Effects of water level variations on the water quality of Huayang Lakes, China

AN Lesheng, LIU Chun, FAN Zhongya, LIAO Kaihua, WANG Wencai, WANG Nan

Journal of Geographical Sciences ›› 2025, Vol. 35 ›› Issue (1) : 173-188.

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Journal of Geographical Sciences ›› 2025, Vol. 35 ›› Issue (1) : 173-188. DOI: 10.1007/s11442-025-2317-4
Special Issue: Climate Change and Water Environment

Effects of water level variations on the water quality of Huayang Lakes, China

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Abstract

To explore water level variations and their dynamic influence on the water quality of Huayang Lakes, the water level from 1967 to 2023 and water quality from 2015 to 2023 were analyzed using the Mann-Kendall trend test, box plots, and violin plots. The results show a notable hydrological rhythm of water level alternation between dry and flood seasons in Huayang Lakes, with an average water level of 12.82 m and a monthly range of 11.21-17.24 m. Since 2017, the water level of Huayang Rivers has shown a decreasing trend of -0.02 m/a. Total phosphorus (TP) has become the primary pollutant. The TP concentrations in Longgan Lake (the largest lake) during the dry, rising, flood, and retreating seasons from 2015 to 2023 were 0.083, 0.061, 0.050, and 0.059 mg/L, respectively. The effect of water level on TP was mainly observed during the low-water period. When the water level in the dry season rose to 12.25 and 13.00 m, the percentage of TP exceeding 0.1 mg/L in Longgan Lake decreased to 55.8% and 33.3%, respectively. During the dry season, wind and wave disturbances caused the release of endogenous phosphorus in Huayang Lakes. This led to drastic fluctuations in TP concentration, reducing the correlation between water level and TP. When external control is limited, the water level during the dry season should be maintained between 12.25 and 13.0 m. Additionally, it is necessary to accelerate the restoration of submerged macrophyte species (such as Hydrilla verticillata and Vallisneria natans) in the Huayang Rivers.

Key words

shallow lake / water level / water quality / total phosphorus / Huayang Lakes

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AN Lesheng, LIU Chun, FAN Zhongya, LIAO Kaihua, WANG Wencai, WANG Nan. Effects of water level variations on the water quality of Huayang Lakes, China[J]. Journal of Geographical Sciences, 2025, 35(1): 173-188 https://doi.org/10.1007/s11442-025-2317-4

1 Introduction

The water level, a crucial hydrological factor in lakes, reflects both the lake’s area and volume, significantly affecting hydrodynamic processes, water quality, and the health of aquatic ecosystems (Evtimova and Donohue, 2016; Kann and Walker, 2020). Under the influence of natural factors, such as climate change, and anthropogenic factors, such as gate control regulation, lake water level changes exhibit complex spatiotemporal characteristics and have important ecological and environmental effects (Wantzen et al., 2008; Mulugeta et al., 2015; Wrzesiński and Ptak, 2016).
High water levels in lakes can increase water storage capacity, enhance self-purification ability, and suppress the occurrence of algal blooms (Li et al., 2019; Geng et al., 2022). Research has shown that the dominance and biomass of cyanobacteria can be effectively reduced to prevent the occurrence of cyanobacterial blooms when the fluctuation of lake and reservoir water levels is higher than 2 m/month (Yang et al., 2016). A high lake water level during a flood year or season indicates an increase in the amount of water entering the lake; consequently, the amount of pollutants entering the lake generally increases (Geng et al., 2021). Furthermore, continuous, stable, and high water levels can inhibit the growth of submerged plants in lakes, particularly those that primarily develop in shallow areas with water depths below 2 m (Yang et al., 2006).
Generally, as the water depth decreases, the ratio of water transparency to water depth increases, allowing the bottom of the lake to receive sufficient light, which favors the reproduction of aquatic plants (Verhofstad et al., 2017; Yuan et al., 2022). During the critical period of spring vegetation germination, proper control of lake water levels and improvement of the underwater light environment are crucial for the restoration of aquatic vegetation (Dong et al., 2021; Xia et al., 2022). However, excessively low lake water levels can cause water level fluctuations (WLF) and increase sediment mobility (Lécrivain et al., 2021), leading to higher nutrient levels and decreased transparency in the water, which increases difficulties in the self-regulation of water bodies and results in compromised water quality, particularly in shallow lakes (Havens et al., 2001; Spears et al., 2013; Yang et al., 2023). Shin et al. (2023) reported that total nitrogen (TN) and total phosphorus (TP) concentrations in Lake Okeechobee, a large shallow lake, are more sensitive to WLF than to climate change.
Numerous shallow lakes, including Huayang Lakes, are distributed along the middle and lower reaches of the Yangtze River (Dai et al., 2018). In the 1980s and 1990s, Huayang Lakes hosted more than 60 species of aquatic plants, with dominant species including Potamogeton wrightii Morong, Hydrilla verticillata, and Vallisneria natans. Submerged plants represented the main vegetation type (Zhu et al., 2006; Yuan et al., 2007). Since 2000, the biomass and biodiversity of aquatic plants have dramatically declined due to excessive aquaculture and other factors, with submerged plants almost disappearing from Huayang Lakes (Tan et al., 2020). Although external pollution and seine aquaculture in Huayang Lakes have been effectively controlled in recent years, the lake ecosystem is still fragile, and water quality is poor (Wang et al., 2021).
In this study, the long-term trends of water levels and the water environment in Huayang Lakes were investigated, and the effects of lake water level changes on key pollution factors and water quality were analyzed. Relevant management and control countermeasures are proposed to provide decision-making references for ensuring the water environmental security of shallow lakes.

