DECOUPLED ICE DURATION AND WINTER STRATIFICATION IN SMALL MOUNTAIN PONDS

Lakes with seasonal ice have an inverted stratification during the winter period; it is assumed that spring mixing closely follows ice-out, delivering oxygenated water at depth where it has been depleted during winter. Evaluation of high-resolution water temperature data from several mountain lakes in western and northern Maine show close agreement in the timing of ice-out among locations, but the data reveal there is sometimes a significant lag before spring mixing begins. Notably, following relatively early ice-out dates at several of our study locations in 2012 and 2013, spring mixing did not begin for a period of days to weeks following ice-out. Once mixing was initiated, it was also very brief in some lakes. Study sites are small (typically ~10 acres), shallow (5-10 m depth) mountain lakes above 2000 feet; previous work at these sites show a range of thermal stability behaviors during the summer. Susceptibility to mixing seems to be influenced by a combination of depth, surface area, and water color. These same attributes may also help to explain the lag between ice-out and onset of mixing. Based on our data and available nearby meteorologic records, we hypothesize that a combination of anomalously warm, calm weather following ice-out promoted persistence of winter stratification for an extended period after the ice cover melted. Rapid removal of the ice allowed the surface water to warm quickly, and the absence of wind promoted continued stratification. The epilimnion of many lakes quickly became much warmer than water below the thermocline, and the stratification switched without a mixing event occurring. Strong winds associated with a storm eventually initiated mixing, either days or weeks later. Because the mixing had been delayed, this meant that at some locations the first spring turnover resulted in atypically warm water being mixed to the bottom. Of our study sites, lakes that are relatively transparent (have low dissolved organic carbon) or are somewhat deeper show winter stratification persisting for longer. Although ice-out has typically been considered a proxy for the end of winter duration, these data show that there may not be a close relationship between the timing of these two major seasonal events, especially for years with anomalously warm temperatures.