FAQs on Cyanobacteria

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Cyanobacteria, cyanotoxins and exposure

What are cyanobacteria? What are blooms?

Cyanobacteria (formerly called blue-green algae) are photosynthetic bacteria that naturally occur in lakes, ponds, and rivers around the world. A cyanobacteria “bloom” (sometimes referred to as a harmful algal bloom or CyanoHAB) is the name given to the excessive growth of cyanobacteria. Blooms can produce one or more types of toxins (cyanotoxins).

Cyanobacteria blooms can occur at any time of year, even under ice in the winter, but are most common in the summer and fall, when there is the right combination of sunlight, warm water and excess nutrients. NHDES recommends performing a self-risk assessment by looking for discoloration or unusual growth prior to recreating or letting your pets in the water. If you see something unusual, take pictures and report it. Remember, when in doubt, stay out!

How are cyanobacteria blooms harmful?

Cyanobacteria sometimes produce toxins that can harm people, pets, livestock and wildlife. When cyanobacteria multiply to form a bloom, they can produce enough toxins to affect human and animal health. Exposure occurs through ingestion, inhalation or skin contact. Symptoms of exposure to cyanobacteria vary but can include skin irritation, stomach cramps, vomiting, nausea, diarrhea, fever, sore throat, headache, muscle and joint pain, mouth blisters, and acute liver damage. Long-term exposure can harm the liver and central nervous system. Cyanobacteria blooms can be fatal to dogs that drink the water or lick themselves after swimming.

How can I stay safe/protect myself?

Protect yourself with a simple, common-sense approach to water use. Perform a self-risk assessment, looking for discoloration or unusual growth prior to recreating. If you see something unusual, take pictures and report it. Do not drink the water or use it for household chores if you suspect a bloom. Do not allow pets or livestock to come in contact with cyanobacteria and rinse them well if they do. Use the Healthy Swimming Mapper to check if the waterbody you are visiting or live on has a current Warning, and stay out of the water if it does.

I think my child, my pet, or I was exposed. What do I do?

Immediately wash with clean water. Monitor symptoms and seek medical attention if necessary. Tell the doctor or vet that you had a potential exposure to cyanobacteria and when it occurred. Consult the CDC’s resources for health care providers and veterinarians. Please also report your exposure to NHDES to help us track waterborne illnesses.

Symptoms of toxic cyanobacteria exposure vary depending on the type and duration of exposure. Exposure occurs through ingestion, inhalation or skin contact. Symptoms can include itching or other skin irritation, stomach cramps, vomiting, nausea, diarrhea, fever, sore throat, headache, or muscle or joint pain, and acute liver damage. The effects of chronic exposure include liver disease and neurological symptoms. Cyanobacteria can be fatal for dogs that drink the water or lick themselves after swimming. 

Can I drink the water and use it for household needs when there is an alert or warning (advisory)?

Do not drink water from a waterbody with an active cyanobacteria warning or if you suspect a cyanobacteria bloom. NHDES also recommends against using it for dish washing, bathing and other household uses, as toxins may be present. Boiling the water does not remove cyanotoxins and may increase toxin levels. Most in-home water treatment filters and purifiers do not remove most cyanotoxins from drinking water. In general, NHDES does not recommend using lakes or streams as a domestic water source.

Are cyanobacteria blooms getting worse in New Hampshire?

Cyanobacteria blooms are becoming more common in New Hampshire, due to increased nutrient availability, warming water temperatures and changing rain patterns. Stormwater runoff due to development and land-use changes in the watersheds around a waterbody is usually the largest source of the excess nutrients that drive cyanobacteria blooms. Unmaintained septic systems can also cause nutrient pollution.

It is important to note that the Cyanobacteria Harmful Algal Bloom Program at NHDES is a response-based program. This means that NHDES only collects samples when potential blooms are reported to us. The increased number of bloom reports in New Hampshire is, in part, a result of increasing public awareness and documentation of blooms.

NHDES cyanobacteria warnings and alerts

What is a cyanobacteria warning? Is an alert different?

Warnings are issued lakewide when the cyanobacteria cell density exceeds the recreational health threshold of 70,000 cyanobacteria cells/mL. This density is associated with the level at which toxins may be present at a concentration that threatens public health. When there is a warning, NHDES advises lake users to avoid contact with the water. Pets and livestock should also be kept out of the water. When NHDES issues a warning, lakes are resampled weekly until the bloom subsides. NHDES issues warnings from May 15 through October 15.

Alerts are issued 1) based on a photo submitted before NHDES can analyze a sample and a bloom that may be health risk is evident; 2) when the cyanobacteria cell density is approaching but does not yet exceed 70,000 cells/mL; or 3) if a bloom is reported, but conditions may have changed. Alerts are intended to serve as statements to be on the watch for a potential cyanobacteria bloom. An alert may develop into a warning but otherwise remain active for a week. Resampling only occurs if further bloom reports are submitted. Alerts are issued year-round as needed. Waterbody users should avoid contact with bloom material and keep pets and livestock out of the water.

All active cyanobacteria warnings and alerts in New Hampshire are posted on the Healthy Swimming Mapper. People can also sign up for a weekly statewide cyanobacteria email update, or to receive waterbody-specific notices of blooms. 

Is it safe to go in the water during a warning or an alert?

NHDES recommends staying out of the water in any part of a waterbody when a cyanobacteria warning is posted, though we do not close waterbodies. This includes swimming, wading and any activity that may result in direct contact with the water. NHDES also recommends against using the water for drinking even when boiled or filtered, dish washing, bathing and other household uses when a warning is in place. Children and pets are particularly vulnerable to cyanotoxins.

