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

During recent decades, climate change and re‐oligotrophication have been affecting many lakes. Most long‐term research focuses on large North American and northern European lakes, but climate forcing south of the Alps seems to be different. Furthermore, lake restoration frequently involves smaller lakes (<10 km²) that are often overlooked in long‐term limnological studies despite their importance for local stakeholders. We investigated the effects of climate change and re‐oligotrophication on the thermal structure of Lake Caldonazzo (Italy – southern Alps; area = 5.6 km²; maximum depth = 49 m) for the years 1973–2014. The lake received untreated wastewaters from its catchment until the mid‐1970s, leading to algal blooms, severe hypolimnetic anoxia and fish kills. Afterwards, local government initiated sewage removal that was completed in 1989. We used transparency, phosphorus and chlorophyll concentrations as trophic indicators, air temperature and global circulation indices as climatic indicators and epilimnion depth and temperature, hypolimnion temperature, thermocline depth and Schmidt stability as indicators of thermal structure. For these time series, we determined trend patterns and timing of change points. Epilimnetic temperatures showed an upward shift in 1985. Here, we present an alternative explanation for this observed change that generally has been attributed to global circulation indices. Epilimnetic depth continually increased until 1989, but less markedly afterwards. We suggest that until restoration continued, the increasingly deeper epilimnion absorbed the incoming heat of climate change without increasing epilimnetic temperature. After sewage removal, however, the epilimnion did not deepen enough to prevent an upward shift in epilimnetic temperature. We linked the deepening of the epilimnion to increased water transparency. Hypolimnetic temperatures showed a downward shift in 1998. Hypolimnetic cooling has been seldom observed and was in our case related to specific interactions between re‐oligotrophication, climate and lake depth. Penetration of incident solar radiation was insufficient to heat the hypolimnion (>50% of lake volume), while deeper mixing released accumulated heat from the previous season and earlier stratification trapped colder water in the hypolimnion. We suggest that these combined effects resulted in a decrease in hypolimnetic temperature. Our study indicated that re‐oligotrophication mitigated the effects of climate change, but when re‐oligotrophication was no longer progressing, the effects of climate on thermal structure were perceivable. These changes were site specific and not tied to atmospheric circulation indices. Epilimnetic warming in particular will have repercussions on plankton dynamics. Management of non‐point sources of nutrients will become increasingly important to limit the eutrophication‐like effects of climate change, especially in the case of a warming epilimnion.