Most lakes in theUpper Midwest US are frozen for 3-5 months per year, and these ecosystems areadapted to predictable cycles of ice and open water. Climate change is shiftingthe timing, or phenology, of ice-on in the fall later in the year and ice-off inthe spring earlier in the year. On top of long-term shifts in ice phenology,increasing climate variability is also making ice phenology much moreunpredictable from year to year. This talk will focus on the importance of lakeice for ecosystem dynamics in temperate lakes, and the implications of anincreasingly unpredictable climate for ice phenology and how it affects thespawning and reproduction of walleye, a highly important Midwest fish.