Water level-driven agricultural nonpoint source pollution dominated the ammonia variation in China's second largest reservoir.

Rainfall-runoff and water flooding are the driving mechanisms of agricultural nonpoint source pollution (ANPSP), but existing research has hardly focused on water level-driven ANPSP. Danjiangkou Reservoir was the second largest reservoir in China, and its water quality was dominated by ANPSP. This study explored the effect of water level on water quality of Danjiangkou Reservoir and aimed to provide basis for water quality management of large reservoirs. The effect of water level-driven ANPSP on the concentration of reservoir ammonia was studied employing the methods of factor decomposition and multiple regression on a extensive time series data of reservoir ammonia, water level, rainfall, fertilizer usage, and inflow river ammonia. The long-term trend revealed the reservoir ammonia peaked in 2011 and the inflow river ammonia peaked in 2012 (Han River) and 2013 (Dan River), which indicated the success of point source control in the past 15 years and the dominant role of ANPSP in the reservoir ammonia in recent years. With the long-term trend series, the multiple regression results showed that 56% of the variation of the reservoir ammonia concentration was due to the water level (standardized regression coefficient 0.422), fertilizer usage (standardized regression coefficient 0.522), and inflow river ammonia (standardized regression coefficient 0.219). However, the rainfall was insignificant. The predominance of water level and fertilizer usage in explanation of the reservoir ammonia variation indicated that water level-driven ANPSP was the primary factor influencing the reservoir ammonia. The effect of water level was primarily reflected in the long-term variation of ammonia concentration rather than the seasonal variation within the year. This study showed that when compared with rainfall-driven ANPSP, water level-driven ANPSP had a greater impact on the reservoir ammonia. Water quality protection should center on the management of the water level-fluctuation zone.

Evidence on the causes of the rising levels of CODMn along the middle route of the South-to-North Diversion Project in China: The role of algal dissolved organic matter.

As the biggest inter-basin water transfer scheme in the world, the South-to-North Water Diversion Project (SNWD) was designed to alleviate the water crisis in North China. The main channel of the middle route of the SNWD is of great concern in terms of the drinking water quality. In this study, we tested the hypothesis that the dissolved organic matter (DOM) derived from the planktonic algae causes the rising levels of CODMn along the middle route by monitoring data on water quality (2015-2019, monthly resolution). The results showed that algal density in the main channel increased along the channel and was significantly correlated with CODMn (p < 0.01). Five fluorescent components of DOM, including tyrosine-like (14.85%), tryptophan-like (22.48%), microbial byproduct-like (26.34%), fulvic acid-like (11.41%), and humic acid-like (24.92%) components, were detected. The level of tyrosine-like components increased along the channel and was significantly correlated with algal density (p<0.01), indicating that algae significantly changed the level of DOM in the channel. Algal decomposition and metabolism were found to be the main mechanisms that drive the changes in CODMn. Therefore, controlling algal density would be an important measure to prevent further increase in CODMn and for the guarantee of excellent water quality.

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.