COMPARISON OF WINTER STRATIFICATION AND ICE DURATION BETWEEN SUBALPINE LAKES AND LARGE, LOW ELEVATION LAKES

The impact of climate change, especially warming winter and spring air temperatures, is not well documented for alpine and subalpine areas of the northeast. The presence/absence and persistence of lake ice is an established proxy for larger, low elevation lakes in the northeast, relying on visual observations of the ice-out resulting in a well documented trend of earlier ice-out over the past 100 years. However, subalpine ponds are frequently too remotely located for visual monitoring and too small to accurately interpret ice coverage from satellite imagery. We deployed data loggers monitoring water temperature and light at sub-hourly intervals in high elevation lakes in western and central Maine to develop a baseline dataset characterizing the temperature variability and seasonal transitions of these lakes. Temperature data is collected year-round at three depths: near surface (0.1 – 0.2m), 2m, and bottom. Characteristic temperature changes indicate patterns of stratification and ice growth (or melt) that allow us to identify the timing of major seasonal events. Data acquired since 2007 are used to reconstruct the duration of winter stratification and ice-out timing for more than a dozen high elevation lakes. These new data indicate that the timing of ice-out at the subalpine locations follows a similar trend to larger lakes but with a lag of several days, depending on air temperatures, snowpack, and other site-specific characteristics. The temperature data are also interpreted to discern the timing and duration of winter stratification. Unlike ice presence which varies slightly from basin to basin in timing because of elevation, aspect, or other geomorphic characteristics, the onset of winter stratification is nearly simultaneous at all study sites. Because differences in basin morphology do not seem to play a strong role affecting the onset of the inverted stratification and it is instead strongly tied to air temperature, this event may be developed as a useful phenological indicator. The onset of winter stratification follows an extended period of sub-zero air temperatures. The termination of winter stratification varies slightly among sites depending on ice cover, but once lakes are ice-free the stratification may not be disrupted for days until a significant wind event induces mixing.