This book provides an in-depth overview of the values of modern and subfossil assemblages of cladocerans to understand the impact of recent climate change on mountain lake ecosystems of northwest Europe. The physical and biological data have been collected from a wide range of lakes of Norway and Scotland. Primarily, how locally-mediated climate influences the lake ecosystem in mountain has been examined using seasonal dynamics of modern cladocderans from a Scottish mountain lake, Loch Coire Fionnaraich (LCFR). Secondly, recent climate change has been examined at a regional scale through the assessment of the modern cladcoerans from the Scottish and Norwegian mountain lakes by developing a training set followed by a construction of a summer lake surface water temperature (LSWT) transfer function. Thirdly, how precisely the transfer function is being developed for a past climate reconstruction in the region has been examined through a taphonomic study of cladocerans in LCFR. Finally, a regional scale climate change of northwest Europe has been reconstructed based on a cladoceran-inferred summer LSWT temperature transfer function in the LCFR.