Water level fluctuation (WLF) in shallow lakes in the middle and lower reaches of the Yangtze River has been a concern of many researchers. This work aims to investigate the effects of climate change and regulation of floodgates and the Three Gorges Dam (TGD) on WLF and lake volume in Huayang Lakes during the past 52 years. The results revealed that precipitation is the dominant factor that leads to seasonal variation of lake levels, whereas regulation of floodgates and TGD are the key drivers of hydrology regime change in the past 20 years. Natural lake regime has higher water level when there is more precipitation and less lake volume. Floodgates and TGD regulations have changed this pattern since 2003, causing less difference in water level in spite of more precipitation and lake recession. Under the combined impacts of floodgates and TGD regulations, Huayang Lakes have experienced a prolonged outflow time since 2003 and the contribution rate caused by the floodgates and TGD regulations has increased by 19.90%. Additionally, the water level of Huayang Lakes decreased by approximately 0.3~0.5 m from September to November, but it showed no alteration from January to March in the past two decades. This indicated that floodgate regulations used for agricultural irrigation and fishery culture dominate the hydrology regime in winter and early spring. This study is beneficial for aquatic ecosystem protection in floodgate-controlled lakes under the circumstance of climate change and vigorous anthropology activities.

Understanding the spatial distribution and pollution characteristics of heavy metals in lake sediment is crucial for studying deposition and migration processes, assessing lake conditions, and determining the extent of remediation dredging. The present work is a case study of heavy metal pollution in Shitang Lake in Anhui province, China. Heavy metal concentrations were determined in sediment at locations across the lake to a depth of 100 cm, and pollution levels were assessed on the basis of the Geoaccumulation Index (Igeo) and Potential Ecological Risk Index (RI). Hg and Cd were the predominant heavy metals in the sediment, and the Igeo and RI indicated high pollution levels in the northern and southern zones of the lake. These findings can be used to guide the prioritization of dredging operations.

Flood events can significantly affect the physical and biological processes of aquatic ecosystems in a short time, leading to rapid changes in phytoplankton community structure. The Huayanghe Lakes experienced extreme flooding in the summer of 2020, with the water level reaching 16.42 m. In order to understand the effects of flooding on phytoplankton diversity and community structure, eight samples were collected in the Huayanghe Lakes from 2019 to 2020. Water-level disturbance has a significant influence on lake-water quality and phytoplankton community structure. The results showed that the Secchi depth increased from 65.36 to 8.52 cm, while the concentration of total nitrogen (from 0.98 to 0.7 mg/L) and total phosphorus (from 0.04 to 0.031 g/L) decreased. In addition, flooding significantly increased the Shannon–Wiener diversity index, the Pielou index, and the Margalef richness index by an average of 43.5%, 36.7%, and 40.21%, respectively. The phytoplankton community structure in the Huayanghe Lakes changed due to the change of physicochemical environment caused by flood. While in the pre-flood period phytoplankton was composed of large diatoms (e.g., Aulacoseira granulata), cyanobacteria (e.g., Microcystis sp., Anabaena sp., and Aphanizomenon sp.) and other multicellular taxa, the flood period showed an increase in the proportion of chlorophytes and diatoms that quickly adapted to settle in new environments. Pearson correlations and redundancy analyses showed that water level fluctuation was the most significant environmental factor affecting the phytoplankton community between the regular hydrological cycle and flood periods. There are few studies on phytoplankton in the Huayanghe Lakes, and the present study provides basic data on phytoplankton diversity and community structure. In addition, it provides a theoretical basis for controlling water level change in the Yangtze River.

Outliers are often present in large datasets of water quality monitoring time series data. A method of combining the sliding window technique with Dixon detection criterion for the automatic detection of outliers in time series data is limited by the empirical determination of sliding window sizes. The scientific determination of the optimal sliding window size is very meaningful research work. This paper presents a new Monte Carlo Search Method (MCSM) based on random sampling to optimize the size of the sliding window, which fully takes advantage of computers and statistics. The MCSM was applied in a case study to automatic monitoring data of water quality factors in order to test its validity and usefulness. The results of comparing the accuracy and efficiency of the MCSM show that the new method in this paper is scientific and effective. The experimental results show that, at different sample sizes, the average accuracy is between 58.70% and 75.75%, and the average computation time increase is between 17.09% and 45.53%. In the era of big data in environmental monitoring, the proposed new methods can meet the required accuracy of outlier detection and improve the efficiency of calculation.

Recovery of tide-receiving is considered to improve the water quality in the Lianjiang River, a severely polluted and tide-influenced river connected to the South China Sea. A tide-receiving scenario, i.e., keeping the tide gate open, is compared with the other scenario representing the non-tide-receiving condition, i.e., blocking the tide flow during the flood phase, by numerical simulations based on the EFDC (Environmental Fluid Dynamics Code) model. The impacts of tide receiving were evaluated by the variation in the concentration of ammonia and its exporting fluxes, mainly in the downstream part of the river. With more water mass coming into the river, in the tide-receiving scenario, the averaged concentration of ammonia reduced by 20–40%, with the most significant decrease of 0.64 g m⁻³. However, the exporting flux of ammonia has decreased in the tide-receiving scenario, as the consequence of the back–forth oscillation of tidal current. In the tide-receiving scenario, the time series of ammonia concentration approximately followed the tidal oscillation, with increased concentration during the ebb tide and reduction in the flood tide. In the non-tide-receiving scenario, the ammonia concentration decreases when the tide gate is open which results in further intrusion of seawater. This was followed by an increase in ammonia concentration again after the currents shift seaward and water mass with higher concentration from the upstream part is transported downstream. Given the identical ammonia input and river runoff, the ammonia concentration stays lower in the tide-receiving scenario, except for short periods after the tide gate opening and neap tides in the downstream part which lasts for around half a day. This study highlights the importance of hydrodynamic condition, specifically tidal oscillation, in the semi-diurnal and fortnight cycles, for the transportation of waterborne materials. Furthermore, the operation of the tide gate was additionally discussed based on potential varied practical conditions and evaluation criteria.