Reducing blooms, growing confidence: Partridge Lake cyanobacteria mitigation a long time in the making
Published by NHDES on January 17, 2026
Tucked away in the low-lying hills just a couple of miles west of Interstate 93, Partridge Lake is a 52-foot deep, 101-acre lake, within an 882-acre watershed spanning the towns of Littleton and Lyman. Its watershed is mostly forested, with about 70 houses along its shoreline and some past mixed agricultural activities. Residents have been monitoring water quality regularly in the lake and tributaries since 1989 through the Volunteer Lake Assessment Program (VLAP), and water quality trends in the lake are well documented.
That includes the lake’s history with cyanobacteria blooms 
. Cyanobacteria have long been a dominant component of the algal community in Partridge, even blooming under the ice in winter months.
Elevated nutrient concentrations were the primary cause. Historic and current land use factors were contributing higher than expected nutrients from the watershed to the lake. Lakes are the low spot in the landscape to which water from the surrounding land drains. Build-up of nutrients, contaminants and other materials can easily wind up in the lake through precipitation events, runoff and groundwater flow, if not well-managed. But, in Partridge Lake, this was exacerbated by a high amount of phosphorus release from lake sediments, called internal phosphorus loading, as documented by VLAP data . An added boost of phosphorus to overlying water often leads to increased algal growth and can trigger cyanobacteria blooms.
Internal phosphorus loading increases when there are low oxygen levels over lake sediments. Normally iron, manganese and other metals in lake sediments will bind the phosphorus, but low oxygen levels can cause a chemical reaction that allows the phosphorus to be released and mix into the water. An added boost of phosphorus to overlying water often leads to increased algal growth and can trigger cyanobacteria blooms.
A plan of action for the watershed
Knowing that water quality was declining and cyanobacteria blooms were a regular occurrence, the Partridge Lake Property Owners Association (PLPOA) began working locally and in partnership with NHDES staff and external contractors, under a mix of federal grants and local match, to conduct detailed analyses of the lake and watershed.
