Water Quality and Lake Health

Among the challenges facing the watershed, "P" is the greatest - how much phosphorus can Lake Winnipesaukee handle, and how does that level affect the health of our communities and our ecosystems?


Introduction Return to Top

While water resource professionals consider the overall water quality of Lake Winnipesaukee “good” and even “pristine”, indicators of water quality in the last 10 yrs show a downward or negative trend.  Some of these indicators are the frequency in beach closings due to bacteria (e. coli), the increase in the spread of milfoil, the frequency and occurrence of algal blooms, some of which are of health concern due to cyanobacteria, and a decline in fish and loon populations.

One of the primary concerns of the Lake Winnipesaukee Watershed Management Plan is phosphorus loading from the land into the lake and its impact on lake water quality. One of the difficulties of discerning problems with excess P loading for Lake Winnipesaukee is that, due to its size and volume, in-lake effects of P loading like reduced clarity and algae blooms are slow to appear.

It is clear from data collected primarily by UNH LLMP that P levels are generally increasing lake-wide, and this should be cause for action. These long term trends are likely to continue unless cities and towns in the lake’s watershed act to adopt land use best management practices that reduce P loadings to the lake.

The Phosphorus Focus Return to Top

Although phosphorus (P) is not the only pollutant of concern affecting water quality, the State of New Hampshire Department of Environmental Services recently set nutrient criteria standards for acceptable levels of phosphorus in surface waters that would support the aquatic life designated use. 

What does that mean?

Think of the lake as a stomach, it receives all the stormwater runoff from the land surrounding it, which contains metals, oil, road salt, sediment, nutrients, etc. Nutrients by definition are good; they provide needed food for growth or energy.

Image of clean water/alge bloom

Schindler, 1973. Ontario, CA
Experiment depicting results from adding C + N to one half of lake, C, N, and P to other half.

Then why is phosphorus considered a pollutant?  Because as with most things, excessive amounts can have a negative impact. Under natural conditions (pre-development), the lake water does not contain an abundant supply of phosphorus. Due to that fact, phosphorus is considered the limiting nutrient in most New Hampshire fresh waters and a necessary nutrient for aquatic productivity.

When excessive amounts of phosphorus enter the lake, it provides the food or energy needed to boost plant productivity, resulting in algal blooms. The lake, like a stomach, gets sick and turns green. The photo at right depicts the results from an experiment conducted by Schindler, 1973 on a lake in Ontario. Nitrogen and carbon were added to the section of the lake in the top of the photo; in the bottom section, phosphorus was added in addition to the nitrogen and carbon, proving phosphorus was the limiting nutrient.

Increased P levels in freshwater, along with other environmental conditions, may result in increased biological productivity, causing increased phytoplankton (algae) and cyanobacteria cell production in the water column. This in turn can cause:

  • Decreased water clarity
  • Increased Chlorophyll-a level
  • Increased turbidity levels
  • Accelerated lake eutrophication
  • Decreased oxygen concentrations
  • Undesirable shifts in relative abundance of aquatic species

Increased levels of P in freshwater may also result in:

  • Decline in property values
  • Economic loss from decline in tourism due to decline in water clarity
  • Public health risk due to potential of increased occurrence of cyanobacteria blooms
  • Decline in swimming, fishing and boating use
  • Increase in public expenditures to address water quality impairments
  • Increase in plant productivity

Phosphorus loading is accelerated through human activities in the watershed. Human and animal waste, residential and agricultural fertilizers, and atmospheric deposition are the major sources of P. P is found in organic and inorganic (“orthophosphate”) compounds. It is bound in soil by adhering to the surface of soil particles. Erosion and sediment transport, including eroding stream banks, roadway runoff, and exposed soil on construction sites are all potential phosphorus sources. High intensity rain events result in untreated stormwater transported from the land and the road network to storm drains and catch basins which discharge directly and indirectly to surface waters.

For more information on phosphorus and the State nutrient criteria, click here for a useful questions and answers paper. (PDF, 1.1mb)

Water Quality Return to Top

Lake Winnipesaukee is currently categorized as oligotrophic (low productivity) and listed as a high quality water by the New Hampshire Department of Environmental Services (NH DES). The trophic status also takes into consideration dissolved oxygen concentration, chlorophyll-a (phytoplankton community abundance), submerged and emergent aquatic plant communities, and thermal stratification (Winnipesaukee stratifies during the summer and typically mixes completely in spring and fall).

