Abstract: In order to understand the long-term variations of water quality in Wuliangsuhai Lake, we used the power function summation type universal index (EI) for eutrophication evaluation based on water quality data of Wuliangsuhai Lake during 1999-2019, and used the Mann-Kendal and STARS mutation test to examine the mutation of water quality in Wuliangsuhai Lake. The results showed that the eutrophic level in Wuliangsuhai Lake showed a fluctuating decreasing trend. Water quality mutation at inlet, center and outlet areas of the lake occurred in 2012, 2014 and 2016 respectively, with a postponed occurrence of mutation time one after another. Among the three areas, the inlet was generally at medium and heavy eutrophic level, the nutrient concentrations decreased significantly before and after the mutation of water quality, EI decreased by 20.56%, but it was still in algae-type turbid water state. External inputs and autochthonous enrichment were the main reasons for the serious pollution of the inlet area. The center and outlet areas of the lake were at a light eutrophic level, with an overall decreasing trend, fluctuating between grass-algae and algaegrass coexistence. The center area of the lake was mainly influenced by the release of autochthonous nutrients from the sediments. In the outlet area, the trend was mainly affected by the decrease of plant quantity and the digestion and metabolism of fish. The sampling rates with nitrogen/phosphorus ratios greater than 16 in three areas of Wuliangsuhai Lake were 44.2%, 65.7%, and 72.8% respectively, indicating that Wuliangsuhai Lake was phosphorus-limited. Our results provide scientific basis for the management and water quality mutation warning in Wuliangsuhai Lake, and a reference for assessing the changes of water quality in shallow eutrophic lakes in arid areas.

Key words: Wuliangsuhai Lake, eutrophication, mutation test method, driving factor, phosphorus limitation.