USING HIGH-FREQUENCY TEMPERATURE DATA TO EVALUATE THE DURATION OF SPRING MIXING

In northern and high elevation lakes, the timing of ice-out, duration of spring mixing, and onset of summer stratification are important controls on productivity during the summer months. These events are also subject to forcing by climate warming, and there is a need to better characterize phenological changes during the winter-spring transition. This study focuses on a group of mountain lakes in western and central Maine that we have been monitoring since 2010. Year-round high-frequency water temperature data collected from the surface and hypolimnion can be analyzed to determine the timing of spring mixing events and onset of summer stratification. The data highlight both spatial and inter-annual variability, and suggest some hypotheses for testing. 1) Does earlier ice-out lead to earlier onset of summer stratification? 2) Does earlier ice-out lead to extended spring mixing? Based on their basin morphology and transparency, each lake tends to develop a consistent thermal structure each summer – some lakes show stronger stratification, and some develop a weak stratification each year. These patterns are consistent with their behavior during the spring, when some strongly stratified lakes show limited spring mixing. However, the timing and actual duration of this mixing varies each year in response to meteorological conditions that operate independently of ice-out timing. Following ice-out, decreased thermal contrast facilitates wind-driven mixing in the spring, interrupting initial attempts at summer stratification. Some years, the lakes experience more than three discrete mixing events before summer stratification is established weeks after ice-out. These data suggest that spring weather events, especially strong winds, can also play a significant role in the onset of summer stratification independent of ice-out.