What are Cyanobacteria? Cyanobacteria are microorganisms that occur naturally in most every water body in New Hampshire and throughout the world. They become problematic when they “bloom” in high abundance and start producing toxins, which can cause sickness in humans who come in contact with the water and even kill pets and wildlife that drink or swim in the water. Young children are especially vulnerable.
How can we control cyanobacteria blooms in Pleasant Lake? Cyanobacteria need light, water, and nutrients to grow. We cannot control light or water in a lake, but we can control the nutrients that go into the lake, and phosphorus, which is found in lawn and garden fertilizers as well as in septic effluent, is the primary nutrient. What’s important to note is that we can reduce phosphorus to help control cyanobacteria blooms.
We are very fortunate that Pleasant Lake has generally had low phosphorus concentrations. Unfortunately, due to human activity, phosphorus concentrations rise to levels that can cause cyanobacteria blooms at unpredictable times and areas of the lake. Pleasant Lake has experienced cyanobacteria blooms every year since 2022, with two blooms in 2025, one in June and one in September. We need to address excess phosphorus inputs from the watershed to stop this alarming trend.
So what can one person or one family do? First, we can embrace the fact that we’re all effectively the managers of the lake, and that our actions count. We can reduce the frequency and severity of future cyanobacteria blooms by avoiding the use of lawn and garden fertilizers, especially those that contain phosphorus, by ensuring that our septic systems are periodically pumped and inspected, and by quickly addressing any problems that are discovered.
And there are other meaningful steps we can take to control phosphorus levels by keeping existing phosphorus down in the lake’s sediments, where its concentrations are thousands of times higher than in the water. It is critical that we avoid stirring up phosphorus from the sediments, which in turn stimulate cyanobacteria to “bloom.” Avoiding activities that disturb the sediments is critical for reducing the risk of cyanobacteria blooms.
What are the PLPA and the Town of New London doing to curb future cyanobacteria blooms? The Town of New London has already passed a septic ordinance that requires the pumping and inspection of septic systems every three years. You can read more about it here and be a conscientious lake steward by complying with the ordinance.
This autumn, PLPA board members John Wilson, Peter Winship, and Craig Williamson worked with FB Environmental engineer Laura Diemer to sample the deep sediments of Pleasant Lake. The objective was to determine whether phosphorus is being released from the bottom sediments due to low oxygen (“internal loading”). High phosphorus levels cause cyanobacteria blooms. The results indicate that internal loading from the deep-water sediments is not a major source of phosphorus. Stirring up of the nearshore sediments by wave action and other human activities, however, continues to be a problematic source of phosphorus.
