Shorter Ice Duration and Changing Phenology Influence Under‐Ice Lake Temperature Dynamics.

Temperate lakes worldwide are losing ice cover but the implications for under‐ice thermal dynamics are poorly constrained. Using a 92‐year record of ice phenology from a temperate and historically dimictic lake, we examined trends, variability, and drivers of ice phenology and under‐ice temperatures. The onset of ice formation decreased by 23 days century−1, which can be largely attributed to warming air temperatures. Ice‐off date has become substantially more variable with spring air temperatures and cumulative February through April snowfall explaining over 80% of the variation in timing. As a result of changing ice phenology, total ice duration contracted by a month and more than doubled in interannual variability. Using weekly under‐ice temperature profiles for the most recent 36 years, we found that shorter ice duration decreased winter inverse stratification and was associated with an extended spring mixing period. We illustrate the limitations of relying on discrete ice clearance dates in our assumptions around under‐ice thermal dynamics by presenting high‐frequency under‐ice observations in two recent winters: one with intermittent ice cover and a year with slow spring ice clearance. Plain Language Summary: Lakes worldwide are losing ice cover in response to climate change. We used a rare and nearly century‐long data set of ice formation and ice clearance records to examine trends, variability, and drivers. We found that ice cover is getting substantially shorter and more variable with winter ice duration about a month shorter now than it was a century ago. Using under‐ice temperature measurements from the most recent three decades, we found differences in under‐ice temperatures affected by lake ice duration. Key Points: Ice duration and ice formation are getting shorter or occurring later, and both are increasing in interannual variabilityIce break up is not trending earlier, but interannual variability is increasingThis dimictic lake is at risk of transitioning toward monomixis in the near future [ABSTRACT FROM AUTHOR]