DIFFERENCES IN WATER ENVIRONMENT CHARACTERISTICS BETWEEN THE NEWLY LAKESHORE ZONE AND THE MAIN LAKE AREA OF QINGHAI LAKE

Qinghai Lake is located at the northeastern edge of the Tibetan Plateau and has important ecological values. Although the intensity of human activities in the catchment is low and there is relatively weak pollution, the water quality of Qinghai Lake has worsened significantly in recent years compared to the 1950s and 1960s. In recent years, some lakeshore areas of Qinghai Lake have experienced a proliferation of Cladophora. It is also accompanied by a continuous rise in the water level of Qinghai Lake since 2004. The large amount of newly lakeshore zone formed by the rising water level may be accompanied by soil nutrient release. However, there is a lack of specific knowledge about what specific differences exist between the water environment conditions in the newly lakeshore zone and the main area of Qinghai Lake, as well as the spatial and temporal patterns of water quality in the lakeshore zone area. Based on the field monitoring data of the lakeshore zone and the main lake area in different seasons, this study compares the differences between the water environment characteristics of the newly lakeshore zone and the main lake area after the rising water level of Qinghai Lake, and analyzes the causes and potential impacts. It could lay the foundation for further establishing the correlation between the outbreak of Cladophora and the rising water level process of Qinghai Lake. In this study, water samples were collected in the lakeshore zone and the main lake area of Qinghai Lake in June and August 2020, respectively. Water environmental indicators such as water temperature (WT), conductivity (EC), pH, dissolved oxygen concentration (DO), dissolved oxygen saturation (DO%), turbidity (Turb), total phosphorus (TP), dissolved reactive phosphorus (SRP), total nitrogen (TN), nitrate nitrogen (NO₃-N), ammonia nitrogen (NH₃-N), nitrite nitrogen (NO₂-N), dissolved inorganic nitrogen (DIN), and COD_Mn were obtained by field measurements and laboratory analysis. In the main lake area of Qinghai Lake, pH ranged from 9.05—9.20, DO ranged from 6.78—8.30 mg/L, DO% ranged from 99.5%—111.4%, WT ranged from 9.3—16.9℃, EC ranged from 15.82—17.77 ms/cm, and turbidity ranged from 1.48—4.94 NTU, TP ranged from 0.009—0.136 mg/L, SRP ranged from 0.001—0.024 mg/L, TN was at 1 mg/L level, NO₃-N ranged from 0.09—0.25 mg/L, NH₃-N ranged from 0.10—1.10 mg/L, NO₂-N was between undetected to 0.0048 mg/L, DIN was between 0.20—1.27 mg/L, COD_Mn was between 1.75—4.79 mg/L; While in the lakeshore zone, pH ranged from 8.76—9.46, DO ranged from 1.14—11.25 mg/L, DO% ranged from 15.7%—190.6%, WT ranged from 8.5—22.0℃, EC ranged from 0.85—18.82 ms/cm, and turbidity ranged from 2.09—196.00 NTU, TP was between 0.014—0.462 mg/L, SRP was between 0.002—0.044 mg/L, TN was at the level of 1.4 mg/L, NO₃-N was between 0.09—0.91 mg/L, NH₃-N was between 0.13—1.23 mg/L, NO₂-N was between undetected to 0.0099 mg/L, DIN was between 0.31—2.14 mg /L, COD_Mn was between 1.78—48.67 mg/L. Based on the significance of the spatial and temporal differences of the indicators between the main lake area and the lakeshore zone, water environment indicators involved in this study can be divided into four categories: there were no significant spatial and temporal differences in pH, TN and DO%; there were significant seasonal differences in WT, DO, SRP, DIN, NH₃-N, NO₃-N, and NO₂-N, but no significant spatial differences; there were significant spatial differences in turbidity and COD_Mn, and the spatial differences were the same in different seasons; there were also significant spatial differences in TP and EC, but the spatial differences were different in different seasons. Except for NO₂-N, the maximum values of coefficients of variation of water environment indicators were found in the lakeshore zone. Most indicators had higher coefficients of variation in August. The results of the principal component analysis suggested that the differences between different types of lakeshore zones might also be enhancing or weakening the influence of lakeshore zones on water environment indicators. The PCA plot shows that the distribution of sample sites in the lakeshore zone of Qinghai Lake disperse along different principal component axes in different seasons. Therefore, the dispersion could be the cause or the result of the outbreak of the algal bloom in the lakeside zone of Qinghai Lake. The results of this study suggest that the water environment characteristics in some lakeshore zone areas of Qinghai Lake are favorable for the growth of Cladophora, which may be partly responsible for the outbreak of Cladophora. The continued impact of the new lakeshore zone formed by the rising water level on the Tibetan Plateau lakes represented by Qinghai Lake needs further attention in subsequent studies.