2 Materials and methods

2.1 Study area

Huayang Lakes, located in Susong county, Anhui province, China, lies in the middle and lower reaches of the Yangtze River. The watershed area covers 5511.4 km2 and experiences a subtropical humid climate, with an average annual temperature of 16.6℃ and annual precipitation of 1410 mm. Huayang Lakes consists of four interconnected shallow lakes: Longgan Lake, Daguan Lake, Huanghu Lake, and Pohu Lake, with water surface areas of 420, 156, 133, and 180 km2, respectively. The main rivers feeding the lakes are the Erlang and Liangting rivers (Fang et al., 2020). The elevation of the lake bottom in Huayang Lakes ranges from 9.5 to 11.5 m. Water flows from west to east, beginning in Longgan Lake and moving eastward into Daguan Lake, Huanghu Lake, and Pohu Lake, before finally draining into the Yangtze River through the Yangwan and Huayang sluices in Wangjiang county. The area is rich in wetland and biodiversity resources and includes the Huayang Lakes Wetland Provincial Nature Reserve (total area: 504.96 km2), which serves as a resting place for important species such as Grus leucogeranus and Ciconia boyciana (Wang et al., 2021).

2.2 Data sources and research methods

2.2.1 Data collection

Hydrological data, including daily water levels and rainfall from 1967 to 2023, were obtained from the Xiacangbu Hydrological Station at Huanghu Lake, a well-established and highly representative station in the basin. Water quality data were collected through monthly manual monitoring at four state-controlled surface water sections in Huayang Lakes from 2015 to 2023, measuring parameters such as pH, dissolved oxygen (DO), Secchi depth (SD), CODMn, NH3-N; TN; TP; and Chl-a). The daily average TP concentration and turbidity of the surface water were measured at an automatic monitoring station in the central section of Longgan Lake from December 1, 2019, to December 31, 2022. The years 2020, 2021, and 2022 in this watershed were associated with wet, normal, and dry conditions, respectively. The bottom elevations of the automatic monitoring stations at Longgan Lake, Daguan Lake, Huanghu Lake, and Pohu Lake are 11.19, 11.06, 10.85, and 10.01 m, respectively. The study area and monitoring stations are shown in Figure 1.
Figure 1 Location of Huayang Lakes, Anhui province, China