Data on water quality of the lake has been collected for over 20 years through University of New Hampshire’s Lakes Lay Monitoring Program (UNH LLMP). The LLMP program measures water clarity, temperature, phosphorus, chlorophyll-a, alkalinity, and dissolved color. However, not all areas of the lake have consistently been sampled, and therefore monitoring needs to expand and continue in the Meredith, Paugus, and Saunders Bay subwatersheds to assess current in-lake phosphorus concentrations.

Phosphorus water quality analyses (in-lake productivity) were conducted on data available from NH DES, UNH, PSU, Lake Winnipesaukee Watershed Association, and town records. The data was divided into two categories – historical data over 10 years old,  and summer data collected within the last 10 years. Review of those data show that the phosphorus trend in Lake Winnipesaukee increased (worsened) 1.1ppb in ten years.

State Standard for Phosphorus Return to Top

In New Hampshire, designated uses and the water quality to protect those uses are regulated through the Water Quality Standards, which include RSA 485-A:8 – the Classification of Water, and Env-Wq 1700 – the Surface Water Quality Regulations. RSA 485-A:8 establishes that all New Hampshire surface waters are classified as either Class A or Class B waters, and specifies certain minimum surface water quality criteria for each classification. The Surface Water Quality Regulations further protect and maintain New Hampshire’s waters through the identification of designated uses, antidegradation provisions, and additional numeric and narrative water quality criteria. The designated uses for New Hampshire waters are:

1. Aquatic Life
2. Fish and shellfish consumption
3. Drinking water supply
4. Primary and secondary contact recreation (swimming and boating)
5. Wildlife

In 2010, the State of New Hampshire set water quality standards for nutrients based on the aquatic life designated use of the water body. The total phosphorus and chlorophyll-a criteria for supporting aquatic life designated use are:

TP and Chl-a Criteria for Aquatic Life Designated Use

Trophic StateTP (ug/L)Chl-a(ug/L)
Oligotrophic< 8.0< 3.3
Mesotrophic<= 12.0<= 5.0
Eutrophic<= 28<= 11

As mentioned above, Lake Winnipesaukee is categorized as oligotrophic. The new state standard for total phosphorus (TP) and chlorophyll-a (chl a) that would apply to determine if Lake Winnipesaukee is supporting the aquatic life designated use is < 8 ug/L TP and < 3.3 ug/L Chl-a.

This standard means that if the lake median phosphorus level reaches above 8 ug/L, the lake would be considered impaired and must meet rigorous antidegradation requirements for any level of development to occur. For information regarding antidegradation provisions, refer to the NHDES Fact Sheet WD-WMB-23 “What is Antidegradation?” The existing averaged in-lake phosphorus concentration for Lake Winnipesaukee places the lake within 2 ug/L of reaching that threshold.

Currently Lake Winnipesaukee is listed as one assessment unit; however, beginning with the next assessment period (2012), Winnipesaukee will be divided into 10 Assessment Units (AU).

Water Quality Data Analysis Return to Top

As part of the management plan process, analysis was done on the existing water quality data available for the last ten years (1999-2009) for each of the three subwatersheds to determine the median total phosphorus (P) and mean chlorophyll-a values, and to determine the assimilative capacity of the subwatersheds as compared to the State Standard for P. Water quality data for Lake Waukewan was also included as it contributes a significant volume of water to Meredith Bay.

Water Quality Analysis (1999-2009)

WaukewanMeredith Bay Paugus Bay Saunders Bay
Total P (ug/L)
Chl-a (ug/L)

Assimilative Capacity Analysis

The assimilative capacity of a water body describes the amount of pollutant that can be added to that water body without causing a violation of the water quality criteria. The water quality nutrient criterion for phosphorus has been set at 8ug/L for an oligotrophic water body (high quality water). The NH DES requires 10% of the state standard to be kept in reserve; therefore phosphorus levels must remain below 7.2 ug/L to be in the Tier 2 High Quality Water category.

