.

How can I reduce the occurrence of HABs?

Reduce nutrient loading of local ponds and lakes by using only the recommended amounts of fertilizers and pesticides on your yard. Properly maintain your household septic system. Maintain a buffer of natural vegetation around ponds and lakes to filter incoming water.

Can I use chemicals to treat water with blue-green algae in it?

Most chemicals work to PREVENT an algal bloom. Water in small dams can be protected from blue-green algae by dosing with gypsum and alum. These chemicals work by removing phosphorus from the water. Algaecides can be used to safeguard water for agricultural use in farm dams BEFORE algal blooms occur. However, if used to treat a bloom, the algaecide may cause a release of toxins into the water when it destroys the algae. Algaecides may be toxic to organisms that naturally control algal blooms or, if not correctly applied, to livestock and humans. Before using algaecides seek advice from the relevant authority.

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