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Mountain lakes experience interannual variability in spring snowpack and ice cover that can lead to differences in physical, chemical, and biological properties in the succeeding summer. Lake studies that capture extreme years of snow and ice would be useful to understand and anticipate effects of climate change, but such data are rare for remote mountain lakes. Monitoring of lakes in Olympic, North Cascades, and Mount Rainier National Parks from 2007 to 2018 allowed us to examine limnological differences along interannual and elevation-driven climate gradients that included unusually high (2011-2012) and 100-yr record low (2015) snowpack years. Years with lower spring snowpack had earlier ice-out. Across lakes, our analysis suggested an average of 0.075 C lake warming per day of lost ice duration (0.525 C per week), giving rise to other ecosystem changes linked to temperature such as lower dissolved oxygen, higher total dissolved N, higher chlorophyll, and higher abundance of cladoceran zooplankton. Conversely, in years with higher snowpack and a shorter ice-free season, lakes were colder and clearer (1 m deeper Secchi depth for every 1 m May snow water equivalent), with more dilute ions as well as lower algal biomass and zooplankton abundance. These results add to evidence that changes in snowpack or ice-out dates alter mountain lake ecology through multiple processes associated with hydrology, terrestrial-aquatic connection, water temperature, productivity, ion composition, and plankton communities.

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