Effects of re-oligotrophication and climate change on lake thermal structure

1. Re-oligotrophication and climate change have affected many lakes during the last decades. Most climate research has focused on large lakes in Northern Europe and North America while restoration frequently involves small lakes. We investigated the effects of climate and re-oligotrophication on lake thermal structure of Lake Caldonazzo (A = 5.6 km2; zmax = 49 m), located in the southern Alps. Eutrophication-like effects of climate change might be expected, but considering the size and location of Lake Caldonazzo, we expected different responses of lake thermal structure than that seen in large northern lakes. 2. We used transparency, phosphorus and chlorophyll concentrations as trophic indicators, air temperature and global circulation indices as climatic indicators and epilimnion thickness and temperature, hypolimnion temperature, thermocline depth, Schmidt stability as indicators of thermal structure. For these time-series, we determined trend patterns and timing of change points in the 40-year dataset. 3. Re-oligotrophication in Lake Caldonazzo increased water transparency that led to greater epilimnion depth. Until restoration continued, this deeper layer absorbed the incoming heat of climate change without increasing epilimnetic temperature. After completion of sewage removal, however, epilimnion thickness did not increase enough to prevent an upward shift in epilimnetic temperature. 4. The hypolimnetic temperature instead, tended to decrease. We attribute this to deeper mixing and earlier stratification. Penetration of incident solar radiation was insufficient to increase hypolimnetic temperatures. 5. Our study indicated that in Lake Caldonazzo the hypolimnion mainly tracked climate while the epilimnion tracked the interaction between climate and restoration. This will have important repercussions on ecosystem services. Nonpoint sources of nutrients will become increasingly important to manage in order to limit the eutrophication-like effects of climate change, especially in the case of a warming epilimnion.