Unraveling environmental influences on the spatial and temporal dynamics of cyanobacterial blooms in Lake Erhai during its early stage of eutrophication

Lake Erhai, a lake in the early stage of eutrophication, has been threatened by algal blooms (particularly the overproliferation of blue-green algae), which can have an impact on drinking water safety and the lake's ecosystem. Understanding the governing factors of cyanobacterial blooms is critical for taking timely and effective action during this key eutrophication-transition period. However, long-term records of cyanobacterial bloom and its key dominating factors remain unclear. It is, therefore, essential to understand the bloom dynamics and the driving forces before any control strategies can be determined. We investigated the cyanobacterial phycocyanin concentration variability based on satellite observations from 2003 to 2019, by using the empirical orthogonal function analysis. We observed a decrease in the coverage of the dominant mode of variability in phycocyanin magnitudes compared to the period 2003 to 2011, with variations primarily occurring in the northern bays. The largest variability was identified to be predominant in July, and an apparent timing shift in variability was observed in December 2016 and 2017. The 95% quantile regression model indicated a distinct upper boundary response in cyanobacteria proliferation to the joint Total Nitrogen (TN) and Total Phosphorus (TP) concentrations, which occurred in summer from 2003 to 2011. An apparent response of cyanobacterial bloom to TP was observed during the winters from 2016 to 2019. Additionally, water level and TN: TP ratio played a central role in summer from 2003 to 2011, while from 2016 to 2019, TN: TP ratio was found to dominate in the summer months. In winter, air temperature turned out to be a significant modulating factor compared to water level. Our results suggest that implementing a phosphorus reduction strategy, while controlling TN: TP ratio and suitable water level manipulation, should be considered to ensure the sustainability of Lake Erhai, especially under a consistent global warming scenario.