Assimilative Capacity (AC) for Total Phosphorus (TP)

  • Total AC = (Water Quality Standard (8 ug/L TP) – Best Possible WQ (0 ug/L) = 8.0 ug/L TP
  • Reserve assimilative capacity = 0.10 x Total AC = 0.8 ug/L P
  • Remaining assimilative capacity = 7.2 ug/L – Existing WQ

An analysis of a waterbody’s assimilative capacity is used to determine the total assimilative capacity, the reserve assimilative capacity, and the remaining assimilative capacity of each water quality parameter being considered. This information is then used to determine water quality goals and actions necessary to achieve those goals. The assimilative capacity analysis is conducted in accordance with the Standard Operating Procedure for Assimilative Capacity Analysis for New Hampshire Waters.

Results of Assimilative Capacity Analysis

The results of the assimilative capacity analysis for Lake Waukewan, Meredith Bay, Paugus Bay, and Saunders Bay are shown in the graph below. The existing median TP values provide an indication of current water quality; however, limited data emphasizes the need for continued monitoring in Lake Waukewan and the three bays.

The water quality data and assimilative capacity analysis support Lake Winnipesaukee’s designation as a high quality water and oligotrophic classification.

Assimilative Capacity Analysis

In-Lake Models Return to Top

The phosphorus loads estimated from the STEPL (Spreadsheet Tool for Estimating Pollutant Load) land use model (refer to the Issues of Concern tab for results of the STEPL analysis) were input into trophic lake models that predict the in-lake response to the loading.

In other words, how will the lake respond to X amount of phosphorus?

This approach provides a useful tool to communities in planning for future growth as they can use the model to predict the in-lake response from future increased loadings. Communities need to be aware that once in-lake phosphorus levels reach 7.2 ug/L, the water body will fall into the Tier 1 category, be considered impaired, and no further phosphorus loading will be allowed by the State. This has potential implications on development – a community would need to demonstrate that a proposed project would not result in any additional nutrient loading to the receiving waterbody.

The in-lake trophic models predict in-lake phosphorus levels at spring overturn, when the lake is fully mixed throughout the water column. The results of several models were averaged to predict in-lake P; including two well known models used by lake managers, limnologists, and researchers, the Dillon-Rigler and Vollenweider models. Refer to the MPSB Site Specific Project Plan (PDF, 61kb) for more details on the modeling.

Spring overturn occurs shortly after ice out; the lake water begins to thermally stratify in a relatively short time thereafter providing a small window of opportunity to collect water samples. Ice out occurred on March 24, 2010, the earliest ever recorded in the 120 years since it’s been tracked. On April 2, 2010 a group of volunteers, DES, UNH, and PSU scientists collected over 150 water samples from Lake Winnipesaukee at twelve (12) designated deep spots to determine phosphorus levels at spring overturn (see map). As water samples at ice out had never been collected on the lake before, this offered a great opportunity to verify the models.

The estimated in-lake P values for each subwatershed are based on the results of the STEPL and In-Lake Trophic Response models. As mentioned under the Land Use and Phosphorus Loading section of the Issues of Concern page, land use characteristics, rainfall and runoff data, and hydrologic soil type are factors that impact pollutant load estimates.

Factors that influence a lake or water body’s response to pollutant loading from the land are lake volume, mean depth, flushing rate, and the watershed area to water area ratio. Determining land use impacts for each of the subwatershed basin areas is more difficult than if each were a stand alone water body. Annual water loads and volume inputs from other areas of the lake need to be calculated for each subwatershed basin, along with the morphometric characteristics mentioned above. Andy Chapman, Clean Lakes Program, NH Department of Environmental Services provided the much needed assistance in calculating the estimated flows, water volumes and flushing rates for all the subwatershed basins of Lake Winnipesaukee.

A summary of the STEPL modeling analysis and the in-lake P trophic model response is provided below.

The graph below compares the spring overturn data results to the predicted in-lake P from the models. Although the Waukewan Watershed was included in the modeling, ice out data was not collected for Lake Waukewan.

To provide further verification of the models, and as a base line for future comparison of in-lake P values, a water quality monitoring component was added to the project. The full 2009 Water Quality Report can be found on the Monitoring the Lake page; however, the major results are shown in the table below.