Warming surface and Lake heatwaves as key drivers to harmful algal Blooms: A case study of Lake Dianchi, China

Extreme heat events such as marine and atmospheric heatwaves, have catastrophic impacts on marine and terrestrial ecosystems. Recent studies have shown that the continuous warming of lake surfaces and enhancement of lake heatwaves (LHWs) have generated great research interest globally. However, the effects of LHWs on lake ecosystems, especially cyanobacterial harmful blooms (cHABs), remain unclear. This study reconstructed the daily Lake surface water temperature (LSWT) time series in Lake Dianchi from 1951 to 2020 using random forest. Trends and characteristics of LSWT and LHWs were identified, and their critical driving effects on cHABs were analyzed. Results showed that the LSWT in Lake Dianchi significantly increased with a rate of 0.26 C-degrees/10a and with an abrupt changing point in 1992, coinciding with the first report of severe cHABs in Lake Dianchi. From 1951 to 2020, 141 LHWs were detected, of which 94.3 % occurred after 1992. Among all the potential climatic and water quality driving factors on Chlorophyll-a (Chla) variation, LSWT is the most important. Chla concentration and the intensity of LHWs were significantly correlated. The complex and strong interactions between LHWs and cHABs enable the suppression of cHABs only when both remain low. A case study in Lake Dianchi highlighted the critical driving effects of LSWT and LHWs on cHABs, suggesting that continuously warming surface and enhancing LHWs in the future will further challenge more specific and effective lake management strategies to suppress cHABs.