Algal blooms in the middle and lower Han River: Characteristics, early warning and prevention.

Eutrophication with the sustained occurrence of harmful algal blooms (HABs) has been widely recognized since the 1990s as one of the largest drinking water quality issues in the middle and lower reaches of the Han River (MLHR, the largest tributary of Yangtze). Due to impoundment of the middle route of the South to North Water Diversion Project (SNWDP), the height of the Danjiangkou Reservoir Dam was raised in 2014, resulting in lower discharge (or flow rate) for the MLHR, and subsequently enhancing the potential occurrence probabilities of HABs. Here, the characteristics of HABs that took place during the 1992 to 2018 period were summarized based on the historical water quality and meteorological data. Then, some key factors governing the HABs were analyzed to explore the underlying mechanisms. The results showed that diatoms (Cyclotella sp.) were the dominant species of the HABs. The simultaneous increase of algal density and total phosphorus (TP) concentration during algal blooms revealed that phosphorus was the key limiting nutrient for algal growth. The algal density was significantly negatively correlated with water discharge (R2 = 0.764), or flow rate (R2 = 0.711), indicating that the increase of water discharge from Danjiangkou Reservoir could possibly mitigate the occurrence of HABs. The threshold level of water discharge for HAB prevention was estimated as 500 m3/s in Xiantao Gauging Station before the height of the Dam was raised in 2014, but it subsequently increased to 800 m3/s afterwards. Our study clearly revealed that the occurrence probabilities of HABs in the MLHR were higher than before because of the increase in the phosphorus concentration and the decrease in water discharge under the new engineering conditions of the SNWDP. Therefore, it is urgent to undertake strategies for the HABs prevention in the MLHR.