Answer
A few individuals of the various species, or their ancestor species, would have reached the new kettle lake in a founder event. Populations would have been small at first, allowing genetic drift to change genotype and phenotype in unpredictable ways. However, natural selection would promote phenotypes which were better adapted to conditions in that particular lake, such as pH and nutrient availability. Mutations would add genetic variability for natural selection to work on, as would any gene flow from other such lakes, perhaps on the feet of water birds moving from lake to lake.
Work Step by Step
Remember that the lake starts devoid of life, so a few critters must manage to get there somehow. Genetic drift will be important, especially at first when populations are small, but natural selection always acts to pick winners and losers. Review when mutations arise and how gene flow affects populations.
