The management of freshwater ponds that abut our protected conservation lands has been an increasing area of focus over the past several years for the Nantucket Conservation Foundation’s Science and Stewardship Department. Working with SWCA Environmental Consultants and the Nantucket Land Council, we are developing and implementing management plans to address harmful algae blooms (or HABs) have become increasingly frequent within the island’s freshwater ponds.

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

Warm spring weather followed by hot, dry summers create 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 will detrimentally impact our pond water quality more often and for longer periods of time going forward than they have in the past.


Excess nutrients within a pond system are a result of what is currently entering the pond within the groundwater, from surface water run-off and from precipitation. Groundwater and surface water in particular 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. 


Phosphorus deactivation to reduce and control the “food source” of excess algae growth is one way to prevent HAB’s from forming. High levels of phosphorus have been shown to be present in both sediment and the water column of Nantucket ponds. Alum is a 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. It is important to note that freshwater ponds and their adjacent wetlands are protected under the Nantucket Wetlands Protection Act. Therefore, all management within these wetland resource areas requires permitting from the Town of Nantucket Conservation Commission.

Setting up for the liquid alum treatment at Gibbs Pond (photo: Karen Beattie)

Addressing this important problem with our partners is a high priority for us, and we just made some significant progress. We contracted with SOLitude Lake Management (a nationwide environmental firm) to implement phosphorus inactivation aimed at controlling harmful algae species and phosphorus levels within Gibbs and Capaum Ponds. Both projects were permitted through the Nantucket Conservation Commission (Gibbs Pond: MassDEP SE48-3519; Capaum Pond: MassDEP SE48-3400). The management was informed by detailed pond assessment and management plans prepared by SWCA and many years of water quality data collection performed by the Nantucket Land Council.

Phosphorus deactivation was performed through liquid alum (or Polyaluminum Chloride) application by SOLitude, pond management specialists operating under licensing issued by the Massachusetts Department of Environmental Protection. The liquid alum was brought to Nantucket via tanker truck and applied to the ponds on April 27 and 28, 2023 as a water column treatment via boat with a trailing application hose – an efficient way to administer the alum across the pond. All work was performed consistent with the products and methodologies stipulated in permitting through the Nantucket Conservation Commission.

The liquid alum treatment at Capaum Pond (photo: Dominic Meringolo, SOLitude Lake Management)

This is the first time that this specific type of pond management technique has been done on Nantucket. It has taken several years of assessment and permitting to get to this implementation phase. We are very excited to be testing a new method to help address this problem. Water quality testing was done by the Nantucket Land Council just prior to the alum application and will continue throughout the season following pond management. The results will be used to evaluate the effectiveness of the treatment and inform future management.

We are very grateful to the Gund Family, who established the Lulie Gund Nantucket Pond Management Fund several years ago in memory of Lulie, who was a former NCF Board of Trustees member. A number of abutting property owners and island wide residents have also contributed to the fund to help make this project possible.

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!