Algal blooms pose serious threats to water environments and the balance of aquatic ecosystems. Moreover, environmental factors may be the primary driver of bloom outbreaks. Studying the characteristics and driving factors of the evolution of cyanobacterial blooms can provide a scientific basis for the restoration of lake water environments and aquatic ecosystems. Based on the Landsat extended time series and practical ecological investigation, this study determined the bloom area and dominant species of cyanobacterial blooms in Hulun Lake, and analyzed their evolution characteristics and primary influencing factors. The results show that the area of bloom outbreak tended to decrease from 2018 to 2020, with the peak area remaining below 150 km²; in contrast, the bloom area showed an increasing trend in 2021–2022, with a maximum of 1970.55 km² in June 2022. Pearson correlation analysis of bloom area and driving factors showed that the wind speed was the main influencing factor of bloom outbreaks. In 2022, there were five dominant species of cyanobacteria in summer and three in autumn. Redundancy analysis of the dominant species of cyanobacteria and water quality indicators showed that COD, DO, TP, WT, pH, and TN were the main influencing factors of the dominant species of cyanobacteria in summer, and COD, DO, TP, and WT were the main influencing factors in autumn. In general, cyanobacteria in Hulun Lake are sensitive to changes in wind speed and nutrient concentration. An important approach towards effectively reducing the intensity of cyanobacterial blooms in Hulun Lake and coping with the ecological risks associated with the changing environment would be to reduce external loads and regulate the ecosystem structure of the lake. In the future, while studying the law of cyanobacterial bloom outbreaks, attention should be paid to the impact of the cyanobacterial physiological status on bloom outbreaks.

Hulun Lake is the largest lake in northeastern China, and its basin is located in China and Mongolia. This research aims to analyze the dynamic changes in the water volume of Hulun Lake and to estimate the groundwater recharge of the lake during the past 60 years. Multi-source data were used, and water-level-data-interpolation extrapolation, water-balance equations, and other methods were applied. The proportion of the contribution of each component to the quantity of water in Hulun Lake during the last 60 years was accurately calculated. Evaporation loss was the main component in the water loss in Hulun Lake. In the last 60 years, the average annual runoff into the lake was about 1.202 billion m³, and it was the factor with the largest variation range and the leading factor affecting the changes in the quantity of water in Hulun Lake. There was groundwater recharge in Hulun Lake for a long period, and the average annual groundwater recharge was about 776 million m³ (excluding leakage). The contribution ratio of the river water, groundwater, and precipitation to the recharging of Hulun Lake was about 5:3:2. The changes in the quantity of water in Hulun Lake are affected by climate change and human activities in China and Mongolia, especially those in Mongolia.

Bacteria play a critical role in the material and energy-cycling processes of lake ecosystems. To understand the characteristics of the bacterial community in Wuliangsuhai Lake in spring, we explored the influence of environmental factors on the community structure of particle-attached bacteria (PA) and free-living bacteria (FL) in the water column of Wuliangsuhai Lake. In this study, we analyzed the bacterial community characteristics of 10 sampling sites in Wuliangsuhai Lake in April 2019 based on the high-throughput sequencing of 16S rRNA genes. Redundancy analysis (RDA) was used to analyze the influence of environmental factors on bacterial communities in lake water. The results showed the following: (1) The relative abundance of bacteria in Wuliangsuhai Lake did not significantly differ among the 10 sampling sites, and the dominant bacterial phyla were Actinobacteria, Proteobacteria, and Bacteroidetes. In addition, the community diversity of particle-attached (PA) was higher than that of free-living (FL). (2) The relative abundance of Bacteroidetes in PA (28.83%~54.67%) was significantly higher than that of FL (10.56%~28.44%), the relative abundance of Actinobacteria in the number of PA (20.02%~61.61%) was lower than that of FL (8.18%~16.71%), and the relative abundance of Verrucomicrobia in the PA (0.55%~13.11%) was higher than that of FL (0.05%~6.31%). (3) The redundancy analysis (RDA) showed that transparency, total nitrogen, total phosphorus, and ${\mathrm{NH}}_4^{+}$-N were the main factors influencing the dominant bacterial communities in Wuliangsuhai Lake. This study provides the basis for further research on bacterial communities in freshwater lakes and may help local governments in the management of the water resources of Wuliangsuhai Lake.