It’s mid-July on Nantucket, and a number of harmful algae blooms (or HABs) have already been documented within the island’s freshwater ponds. In past years, this phenomenon has occurred later in the summer. However, the unusually warm spring weather seems to have created earlier-than-usual conditions that favor the excess growth of Cyanobacteria, a particular type of blue-green algae that is responsible for HAB’s. Cyanobacteria populations sometimes produce toxins that can cause serious health risks to people and pets. If the current trend continues, HAB’s may be detrimentally impacting our pond water quality more often and for longer periods of time going forward than they have in the past.

Gibbs Pond (photo: Danielle O’Dell).

The management of freshwater ponds that abut our protected conservation lands has become a recent, expanded focus of the Nantucket Conservation Foundation’s Science and Stewardship Department. Working with SWCA Environmental Consultants and the Nantucket Land Council (NLC), we are in the process of developing and implementing management plans to address these increasingly-frequent events at several sites, including Pest House Pond, Capaum Pond and Gibbs Pond. Address this important problem with our partners is a high priority for us. Read on to learn more about what causes a HAB, how they are being monitored, the science of pond management, the various options available for preventing and treating HAB’s, and what you can do to help.

What conditions trigger a HAB? Although algae are a normal component of all freshwater ponds, Cyanobacteria populations can rapidly increase when the water is warm and when excess nutrients, which serve as a source of “food” for algae growth, are present. Warm water holds less oxygen than cold water, which can decrease microbial activity and stress or kill pond life such as fish. Warmer pond water is determined by the seasonal weather patterns occurring during a particular year, as well as the overall trend of increasing temperatures due to a changing climate.


Excess nutrients within the pond system are a result of what is currently entering the pond within the groundwater, from surface water run-off and from precipitation. In particular, groundwater and surface water can carry excess nutrients such as phosphorus and nitrogen into the pond from nearby fertilizer use, agriculture and septic systems. Waterfowl such as ducks and geese can add nutrients through their droppings. Additionally, excess nutrients accumulate and are stored within the sediments at the bottom of the pond, which is essentially a legacy of what has occurred in the past and accumulated over many years. Disturbance of these sediments – even just from high wind events – can mobilize these nutrients into the pond water column, making them available as a source of fuel for algae growth. 

How are HAB’s detected and monitored? In response to more frequent HAB’s taking place on Nantucket within the past several years, the Town of Nantucket developed a Harmful Algae Bloom Monitoring Program in collaboration with the Nantucket Conservation Foundation, Nantucket Land Council, Nantucket Land Bank, Linda Loring Nature Foundation, UMASS Boston and Mass Audubon. On a weekly basis from June through September, representatives from these partner organizations visually monitor the island’s major ponds for evidence of HAB’s (check out our past blog post regarding the Foundation’s participation in this program). If a HAB is detected, it is reported, bi-lingual warning signs are placed at public access locations, information is posted on the Town of Nantucket’s website and the public is notified via social media postings. As part of their Island Pond Sampling Program, the Nantucket Land Council may collect pond water samples to identify and enumerate the specific species present when a HAB is detected in order to determine it is dangerous to public health.

Harmful algae bloom warning sign at West Hummock Pond (photo: Jen Karberg).

What are the options for managing and preventing a HAB, and how do they work?

There are various management options available for managing HAB’s. Because freshwater ponds and their adjacent wetlands are protected under the Nantucket Wetlands Protection Act, all management within these wetland resource areas requires permitting from the Town of Nantucket Conservation Commission. Below are some of the options that are currently being explored here on Nantucket, as well as additional management treatments that have been used elsewhere and may be considered for use here sometime in the future.

Phosphorus Inactivation: Nutrient management to reduce and control the “food source” of excess algae growth is one way to preventing HAB’s from forming. High levels of phosphorus have been shown to be present in both sediment and the water column of Nantucket ponds, which appear to be fueling excess algae growth. Alum (or aluminum sulfate) is a granular chemical that, when combined with water, forms a solid precipitate after a series of chemical reactions. This “flocculant” settles out of the water column and binds with phosphorus, thus making it unavailable as a food source for algal growth. Alum applications are most effective when applied in the spring and early summer to remove the phosphorus available to fuel a bloom later in the season. After several years of planning and permitting in collaboration with the Nantucket Land Council and SWCA Environmental Consultants, we were able to complete an alum treatment this June at Capaum Pond to address severe HAB’s that have been occurring there for several years. To-date, the treatment has had positive results in terms of water clarity and lack of HAB’s detected. However, we have many weeks to go before the HAB season is over for the year.


Algaecide Application: An additional means of algae control is to use algaecide (an aquatic herbicide that specifically targets algae) to directly kill the algae causing the HAB. There are many different types of algaecides that target different species. In the United States, algaecides must be registered for use in wetlands by the Environmental Protection Agency, which has strict regulations regarding how they can be applied. It is important to note that, while algaecides can be effective at immediately killing the algae causing a HAB, they only address the symptom of the problem, not the source of the problem. Therefore, algaecide application is most effective as a component of an overall management plan that includes other options. For example, an alum treatment does not usually result in the complete removal of excess phosphorus because it is difficult to very precisely calculate an exact dosage rate. Therefore, algaecide application can be used as a follow-up treatment to reduce populations of algae that pop up later in the season. The Foundation has used algaecide treatments at Pest House Pond, and may need to apply algaecide at Capaum Pond later this summer if a HAB is detected.

