Harmful Algal Blooms Monitoring Program

The Town of Nantucket, in collaboration with Nantucket Land Council, Nantucket Conservation Foundation, and Nantucket Land Bank monitors the following ponds for harmful algal blooms (HABs) weekly from June through September:Hummock, Long, Miacomet, Sesachacha, Capaum, Gibbs, Clark’s Cove (West Hummock Pond), Maxcy, and Washing Ponds.

HABs monitoring has ended for the season and will begin again in June 2021.

What are harmful algal blooms (HABs)?

HABs are algal blooms with the potential to harm human health or aquatic ecosystems. In freshwater systems, cyanobacteria (also called blue-green algae) are microorganisms that can produce HABs. Some cyanobacterial HABs, or cyanoHABs, can produce toxins (cyanotoxins) that are harmful to human and aquatic health.

What causes HABs?

Environmental factors such as the presence of nutrients, warm temperatures, and increased periods of sunlight can encourage the natural increase of cyanobacteria which is why blooms occur more frequently during the summer time. Agricultural (fertilizer) runoff and wastewater effluent are sources of nutrients that have been linked to higher rates of these bacteria.

Harmful Algal Blooms Monitoring Program
Algal survey SP

What are the potential effects of HABs?                                                                

HABs have been associated with human health impacts including skin rashes, gastrointestinal and respiratory disease, and liver damage. Animals, including dogs, can also be impacted by HABs, having more pronounced effects that can potentially be fatal.

How do I know if a HAB is occurring?

A harmful algal bloom can produce dense mats and may look like green paint or scum on the surface of the water; they can also cause foul odors.

What to do if you notice a HAB?

HABS might not always be visible but are typically common during warmer months of the year and after rain events.

If you notice a HAB occurring or are not sure, please use caution and do not recreate in the pond. Keep your pets on leashes to control your animal’s contact with potential HABs.


The Nantucket HAB monitoring team performs VISUAL HAB inspections for the above mentioned ponds. HAB sampling and testing is only performed by the TON for Hummock, Long, Miacomet, and Sesachacha Ponds.

The duration of a HAB occurrence is dependent on a number of factors, including physical and biological conditions, and therefore is not possible to predict in advance.

As monitoring is performed weekly and at specific locations and times, blooms may occur and disappear throughout the week.

Not all algal blooms that occur in waterbodies are harmful. If you are unsure whether a bloom is harmful or not, please keep away from the pond.

For more information, please visit the Environmental Protection Agency website at https://www.epa.gov/cyanohabs.