Many lakes in New Hampshire are dimictic *; they go through a process of turnover twice a year. As the ice melts in the spring, the winds begin to mix the waters and, as a result, temperature, density, and the amount of dissolved oxygen become relatively equally distributed throughout the water column (from the surface waters to the bottom waters).
As the spring progresses, the sunlight warms the upper surface waters while the bottom waters remain cool. As a result, the waters at the surface become less dense than the cooler underlying bottom waters. The density difference between the surface waters and the underlying bottom waters prohibits the mixing of the water column and the diffusion of dissolved oxygen to the bottom waters.
By mid-June, many of the deeper lakes in New Hampshire have thermal stratification *; in deeper lakes there are usually three layers, determined by water temperature. The bottom layer (hypolimnion *) has cooler waters; the temperature of the middle layer (metalimnion *) quickly increases temperature up through the water column; the surface (epilimnion *) has the warmest waters.
In the fall, the winds and cooler air begin to mix the lake again. The surface waters cool and are mixed with the bottom waters. During the winter, the water is typically close to 0oC just under the ice and approximately 4oC at the bottom of the lake. Although there is only a 3oC to 4oC temperature difference between the top and the bottom the water column, the water column does not mix under the ice. This is called winter or inverse stratification *.
DES biologists use a model that considers the following parameters to classify * the state’s lakes: chlorophyll-a *, Secchi disk * transparency, plant abundance *, and dissolved oxygen *. There are three classifications that the lakes can be reported as: oligotrophic *, mesotrophic*, and eutrophic *.