Pond assessment work being undertaken by SWCA Environmental Consultants at Capaum Pond in August 2020 (photo: Neil Foley).

Aeration: Although not a management technique that has as yet been employed on a large scale on Nantucket, adding oxygen to pond water via the installation of aerators, pumps and/or fountain-like structures allows beneficial bacteria to thrive and compete with algae for nutrients, increases the decomposition process and benefits wildlife such as fish. Aeration has the benefit of being a non-chemical management technique, but it is generally employed in more developed settings such as golf course ponds. For larger water bodies, a series of aeration devices would be needed and would be very costly to install. Additional considerations include the need for a source of power to run the pumps (often lacking in more natural settings) or solar panels, adding additional costs.

Sediment Removal: In situations where high levels of nutrients have been documented in the sediments of a pond due to past land use, a more drastic management action may be recommended: the use of dredging to directly remove and dispose of these sediments. Dredging has not yet been employed on Nantucket as a pond management technique; it’s use has been limited to marine habitats in order to improve and maintain navigation. It is extremely cost prohibitive and there can be issues regarding where and how to dispose of the dredged materials, especially if there is contamination from past adjacent land use. It is also very disruptive and damaging to wildlife present in the pond such as fish, turtles and invertebrates. Additional considerations on Nantucket include the possibility of disturbing Native American artifacts that may be present.

A harmful algae bloom at Pest House Pond in June 2017, prior to the initiation of pond management (photo: Karen Beattie).

What can you do to help?

Nantucket is just beginning to explore and pursue active management of its freshwater ponds in order to improve pond water quality and reduce the threat of HAB’s and their associated potential health risks to humans and pets. In addition to the active management treatments described, there are steps that can be taken by individuals to help reduce nutrient inputs that facilitate HAB formation, assist the island’s pond managers with HAB detection and prevent the risk of exposure to HAB’s. 

Comply with Nantucket’s Fertilizer Use Program: The Town of Nantucket Board of Health Local Regulation 75.00 regulates the content and use of fertilizer on Nantucket as a means of preventing poor water quality due to excess fertilizer use in the island’s watersheds. Individual homeowners can assist in this effort by following these regulations and/or confirming that their landscape contractor does. Best Management Practices for Landscape Fertilizer Use on Nantucket Island is an excellent reference developed in 2010-2012 by the Town’s Article 68 Working Group “to provide science-based guidelines for fertilizer use and other landscape practices that, when followed, reduce the loss of soil nutrients from excessive, incorrectly timed, or inappropriate fertilizers.”

Maintain your septic system: The Town of Nantucket has a limited sewer system. A large number of homes on the island, especially in remote areas, are still not able to connect and instead rely on individual septic systems to manage their wastewater. Even properly-maintained septic systems leach nutrients into the soil, which eventually make their way into Nantucket’s ponds, harbors and the ocean. Maintaining your septic system by having it pumped annually and inspected regularly is an important way to keep excess nutrients out of our watersheds.

A harmful algae bloom at West Hummock Pond in 2013 (photo: Kelly Omand).

Report HAB’s and Adopt a Pond: If you notice what you think may be a HAB, the Town’s of Nantucket’s Harmful Algae Bloom Monitoring Program website page has a portal for reporting your observations. Because HAB’s can form very quickly, these reports from the public are very useful for detecting blooms and posting sites when necessary. Additionally, the Nantucket Biodiversity Initiative has an Adopt a Pond Program where you can volunteer to visit a pond and report your observations. 

When in doubt, stay out: If you notice anything that could be a HAB when visiting or recreating near a pond, please use caution. Stay out of the water and keep your pets on a leash.

Support NCF’s pond management work: Two separate funds have been established in memory of a previous Foundation Board of Trustees member by her family: The Lulie Gund Capaum Pond Restoration Fund and the Lulie Gund Nantucket Ponds Restoration Fund. These funds support the analysis, planning, permitting and management actions needed to restore and maintain the health of Capaum Pond and other Nantucket Ponds managed wholly or in part by the Foundation. Matching support for the Capaum Pond Restoration Fund has been generously committed on a dollar-for-dollar basis up to a total of the first $500,000 raised, and similarly for the Nantucket Ponds Restoration Fund up to a total of the first $1,000,000 raised. We welcome your support of these efforts! If you would like to contribute a matching gift towards either of these funds, please contact Cormac Collier, the Foundation’s CEO at 508-228-2884 or Thank you!

The Nantucket Conservation Foundation is a private, non-profit land trust that depends on contributions from our members to support our science projects, conservation property acquisitions and land management efforts. If you are not already a member, please join us now!