Under-ice thermal stratification dynamics of a large, deep lake revealed by high-frequency data

We measured under-ice thermal stratification from before ice-on through after ice-off in Lake Sunapee, New Hampshire, a large, deep, north temperate lake, using a high-frequency monitoring buoy in the winter season of 2007–2008 to quantify how lake thermal stratification varies throughout the under-ice season. We examined potential drivers of variation in under-ice stability, identified diel-scale patterns in under-ice stratification, and used this dataset to test the hypothesis that there are two distinct under-ice phases driven by heat flux from the sediment followed by increased solar radiation as winter progresses. High-frequency measurements demonstrated that only a small fraction of the under-ice period exhibited the traditional inverse stratification previously thought to prevail, based on temporally discrete under-ice temperature profiles. Local short-term weather conditions altered under-ice conditions throughout the ice season with brief periods of snow melt, resulting in several days of disrupted thermal stratification. Our data indicate that thermal structure under the ice in Lake Sunapee is dynamic, and in contrast to smaller, shallower lakes, may be categorized in three, not two, distinct phases. As the under-ice season continues to become shorter due to climate change, under-ice thermal stratification in lakes will likely decrease further.