Lakes have a variety of functions, including water supply, flood prevention, aquaculture, transport and tourism [1
]. As the water exchange of lake water is long, it has weak self-purification abilities [2
]. Lakes are easily contaminated because they receive water from surrounding areas. The spatial and temporal changes in the water quality parameters can directly reflect the environmental water conditions of a lake, as variations in water quality parameters can lead to changes in lake trophic status [3
]. Therefore, it is of practical significance to strengthen the monitoring and analysis of water quality parameters, especially for preventing and controlling eutrophication and protecting water quality and safety.
Mazhar et al. studied the changes of water bodies in different regions under different conditions. The results show that the river water profile has different laws in different climatic regions, which has an impact on DO dynamics [4
]. The Ara Waterway, located in the wet regions, has a higher water quality variation in seasonal scale than that of the Yamuna Waterway, which is in the dry region [5
]. In the southern estuarine water ecosystem of the Boseong County in Korea, there is a high Carlson Trophic State Index in the cropping area and land settlements in summer and autumn [6
]. Through the groundwater monitoring data in Pakistan, it can be found that excessive groundwater abstraction has caused adverse impacts on groundwater quality [7
Thermal stratification is the result of geographic location and summer–winter climatological differences and the depth of the lake. The seasonal vertical distribution of nutrients accompanies the thermal structure dynamics [8
]. The thermal stratification of lakes may lead to a lack of oxygen in the hypolimnion and a boom in algae growth in the epilimnion in summer. This would destroy the balance of aquatic ecosystems and damage water quality.
Algal growth rate and other physiological characterization of living organisms are responding temperature and not vice versa. Chlorophyll-a
is one of the important parameters of the ecological water system. Its content reflects phytoplankton biomass in water and is the basis of determining eutrophication. Phycocyanin is a kind of phycobiliprotein usually found in the body of cyanophyte, red alga, cryptophyta, and dinoflagellate. In these kinds of algae, red algae are usually found in the matrix of an oligotrophic stream. Cryptomonas mainly live in water with poor or moderate levels of nutrition. It is a common alga in tropical and polar waters, contributing less to the abundance of phytoplankton species. The amount of dinoflagellate is also lower in fresh water. However, cyanobacteria have remarkable heterogeneous structures and functions and a relatively broad ecological niche, with an absorption peak near a wavelength of 620 nm [10
]. Cyanobacteria include many species, which are often the dominant species in lakes. The algal toxins contained in it will have a serious impact on organisms [11
]. According to a study by Xie et al. [12
], there are 13 genera (33.3%) of cyanophytes, 4 genera (10.3%) of dinoflagellate and 1 genus of cryptophyta in the water body of Lake Yangzong in 2013. Based on the above, we can preliminarily determine that cyanobacteria are the main algae in Lake Yangzong. Chlorophyll-a
is usually used as an indicator to evaluate the trophic status of the water in most previous studies, but the use of phycocyanin is rare. Normally, eutrophication is often accompanied by the outbreak of cyanobacteria bloom; therefore, strengthening the estimation of cyanobacteria quantity is of great significance for preventing eutrophication.
Since cyanobacteria contain both Chlorophyll-a
and phycocyanin, the ratio of the two pigments in lakes may indicate the relative amount of cyanobacteria in the lake water. The previous Cyanophyte Relative Quantity Index (CRQI) estimation mainly relied on the data from remote sensing rather than the water quality data measured in situ. According to the calculation formula established by Zhang et al. [13
and phycocyanin were measured in situ to estimate the relative quantity index of cyanobacteria in this study. This method could avoid the estimation error caused by the separate use of Chlorophyll-a
as an indicator. