Water quality assessment using optimized CWQII in Taihu Lake.

To improve the rationality of weight allocation and weight proportion of different periods in the process of water quality assessment, the comprehensive water quality identification index (CWQII) model was optimized in this study. A new improved comprehensive water quality identification index (ICWQIIG) model based on game theory was established to combine subjective weight and objective weight. Based on ICWQIIG, an improved comprehensive water quality identification index (ICWQIIP) model with phased period combination weights was established to determine determined the weight proportion of phased periods was established. In this study, CWQII, ICWQIIG, and ICWQIIP were used to evaluate the water quality of seventeen sites in Taihu Lake in 2020. The models selected nine water quality parameters and six water quality indicators. The assessment results of water quality classification were between "slightly polluted" and "moderately polluted". The pollution level on the east bank was lower than that on the west bank and north bank. Furthermore, it was also affected by seasonal change, water quality was worse in January and February but better in October and November. The mean value of Iwq calculated by CWQII, ICWQIIG, and ICWQIIP were 2.405, 2.833, and 3.000, respectively. The compared results showed that the ICWQIIG method can more representative identify the location of polluted water than CWQII. Moreover, the ICWQIIP method calculation results not only retained the representative polluted water samples in the ICWQIIG method but can also identify more pollution sites and worse polluted water bodies. Both ICWQIIG and ICWQIIP had high reliability and accuracy in assessment results, and ICWQIIP was more accurate under sufficient data conditions. This study can offer a scientific basis for local water resource management in Taihu Lake, while simultaneously proposing a science-based and valid methodology for the assessment of other similar water bodies.

Strong spring winds accelerated the recruitment and reinvasion of cyanobacteria.

The reinvasion and recruitment of overwintering cyanobacteria in sediments in spring have an important impact on cyanobacterial blooms in summer and autumn, but until now, this process has not been observed in natural water bodies. In this study, wireless sensors and automatic water sample collection systems were used to carry out continuous high-frequency monitoring of cyanobacteria and related environmental indicators in Meiliang Bay, Lake Taihu, during a northwest wind event in spring. The results showed that there were many dormant cyanobacteria seeds in the sediments of Lake Taihu. These cyanobacterial seeds were easily resuspended from sediments, allowing them to reinvade and remain in the overlying water. Simultaneously, the water temperature exceeded the recovery temperature after the northwest wind event and the available light was sufficient to allow the reinvading cyanobacteria to recruit. The circadian photosynthetic rhythm of these recruited cyanobacteria eventually led to a rapid increase in dissolved oxygen in the water body during the south wind phase to a supersaturated state, and periodic diurnal fluctuations of the water body dissolved oxygen saturation curve. This study provided direct evidence for fully understanding the annual cycle of cyanobacterial blooms.