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2.2.2 Data analysis

To understand the annual and interannual variations in water levels in Huayang Lakes, a time series analysis of water levels over the past 50 years was conducted. The Mann-Kendall mutation test (Wang et al., 2012; Han et al., 2022) and wavelet analysis (An et al., 2021; Han et al., 2022) were used to test the abruptness and periodicity of water level changes, respectively. Box plots were utilized to summarize manual monitoring results for each lake from 2015 to 2023, helping identify the water quality and primary pollutants in each lake. The comprehensive trophic level index (TLI) calculation and grading method for the lakes were described by Yang et al. (2023). The concentrations of primary pollutants during different hydrological periods were examined using violin plots, and the correlation between daily water quality and water levels over the past three years was analyzed. In combination with manual monitoring results and data from automatic monitoring stations (using box plots to filter out outliers), quantitative correlations between primary pollutant concentrations and water levels were established using regression analysis to clarify the effects of water level changes on water quality and to inform management strategies.

3 Results

3.1 Water level variation

The average water level at the Xiacangbu Station in Huayang Lakes from 1967 to 2023 was 12.82 m. The highest and lowest historical water levels were 17.35 m (September 6, 1999) and 11.56 m (April 1, 1963), respectively. Figure 2a shows that the lake’s water level has slightly increased over the past 50 years. The water level exhibited a fluctuating decline from 1967 to 1988 and from 1989 to 2007, with a greater rate of decline from 1989 to 2007 (-0.24 m/10 a). It then increased between 2008 and 2016 at a rate of 1.1 m/10 a. However, since 2017, there has been a declining trend in the water level (-0.02 m/a). The Mann-Kendall test results for the annual average water level in Figure 2b reveal two inflection points where the upward and downward trends intersect with the significance level trend line, occurring in 1988 and 2016, respectively.
Figure 2 Multi-year variation of the water level in Huayang Lakes, China

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The monthly average water level in Huayang Lakes ranged from 11.21 to 17.24 m, with the lowest and highest monthly average values occurring in April and September, respectively. The water level exhibited a single peak variation with significant fluctuations throughout the year, demonstrating a notable seasonal alternation between the flood and dry seasons (Figure 3). Generally, the water level in a hydrological year can be divided into four stages: dry season (December to April of the following year), rising season (May to June), flood season (July to September; July and August constitute the main flood period in the basin), and retreating season (October to November). The average water levels during the four hydrological periods were 11.99, 12.72, 14.00, and 13.24 m, respectively. The average difference in water levels between the wet and dry seasons was 2.01 m, and the rates of water-level rise or fall during the rise and recession periods were approximately 0.6 and 0.7 m/month, respectively.
Figure 3 Box and whisker charts of the monthly water level in Huayang Lakes from 1967 to 2023

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The wavelet variance analysis revealed that, on the time scales of 18 and 64 months (corresponding to the first and second main periods, respectively), the average periods of water level “rise-fall” changes were 12 months (i.e., 1 year) and 48 months (i.e., 4 years), respectively. The results of the wavelet energy spectrum analysis in Figures 4b and 4c show the first main period of 12 months, followed by 48 months. The high wavelet energy values in the primary 12-month period span the entire study time domain (from 1967 to 2023), whereas the high energy values in the 48-month main period are distributed in the early (approximately from 1967 to 1985) and intermediate stages (approximately from 1994 to 2003) of the study time domain. In addition, the distribution of high wavelet energy values at different timescales in Figure 4b can also be used to divide the monthly water level changes into four stages, which are generally consistent with the periods shown in Figure 2.
Figure 4 (a) Wavelet coefficients as a function of scales; (b) Wavelet power of the monthly water level in the period of 1967-2023; (c) Average wavelet power of the water level in Huayang Lakes, China, at different temporal scales

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3.2 Water quality trend

The monthly manual monitoring results from 2015 to 2023 show that the pH, DO, and NH3N) levels in Huayang Lakes meet the Class III standard for surface water (Figure 5; the dotted line indicates the standard limit for water quality). The pH exceeded the standard only once, in August 2018, with a value of 9.38. The DO concentration consistently exceeded 5 mg/L. The concentration of CODMn ranged from 0.6 to 8.8 mg/L. The maximum concentrations of TP, NH3-N, and TN were 0.350, 0.88, and 2.98 mg/L, respectively, all showing varying degrees of exceedance of the standards.
Figure 5 Box plots of the monthly water level from 2015 to 2023. The dashed line represents the Class III standard limit of surface water in China

