GCD member Julie Crocker initiated the first water quality tests of Dublin Lake in 1989. Every summer for the past 30 years GCD Conservation Committee members have continued to collect water samples for testing in order to keep a record of long-term health changes to the lake. Initially, water samples were sent to the University of New Hampshire lab. Today, we work with the Volunteer Lake Assessment Program, a NH Department of Environmental Services program that coordinates 500 volunteers on over 150 NH lakes.
Dublin Lake is an oligotrophic lake meaning the water has a low nutrient value. This limits the lake’s ability to support animal life and is the reason why the water is so clear. All oligotrophic lakes eventually age and become higher in micronutrients and algae resulting in increased plant and animal life. In a pristine environment this process can take thousands of years. Our concern is that human activities such as runoff from roads, septic fields and fertilizer use might be contributing to a more rapid aging of Dublin Lake. Annual water testing lets us keep track of how the water is changing and formulate strategies for keeping the lake clean and the water clear.
The waters of Dublin Lake stratify by temperature into three distinct layers that remain separate for most of the year. Summer stratification is caused by sunlight warming the top 20 feet. As warm water is less dense it floats at the top while the cold, dense, bottom layer stays at a constant temperature of about 39 degrees Fahrenheit. A mid layer of rapidly decreasing temperatures separates the two. Twice a year the lake becomes isothermal for one week when the temperature of each layer reaches 39 degrees and the three layers mix. In the spring this takes place once all the ice has melted but before the sun begins to warm the top layer of water. In the fall it happens just before the ice begins to form. Because each layer has different chemical and biological properties we take water samples from three different depths.
To collect the samples we anchor our boat at the deepest spot on the lake. We lower an open Kemmerer bottle approximately three meters into the water, snap it shut using a sliding weight on a string and haul up a full bottle of water. This process is repeated at seven and eighteen meters. The collected water is tested for PH, conductivity, turbidity, phosphorus and chlorophyll. Water transparency measurements are determined with a standardized eight-inch diameter black and white disk that is lowered into the water column until it can no longer be seen. We also collect water samples from the Dublin Lake Club and Women’s Club beaches and the Sailing Dock. These shoreline samples are tested for bacteria.
The lab results of the 2019 water testing indicate that Dublin Lake continues to score in the good to very good categories on most parameters. Two areas of concern are conductivity and PH. Conductivity is an indicator of salt ions in the water and over the past ten years the lake measurement has increased to where it is now slightly greater than the state median. This trend illustrates the importance of continuing our efforts to decrease erosion and runoff from Rt. 101 and Lake Road.
Dublin Lake PH levels have been improving since the acid rain effects of the 1980’s, however, 2019 saw a dip in PH from neutral 7.1 to more acidic 6.5. We believe this may have been a result of the early summer storm that pushed seven dump truck loads of mud and gravel into Cemetery Cove. Hopefully, that number will rise back to neutral levels this summer.
Testing the water of Dublin Lake is one component in a comprehensive effort to preserve the health and beauty one of our town’s most important assets. Other entities keeping “eyes on the lake” are the Dublin Lake Preservation Committee, the Beach Hill Dublin Lake Watershed Association, the Lake Host Program, the Weed Watch Committee and the NH Department of Environmental Services.
By Katy Wardlaw, Conservation Committee Chair