Lake Clarity

Maine lakes continue to be some of the finest in the country and we are fortunate that most of them are considered *Stable* in terms of trophic state based on statisitical analyses.

Maine statute classifies lakes and ponds as "GPA." Lakes and ponds are described by their trophic state or productivity based on measures of Secchi disk transparency, total phosphorus content and chlorophyll. In addition, Class GPA waters must have a stable or improving trophic state, subject only to natural fluctuations, and must be free of culturally induced algal blooms.

The chart below shows the percentage of lakes that are less clear, the same or more clear than their long-term average clarity summarized by year. The chart also shows January through July precipitation levels as recorded at the Portland International Jetport. In 2006, 2009 and 2012, with a higher level of precipitation, most lakes were less clear versus their long-term average. This illustrates the effect that increased precipitation levels and associated stormwater runoff has on the water quality of Maine lakes.

Statistical analysis reveals that at least one-third of the variation seen in Maine lake transparencies is due to precipitation.

Average transparency (measured using a Secchi Disk) of Maine lakes monitored in 2012 is illustrated on a map of Maine, below. The map is based on data collected by the Maine DEP and the Maine Volunteer Lake Monitoring Program (VLMP). Unmonitored lakes are indicated in light blue. Monitored lakes are colored according to their transparency or clarity as indicated in the legend. Lakes that have average transparencies less than 2 meters are often lakes that are brown in color due to natural organic substances exported from wetlands in their watersheds. Lakes with average transparencies between 2 - 3.9 meters may support annual algal blooms which impair their use. Lakes with transparencies 4 meters or greater are some of the clearest in the country.

The map was produced for the VLMP’s Maine Lakes Report 2012 which is available on their website under ‘Publications and Resources’ at:

Contact: Linda Bacon (207) 441-0462

Yearly Comparisions of Lake Clarity and January through July Precipitation and 2012 average lake transparancy map