Climate change is already having significant effects on aquatic ecosystems with recent scenarios indicating greater change in the Alps. Climate models are predicting significant changes in precipitation patterns, with mid-altitude lakes particularly vulnerable. These lakes provide valuable ecosystem services such as irrigation, power, recreation and fishing. LTER site Lake Tovel, Italy (46.26137 N, 10.94934 E, 1178 m asl) is a small, Alpine lake located in the Adamello Brenta Natural Park. In addition to standard limnological parameters since 1995, stable isotopes of oxygen (δ18O) have been sampled monthly along a depth gradient since 2009. δ18O can be used to successfully investigate many aspects of lake hydrology; for example Lake Tovel δ18O values between -13 and -12 ‰ indicate that the aquifer feeding the lake originates at ca. 1000 m above the lake. Long term data show a reduction of N, in line with general trends in the Alps, an increase in surface water temperature coherent with other lakes in the region, but no significant change in total phosphorus. Isotope values reflect winter precipitation. Recharge of the aquifer with rain instead of snow may result in a less steady inflow with repercussions on the lake ecosystem and services provided. Isotopes can contribute to a better understanding of a changing hydroclimate.