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Combined with the distribution information of quartiles and medians in the box plot, CODMn occasionally exceeded the standard (except for Pohu Lake), whereas the TP and TN contents frequently exceeded the standard, especially TP. From 2015 to 2023, the average TP concentrations in Longgan, Huanghu, Daguan, and Pohu Lakes were 0.071, 0.045, 0.041, and 0.032 mg/L, respectively. Table 1 indicates that the TP concentration in Longgan Lake has consistently remained high over the years. It was only in 2020 that it barely met the Class III standard limit (0.05 mg/L). From 2021 to 2023, the monthly monitoring data for TP in Longgan Lake showed that TP exceeded the Class III standard limit 20 times, with a maximum value of 0.133 mg/L in January 2021.
Table 1 Annual mean changes of the main water quality indexes in Huayang Lakes, China (2015-2023)
Lake name Water quality factor (mg/L) Limit (mg/L) Time (year)
III IV 2015 2016 2017 2018 2019 2020 2021 2022 2023
Longgan Lake CODMn 6 10 4.8 5.2 4.4 5.3 4.8 3.4 5.6 5.8 5.3
TP 0.05 0.10 0.065 0.060 0.066 0.133 0.073 0.050 0.062 0.060 0.071
TN 1.0 1.5 0.822 0.900 1.022 1.264 1.015 0.713 0.792 0.897 0.972
TLI (dimensionless) 52.9 53.3 53.9 55.0 58.8 51.1 55.7 57.6 58.9
Daguan Lake CODMn 6 10 / 4.0 3.4 3.6 3.4 3.1 4.6 4.6 4.9
TP 0.05 0.10 / 0.050 0.040 0.060 0.039 0.038 0.034 0.032 0.038
TN 1.0 1.5 / 0.600 0.845 0.945 0.737 0.490 0.735 0.596 0.363
TLI (dimensionless) / 46.9 50.5 46.9 46.5 47.6 49.6 50.0 45.7
Huanghu Lake CODMn 6 10 4.1 4.1 3.3 3.3 3.5 3.5 4.3 4.1 4.1
TP 0.05 0.10 0.053 0.040 0.046 0.067 0.045 0.036 0.038 0.044 0.033
TN 1.0 1.5 0.610 0.550 0.824 0.785 0.696 0.468 1.185 0.842 0.518
TLI / / 48.1 46.9 50.6 45.7 48.0 47.9 49.4 53.9 44.2
Pohu Lake CODMn 6 10 2.0 2.1 2.1 2.7 2.0 1.7 1.8 2.9 3.0
TP 0.05 0.10 0.038 0.020 0.033 0.033 0.031 0.027 0.040 0.041 0.040
TN 1.0 1.5 0.905 1.310 1.143 0.980 0.597 0.568 0.752 0.399 0.627
TLI (dimensionless) / / 45.0 44.9 39.2 41.7 44.1 45.4 47.4
Note: In 2015 and 2016, the TLI values of the Daguan and Pohu lakes were not calculated because of unmeasured indicators such as SD and Chl-a.
Among the four lakes, the SD of Pohu Lake was the highest (mean value of 52 cm over several years), whereas that of Longgan Lake was the lowest (mean value of 31 cm). The highest mean Chl-a value in Longgan Lake was 20.2 μg/L. From 2015 to 2023, the TLI in Longgan Lake consistently exceeded 50, indicating mild eutrophication, while eutrophication in the other lakes was moderate. Among the four lakes, the water quality of Pohu Lake was relatively good, whereas that of Longgan Lake was relatively poor. Due to the connectivity of the water bodies, the water quality of Huanghu and Daguan Lakes was similar and moderate. Since 2019, the water quality of Huayang Lakes has generally improved com-pared to the previous period (2015-2018); however, fluctuations remain.

