Analysis on the variation of total phosphorus concentrations and loads of inflow and outflow rivers from different water resources zones around Lake Taihu

The variation in inflow and outflow total phosphorus (TP) loads is a key factor affecting the balance of the phosphorus budget for Lake Taihu. Based on the monitoring data of water quality and quantity from 2012 to 2018, the TP load of the rivers in each water resources zone around Lake Taihu were calculated, the flow-weighted mean annual concentrations of TP were attained, and their spatio-temporal variations were analyzed; the double accumulation curve method was adopted to analyze the relationships between water quantity and TP load in different zones; the Pearson correlation coefficient was utilized to reveal the correlation between the TP load and water quantity as well as TP concentrations of the inflow at the monthly time scale; through discussion and analysis, the calculation method of the average TP concentration of inflow applicable to Lake Taihu, the influencing factors of TP loads and measures of controlling the TP load were investigated. The results showed that: (1) The flow-weighted mean concentration of TP is the preferred approach for calculating average concentrations of tributaries with variable flows for Lake Taihu; The average annual TP concentration of inflow rivers of Lake Taihu was 0.189 mg/L, significantly higher than that of outflow rivers (0.079 mg/L). The average annual TP load of the inflow was 2206.51 t, which was also much higher than that of the outflow (873.73 t); in the high flow year of 2016, the TP loads of the inflow, outflow and net inflow of the lake were the largest among 2012-2018, and their values were 2993.9, 1341.1 and 1652.8 t, respectively. (2) The average annual TP concentration of inflow rivers in the Huxi zone was 0.226 mg/L, which was higher than that in other zones; the average annual TP concentrations of outflow rivers in the Zhexi and Wuchengxiyu zones were 0.114 and 0.109 mg/L, which were higher than those in other zones. The interannual variation of the relationships between water quantity and TP load in different water resources zones behaved differently, suggesting that TP pollution was not effectively controlled on the whole except in the Hangjiahu zone. (3) The average annual TP load of the inflow rivers in the Huxi zone and Zhexi zone were 1748.71 and 278.61 t, respectively. The Huxi zone accounted for above 67.88% of the annual total inflow TP load, and the percentage for the Zhexi zone was between 7% and 19%; the average annual outflow TP load of Taipu River was 272.27 t and accounted for more than 30% of the total outflow TP load since 2014. (4) There were good correlations between the monthly TP load and water quantity of the inflow in different water resources zones. Moreover, except in the Yangchengdianmao zone, the variation of monthly TP load of the inflow in other zones was mainly controlled by the water quantity of the inflow. (5) In the high flow year of 2016, the combined effect of high concentration of TP and large water quantity of inflow rivers in the Huxi zone led to a significant increase in the TP load of the inflow. The research results can provide a scientific basis for external phosphorus load control for Lake Taihu. © 2022 by Journal of Lake Sciences.