If you choose to recreate during a warning, know that you are at risk of exposure to cyanotoxins. If there are enough cyanobacteria in a waterbody to cause a warning, then there is the potential for negative health effects for you, your children and your pets. If you do choose to recreate during a warning, the risk is highest in areas with visible cloudiness, flecks or colored scum, and lower where the water looks clear. Remember that bloom conditions can change rapidly.  

If there is an alert, we suggest taking extra care to visually evaluate the water prior to recreating. Stay out if you see something suspicious and report it.

NHDES encourages everyone to assess their risk before swimming or letting children and pets play in the water, regardless of cyanobacteria warning or alert status.

When will you resample? How are warnings removed?

During the swimming season, waterbodies with warnings are retested weekly. If an alert is issued, resampling only occurs if further bloom reports are received.

Warnings last until the bloom has passed, as indicated by the cyanobacteria cell density falling well below the recreational health threshold of 70,000 cells/mL. When the cell density remains around the recreational threshold blooms often quickly reach the threshold again, making it justified to keep the warning in place.

The cyanobacteria cell density also needs to decline below the recreational health threshold under representative conditions. If it is raining or very windy during resampling the cyanobacteria become physically mixed into the water column, preventing us from determining if the bloom is still present at unsafe levels. NHDES will wait to remove a warning until resampling under representative conditions shows the bloom has fully passed.

How long does a cyanobacteria warning last?

On average warnings last around a month, but each waterbody and each bloom event is different. Warnings can be as short as a week or as long as several months. Warnings are kept in place until the cyanobacteria cell density falls well below the threshold of 70,000 cells/mL. NHDES resamples weekly at waterbodies with warnings from May 15 to October 15 until bloom conditions pass. Following October 15, resampling is performed biweekly. For any waterbody in New Hampshire, you can see its status and most recent sampling date on the Healthy Swimming Mapper or join the email list for a specific waterbody.

Why don’t you publish the exact location of the bloom? Why are warnings issued lakewide?

Due to the nature of cyanobacteria, it can be difficult to track a bloom. Cyanobacteria grow lakewide, even in the deepest parts of the lake. Blooms move around a waterbody based on the wind, weather, waves, lake morphology and boat traffic. In addition, not all areas of a lake are equally used, meaning that blooms could be present but not reported at a specific location. Cyanobacteria blooms are dynamic events, and the risk cannot be characterized perfectly in real time. To best protect public health, NHDES issues lake wide warnings/alerts. NHDES recommends that people and pets stay out of the entire waterbody and avoid using the water for household use when a warning is in place.

Can you test more sites at my lake?

The NHDES Cyanobacteria HAB Program covers the whole state of New Hampshire, so we are only able to accommodate a limited number of samples from each lake during a bloom event. We communicate with locals to focus sampling where the bloom is most severe in order to be protective of public health. Occasionally, we collect additional samples around a waterbody but prioritize the most severe samples for analysis based on current program constraints.

How do I learn where there are warnings or alerts and get updates about a local lake?

Check out the Healthy Swimming Mapper to see all active cyanobacteria notices in the state, the last time a sample was taken, and the cyanobacteria history for a particular waterbody. The map includes both cyanobacteria warnings and alerts, as well as beach advisories issued for high levels of fecal bacteria. You can also sign up to be on a cyanobacteria email list for specific waterbodies. As part of this list, you will receive email updates when bloom warnings or alerts are issued and removed. You can also sign up to receive a weekly statewide cyanobacteria update, every Thursday of the summer.

What can be done about cyanobacteria?

What can I do about cyanobacteria blooms?

Cyanobacteria are naturally occurring and cannot be eradicated. However, there are ways to prevent bloom conditions. The best method for decreasing the likelihood of a cyanobacteria bloom is to reduce nutrient pollution into the lake. Most nutrient pollution is from the landscape around the lake and within the land area that drains to the lake (watershed). In particular, stormwater and erosion contribute a significant amount of the non-point source unnatural nutrient load that reaches our surface waters. Fertilizers and failing septic systems can also contribute nutrients to surface waters. Things you can do include:

  • Be aware of the Shoreland Protection Act and its restrictions on the use of fertilizers along the shoreline. No fertilizer, except limestone, may be used within 25 feet of the reference line (shoreline). Beyond that distance, NHDES recommends either avoiding fertilizer or using fertilizer without phosphorus. Do not exceed label application rates.
  • Properly maintain your household septic system and pick up after your pets.
  • Maintain a buffer of natural vegetation on the water edge to filter water before it enters the waterbody. 

NH LAKES has a free, voluntary, nonregulatory LakeSmart Program, where a qualified professional will come out to your property and talk through ways you could better protect lake health with the decisions you make on your property. Homeowners whose actions protect lake health are recognized with a LakeSmart award. The LakeSmart webpage also has a Resource Library with more information about actions homeowners can take to minimize nutrient runoff into the lake. NHDES’ Soak Up the Rain program is also available to assist homeowners with smaller, self- constructed projects to reduce stormwater runoff.

What is NHDES doing about cyanobacteria blooms?

NHDES created a statewide cyanobacteria strategic plan to prevent the increase of and help control cyanobacteria blooms in New Hampshire’s waterbodies. The plan included input from a 17-member advisory committee. The plan has four strategies that focus on nutrient reduction, education and outreach, monitoring, and addressing cyanobacteria in public drinking water supplies.

How can I learn more?

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