3.3 Correlation between water level and quality

TP had the greatest effect on the water quality categories of Huayang Lakes. The distribution of the monthly TP concentrations in the lakes during the four hydrological periods from 2015 to 2023 is shown in Figure 6. In terms of the median, the TP concentrations of the four lakes were significantly higher during the dry season (reaching a maximum value) and lower during the flood season. Additionally, the TP concentrations in Longgan Lake (~0.07 mg/L) and Huanghu Lake during the dry season exceeded 0.05 mg/L (Class III). From the perspective of quartiles, the TP concentration exceeded 0.10 mg/L (Class IV) in 20% of the months during the dry season in Longgan Lake. The mean water quality values of the four lakes changed significantly with the hydrological rhythm. During the dry, rising, flooding, and retreating periods, the mean TP concentrations in Longgan Lake were 0.083, 0.061, 0.050, and 0.059 mg/L, respectively, whereas those in Pohu Lake were 0.039, 0.038, 0.031, and 0.032 mg/L, respectively.
Figure 6 The TP concentrations of Huayang Lakes, China, during different hydrological periods (DS-dry season, RIS-rising season, FS-flood season, RES-retreating season)

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Figure 7 shows that the lake water level was generally low and exhibited a downward trend after the onset of the dry season, whereas the TP concentration and turbidity fluctuated in high-value areas, with most of the daily average TP concentrations exceeding 0.05 mg/L. When the water level dropped to a low value, especially when it remained at 12.0 m or below for an extended period (the lake water depth was in the range of approximately 1.0-1.5 m), the TP concentration and turbidity were prone to large fluctuations (Figure 7c). After the flood season, the water level of the lake increased from May to June, and both the TP con-centration and turbidity decreased. During the flood season, from July to September, the water level was high, and the TP concentration and turbidity remained low, resulting in significantly reduced water quality changes. When the water level exceeded 14.0 m, the water quality generally improved. After the onset of the retreating season, the water level dropped significantly, and the TP concentration and turbidity began to rise. It is uncommon to observe significant changes in water quality. Figure 7a shows that 2020 was a flood year (the average annual rainfall for the lake was 1236.7 mm). The water levels of the lake during the flood and retreating seasons were higher than those of a normal year (>15.0 m from July to August), whereas the TP concentration was notably lower than that in normal years (e.g., 2021). The year 2022 stands out as a historically rare dry year (the precipitation amount was 1052.5 mm), with significant fluctuations in TP and turbidity occurring throughout the year, except during the wet season (Figure 7c).
Figure 7 Daily variations of the water level and TP concentrations in Longgan Lake, China, from December 2019 to November 2022

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4 Discussion

4.1 Effect of the water level on key pollution factors

From 2015 to 2023, the monthly TP concentration in Longgan Lake exceeded 0.05 mg/L (Class III) 61 times, accounting for 56.5%, significantly higher than the TN and CODMn concentrations (33.3% and 17.6%, respectively). TP became the most important factor af fecting the lake’s water quality grade. A similar phenomenon was observed in the other lakes in Huayang Lakes. TP has become the primary pollutant that restricts the improvement of water quality in the basin and is extremely common in lakes and rivers along the middle and lower reaches of the Yangtze River (Geng et al., 2021). Based on Figure 6 and 7, the effect of the water level in shallow lakes, such as Huayang Lakes, on TP is mainly reflected in the dry season. The fitting of water level and TP concentration data obtained from automatic monitoring revealed that the TP concentration generally decreased with increasing water level. Figure 8b shows that the water level of Huayang Lakes mainly ranged between 11.75 and 13.00 m during the dry season. When the lake water level was at 12.0 m or below, the TP concentration exceeded 0.10 mg/L (Class IV). The percentage of TP exceeding the Class IV standard decreased to 55.8% and 33.3%, when the water level rose to 12.25 m and 13.00 m, respectively.
Figure 8 The relationship between water level, TP, and turbidity in Longgan Lake. Among them, (b) utilized automatic monitoring data during the dry season from 2020 to 2022, while the rest utilized annual data.

