Water Quality Testing

Water Quality Testing Results

Water quality has been continually measured in Sunset Lake since 1984 and is reported with four primary values:

water clarity
microscopic plant life
phosphorus
water color

In summary, Sunset Lake is best characterized as a relatively clear but moderately nutrient enriched, mesotrophic lake. While the measured quality values have varied slightly over the years, they all have been relatively stable since 1984.

Please see the charts and descriptions below on this page for the trends of each of these values throughout the years. You can also see complete UNH Lakes Lays annual water quality reports for Sunset Lake on our Water Quality Reports page.

Bacteria Testing

E. coli Testing: UNH and the Sunset Lake Association does not perform any E. coli bacteria testing on Sunset Lake. NHDES tests NH freshwater beaches for E. coli bacteria throughout the summer, including at the Hampstead town beach. Visit the Beach Information section of our Sunset Lake Info page for more details about bacteria testing at the Hampstead town beach.

Cyanobacteria: Also known as Blue-Green Algae, cyanobacteria is naturally present in all lakes in rivers, but can take two forms of which the Sunset Lake community should be aware:

benthic cyanobacteria mats: these are only harmful to humans and animals if the mat material is ingested or touched. While humans and animals should avoid contact with the mats to avoid rashes, the sparse presence of mats in a lake should not stop recreation in the water. Benthic mats are present in Sunset Lake.
cyanobacteria blooms: these are potentially toxic to humans and animals and if a bloom is present, humans and animals should avoid wading or recreating in the water or coming in contact with the bloom. Sunset Lake has only had one confirmed cyanobacteria bloom on June 3, 2025.

For more details about benthic mats and cyanobacteria blooms on Sunset Lake, see these blog posts:

How Can I Influence the Water Quality?

As members of the Sunset Lake community, we can all do our part to help prevent degradation and possibly even improve the Sunset Lake water quality. The two nutrients that humans contribute to lakes that can most negatively affect lake water quality are phosphorus and nitrogen. The best way to prevent algae and bacteria growth or too many plants/weeds from growing in the lake is to minimize the amount of phosphorus and nitrogen that flows into the lake. Phosphorus is the main nutrient that plants and bacteria need to grow. By fixing eroding areas and soaking up runoff water on the landscape, limiting the use of fertilizer on lawns, ensuring that septic systems are working properly, and avoiding feeding of ducks and geese the amount of phosphorus and nitrogen in the lake will be minimized.

Visit these pages for more information:

Water Clarity

Secchi Disk Transparency is a water clarity measurement and has displayed a slight trend of decreasing water clarity since 1984. On a positive note, the Secchi Disk transparency has been relatively stable since 2014 with the exception of wild variations in 2018 and 2019.

Microscopic Plant Life

Chlorophyll ɑ concentrations, a measure of microscopic plant life within the lake, display a trend of increasing concentrations since 1984. On a positive note, the chlorophyll ɑ concentrations have stabilized since 2010 with lower seasonal average values documented over the past four seasons. From this graph and the previous Secchi Disk Transparency graph, you can see the relationship between plant growth and water clarity; when plant growth goes up, water clarity goes down (2007 and 2009 are good examples).

Total Phosphorus

Phosphorus is the nutrient most responsible for microscopic plant growth. The total phosphorus concentrations have oscillated over the years while the long‐term trend suggest a slight reduction (improvement) in total phosphorus concentrations Phosphorus data is often correlated with the amount of plant growth; increased phosphorous contributes to increased plant growth.

Water Color

Lake water color is a result of naturally occurring “tea” color substances from the breakdown of soils and plant materials. Color has varied annually and displays a relatively stable trend since data has been collected. In most recent years, the color has varied based on wet (darker color) and dry (lighter color) years. In wet years, the water color is darker due to "flushing" of wetlands and stormwater runoff into the lake.

Tributary and Shoreline Phosphorous Samples

Tributary (stream) and shoreline water samples help to determine if there is any localized flow of high phosphorous into the lake from septic, fertilizer, construction, or other runoff.

Tributary (stream) samples are taken by SLA volunteers from 21 streams that flow into Sunset Lake soon after ice-out during the spring rainy season when there is good stream flow. The samples are frozen and given to UNH for phosphorous content analysis. If a particular stream is showing a high phosphorous reading in the spring, additional samples may be taken or the shoreline may be monitored more closely where that stream enters the lake.

Shoreline samples are taken from 21 locations near the shoreline when UNH visits in July and analyzed for phosphorous content. SLA volunteers also monitor the lake shoreline regularly for conductivity spikes which can indicate a localized increase in phosphorous concentrations.

Using the map below, you can see the phosphorous test results over the years for the stream sample (T#) and shoreline sample (S#) test locations. Note that the acceptable phosphorous PPB (parts per billion) ranges for streams are higher than those for water samples in the lake since the stream samples are not diluted by the large water volume of the lake.