As of June 14th and June 15th NHDES has Issued a Cyanobacteria ADVISORY Removed 6/2/23

6/21/2023- We just got word from NH DES that all samples collected this morning from both lakes “Look Good”! They are lifting the Advisory as of this afternoon. These blooms can pop back up, so please report to the state, the office or a weedwatcher in your neighborhood if you see any new accumulations in the areas you recreate.

6/21/2023- Update: Scientists from the state are reviewing samples that were collects around both lakes this morning. We are waiting to hear back from them once they have completed processing all of the samples. We will notify our membership through email as soon as we have news from the state. You can always check for updates by checking the Healthy Swimming Mapper.

6/19/2023- Update: NH DES will be out to collect samples from both lakes on Wednesday- June 21 and recount cyanobacteria in their samples to see if the advisory will remain in effect or be lifted. We will send out an email blast when we learn those results. In the meantime, please continue to follow state recommendations about cyanobacteria blooms.

What is a Warning (Advisory)? 

Advisories are lake-wide warnings issued when cyanobacteria cell counts exceed the recreational health threshold of 70,000 cells/mL. Surface blooms can rapidly change and accumulate in various locations around a waterbody. Please continue to monitor shorelines for changing conditions. NHDES advises lake users to avoid contact with the water in areas experiencing blooms. Pets and livestock should also be kept out of the water.

When an advisory is issued, resampling is performed weekly until the bloom subsides. Advisories are issued from May 15 through October 15. Advisories are not based on toxin evaluation but occur at cyanobacteria cell count densities when toxin production may be likely and are intended as a precautionary measure for short term exposure to cyanotoxins. 

Blooms are dynamic. Always perform a self-risk assessment of the water, looking for discoloration or unusual growth prior to recreating. If you see cyanobacteria develop after an advisory has been closed, please report it

What is an Alert? 

Alerts are issued 1) based on a photo before NHDES can analyze a sample; 2) when the cyanobacteria density is approaching the recreational health threshold but does not yet exceed it; or 3) if a bloom was reported but may have passed by the time a sample was reviewed but could reoccur. Alerts are intended to serve as statements to be on the watch for a potential cyanobacteria bloom. Waterbody users should avoid contact with bloom material and keep pets and livestock out of the water. Sometimes alerts become advisories, and sometimes they pass. Alerts remain active for a week. Resampling only occurs if further bloom reports are submitted. Alerts are issued year-round as needed. 

For more information from NH Department of Environmental Services 

Report a CyanoHAB 

Healthy Swimming Mapper  

Frequently Asked Questions

Advisories issued this week on Lake Wentworth and Crescent Lake

A cyanobacteria WARNING (ADVISORY) has been issued for Crescent Lake, Wolfeboro. The bloom was first reported 14 June, appearing in aggregations that look like dense pollen. See the image below. Samples collected and reviewed on 15 June had cyanobacteria (Dolichospermum and Woronichinia) in concentrations up to 219,333 cells/mL in areas of highest observed accumulations. There was also VERY dense pollen in these samples, masking the presence of the cyanobacteria and making the scums appear yellow.


Image taken 14 June 2023.  NHDES will resample in a week.

A cyanobacteria WARNING (ADVISORY) has been issued for Wentworth, Wolfeboro. The bloom was first reported on 12 June, appearing as bright yellow scums that look like pollen accumulations along some shorelines. See the image below. Samples collected yesterday 6/13 had a high density of pollen and lower density degrading cyanobacteria. Samples collected today 6/14 had cyanobacteria (Dolichospermum) in concentrations up to 86,300 cells/mL in areas of highest observed accumulations. The high density of pollen is visually hiding the presence of cyanobacteria in the water.

Image taken 12 June 2023.  NHDES will resample in a week.

This is a microscopic image of Dolichospermum

The information below is from Woods Hole Oceanographic Institution


Microcystis spp. are one of the most common groups of harmful cyanobacteria impacting New England freshwater bodies.  Cells form round colonies that can move vertically in the water column by use of internal gas vesicles.  Certain species in this genus produce toxins known as microcystins as well as saxitoxins, which are also produced by other freshwater cyanobacteria species, as seen below.  These toxins enter the surrounding waters when cells are ruptured, which can occur naturally at the end of a bloom, or artificially during recreational activities.  When consumed via drinking or eating contaminated food, cyanotoxins enter the body through the intestinal tract and affect the liver, brain, and other tissues, causing symptoms of headache, sore throat, vomiting, stomach pain, diarrhea, and blistering around the mouth.  Symptoms of dermal contact and inhalation include skin irritation, nasal irritation, and asthma.  Livestock and pets are susceptible to the same health problems, and stories of dogs dying after swimming and then licking their fur are not uncommon.

Microcystis is a naturally occurring freshwater organism, but can bloom in extraordinary densities during periods of warm temperatures and nutrient-rich conditions.  A warming climate increases risk for harmful blooms, as does nutrient loading from residential and commercial runoff.  If a regional water regulatory body issues an advisory for Microcystis, avoid entering, consuming, or taking part in recreational activities on or near the water, and do not let your pets drink from or swim in these waters.


Dolichospermum, formerly known as Anabaena, is another common cyanoHAB culprit in New England waters. Cells form filamentous chains, notably with large gas vesicles that allow the cyanobacteria mats to move up and down in the water column in search of favorable growth conditions. Dolichospermum cells also have the capability of forming heterocysts and akinetes, the former being a cell that fixes nitrogen and enables the population to continue when local nitrogen is depleted, and the latter being a thick-walled cyst that can rest dormant in the sediment until conditions are ripe for another bloom. Like MicrocystisDolichospermum produces microcystin, causing the symptoms listed above. Some strains of Dolichospermum produce more diverse toxins, including alkaloids and lipopolysaccharides that function as neurotoxins, cytotoxins, and dermatotoxins. The best way to prevent exposure to these toxins is to avoid contact with and consumption of water where a bloom is occurring, or has recently taken place.

As with Microcystis, eutrophication and warmer waters due to climate change both induce and exacerbate Dolichospermum blooms. A secondary impact of climate change, increased variability in rainfall, is likely to promote Dolichospermum prevalence in freshwater communities as the unique akinete survival strategy allows the species to survive droughts and floods that other less toxic species cannot withstand.

Information from the WWA

What is cyanobacteria?

Cyanobacteria are a phylum of microscopic organisms found naturally in all types of water. These single-celled bacteria live in fresh, brackish, and salt water and photosynthesis to create their own energy from the sun. They are able to multiply quickly in warm, nutrient-rich (high in phosphorus and nitrogen) environments creating blooms that spread across the water’s surface.

A bloom occurs when large quantities of cyanobacteria rise from the sediment on the bottom of a waterbody and form mats on the surface. Blooms are becoming more and more common in New England lakes. The frequency of these blooms is directly related to the amount of nutrients that are entering lakes in stormwater runoff from fertilizers, poorly maintained septic systems, road salts and sands, manmade beaches and shoreline erosion. Water temperatures are also on the rise. Drought conditions with occasional intense rain storms that flush nutrients from the land into the lakes- together create the perfect growing environment for cyanobacteria leading to blooms. Blooms are most common in lake summer and early fall, but can occur anytime.

Why worry about cyanobacteria blooms?

Blooms can be harmful to people, animals, or the environment.

There are many species of cyanobacteria- some of those species can create HABs (Harmful Algae Blooms) from microcystins or cyanotoxins. HABs cannot be predicted. There are 5 species of cyanobacteria that live in fresh water that are known to be the most likely to turn toxic, but there are no predictors on when they will produce toxins. When these toxins are present in the water they are fatal if consumed by dogs and other wildlife. There is little treatment for animals that have consumed toxic cyanobacteria- there livers cannot process the bacteria and it quickly leads to total liver failure- symptom can start in as little as 15 mins or can take several days. DO NOT LET CHILDREN OR PETS NEAR ANY ALGAE BLOOMS!

You cannot tell if a bloom is toxic by looking at it, so assume it is and stay away from the water. Use the contact sheet below if you suspect a bloom. Please also report all suspect blooms to Wolfeboro Police through dispatch.

Check out the flyer below from our friends at the Lake Winnipesaukee Association on how to identify cyanobacteria blooms and who to contact if you see a suspected bloom.

If you use lake water inside your home. DES and EPA advise that you do not use that water during an Advisory. If you “treat” lake water for inhouse use please see this Fact Sheet from the EPA about efficiency. Summary of Cyanotoxin treatment in-house water treatment efficiency from the EPA.

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