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The prominent water environmental problem in Huayang Lakes, namely the total phosphorus (TP) exceeding the standard, primarily occurs during the dry season (Figures 6-8). This is due to the low water level in shallow lakes during the dry season and disturbances from wind and waves, which can easily cause the resuspension of surface sediments and the dynamic release of endogenous phosphorus, inducing the explosive release of endogenous phosphorus in large shallow lakes (Wu et al., 2013; Zou et al., 2020). Figure 7 shows that the turbidity and TP concentrations in Longgan Lake increased or decreased simultaneously throughout the year. Figure 8d demonstrates a strong linear relationship between the two variables (R2=0.73, p<0.05). Figures 8a-8c illustrate the trend of decreasing lake turbidity and total phosphorus with rising water levels. However, the correlation between water level and TP or turbidity is not significant, particularly during periods of low water. This can be attributed to the shallow water depth (1.0-2.0 m with a fluctuation range of about 1.0 m) of Huayang Lakes during the dry season. Additionally, sharp fluctuations in lake turbidity and TP occur due to the frequent influence of cold waves and strong winds in winter. Fang et al. (2020) captured the complete dynamic release process of endogenous phosphorus under wind and wave disturbances in Longgan Lake by the end of 2019. The TP concentrations varied from 0.084 to 0.565 mg/L. Furthermore, phosphorus in the aqueous phase of Huayang Lakes mainly exists in the form of particles (Zhu et al., 2006; Ma et al., 2021), which confirms that wind and wave disturbances cause sediment resuspension and the release of endogenous phosphorus. Shallow lakes, such as Lake Okeechobee in the United States (Jin and Wang, 2010; Shin et al., 2023), Arresø Lake in Denmark (Søndergaard et al., 1992), and Taihu Lake in China (Zhou et al., 2008; Zhou et al., 2021), are characterized by sediment resuspension under wind and wave disturbances and the dynamic release of endogenous phosphorus, leading to sharp changes in TP concentrations.
The indigenous submerged macrophyte species, such as Hydrilla verticillata and Vallisneria natans, in Huayang Lakes, have essentially disappeared (Yang et al., 2006; Wang et al., 2021), resulting in a lack of large aquatic plants, which effectively suppress waves and fix submerged plant roots in the sediment, further exacerbating the adverse effects of wind and wave disturbances (Figure 9). High-density submerged vegetation forms a protective boundary, changing the lake’s flow pattern from mixed laminar to boundary laminar flow. This reduces turbulence and prevents sediment resuspension (Wang et al., 2023).
Figure 9 The conceptual diagram shows how phosphorus is released from sediments in shallow lakes during the dry season, depending on water levels and aquatic plant growth

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4.2 Implications on water quality regulation

In recent years, extreme drought events have frequently occurred in the Yangtze River Basin, with the dry season beginning earlier and lasting longer (Chen et al., 2023b; Lyu et al., 2023). In 2022, the Yangtze River Basin experienced the most severe meteorological drought since 1961, the year when complete meteorological observation records began (Chen et al., 2023a). Rainfall in 2023 was also lower than usual. Although the wavelet analysis in Figure 4 indicates that the periodicity of dry changes is not readily apparent, considering the continuous and frequent droughts in the basin in 2019, 2022, and 2023, as well as the declining trend of lake water levels since 2016, it is anticipated that Huayang Lakes will continue to experience dry periods for some time to come. Long-term low water levels lead to changes in hydrological rhythms, damage water bodies, and reduce biomass and species diversity (Chen et al., 2015; Liu et al., 2023; Xu et al., 2023). Therefore, the stress of low water levels caused by the prolonged dry season on the ecological quality of the lakes must be considered.
Considering that fluctuations in TP exceeding the standard are common in the middle and lower reaches of the Yangtze River during the dry season, it is essential to coordinate water resources, water environment, and aquatic ecological management. In Huayang Lakes, the dynamic release of endogenous phosphorus is currently the largest source of aquatic TP (Fang et al., 2020). The enhancement of exogenous control has a notable marginal effect on the continuous reduction of TP concentration. If endogenous release cannot be effectively controlled, the TP characteristics, with seasonal fluctuations under water level changes and short-term drastic changes due to wind and wave disturbances, will not fundamentally change and will offset the positive effects of exogenous reductions (Qin et al., 2020). Therefore, it is necessary to implement basin water conservancy projects to optimize water volume scheduling during the dry season, particularly by integrating water quality, hydrology, meteorology, and other information while strengthening the control of the water level within the range of 12.25 to 13.00 m to reduce fluctuations in TP concentration. Even if it is necessary to release flood storage capacity before the arrival of the flood season, the water level of the lake should not be lower than 12.25 m.
However, water level regulations alone have been insufficient. Water quality managers target a clear, macrophyte-dominated state over a turbid, phytoplankton-dominated state in shallow lakes (van Wijk et al., 2023). The fundamental pathway for controlling TP concentration in shallow lakes is to carry out aquatic vegetation restoration, particularly of indigenous submerged macrophyte species, guide the restoration of lake ecosystems, utilize the role of aquatic plants in fixing sediment and dissipating waves and slow flow, and reduce sediment resuspension and endogenous phosphorus release (Gao et al., 2021; Li et al., 2021). Currently, a restoration project for submerged plants is being piloted in Datong Lake and other lakes along the middle and lower reaches of the Yangtze River, and its effects are being tracked and monitored (Chao et al., 2022; Yan et al., 2022).

5 Conclusions

The water level of Huayang Lakes exhibits a notable seasonal alternation between flood and drought. Since 2017, the overall trend of the water level has been declining at a rate of -0.02 m/a. There is a good linear relationship between total phosphorus (TP) and turbidity (R2=0.73, p<0.05), and both decrease with the rise in lake water level. Currently, TP is the primary pollutant restricting the improvement of water quality, and the water level mainly affects TP concentration during the dry season. After the water level rises to 12.25 and 13.00 m, the percentage of TP exceeding the Class IV standard limit for surface water in Longgan Lake can be reduced to 55.8% and 33.3%, respectively. During the dry season, the low water level of the lake, combined with wind and wave disturbances, easily causes sediment resuspension and endogenous phosphorus release, which lead to short-term and drastic fluctuations in TP concentration in Huayang Lakes.
In the case of significant marginal effects (limited effectiveness) in external governance, ecological scheduling through gates and dams should be implemented to maintain the ecological water level of the lakes during the dry season within the range of 12.25 to 13.00 m (Xiacangbu hydrological station). Simultaneously, it is necessary to accelerate the restoration of indigenous submerged plants in lakes to achieve a sustained decrease in TP concentrations in shallow lakes in the middle and lower reaches of the Yangtze River, such as Huayang Lakes.

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We compared the nutrient dynamics of three lakes that have been heavily influenced by point and non-point source pollution and other human activities. The lakes, located in Japan (Lake Kasumigaura), People's Republic of China (Lake Donghu), and the USA (Lake Okeechobee), all are relatively large (> 30 km2), very shallow (< 4 m mean depth), and eutrophic. In all three lakes we found strong interactions among the sediments, water column, and human activities. Important processes affecting nutrient dynamics included nitrogen fixation, light limitation due to resuspended sediments, and intense grazing on algae by cultured fish. As a result of these complex interactions, simple empirical models developed to predict in-lake responses of total phosphorus and algal biomass to external nutrient loads must be used with caution. While published models may provide 'good' results, in terms of model output matching actual data, this may not be due to accurate representation of lake processes in the models. The variable nutrient dynamics that we observed among the three study lakes appears to be typical for shallow lake systems. This indicates that a greater reliance on lake-specific research may be required for effective management, and a lesser role of inter-lake generalization than is possible for deeper, dimictic lake systems. Furthermore, accurate predictions of management impacts in shallow eutrophic lakes may require the use of relatively complex deterministic modeling tools.
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Funding

The Joint Research Project for Yangtze River Conservation(2022-LHYJ-02-0504-05-08)
Anhui Provincial Scientific Research Project for Universities, China(2023AH050508)
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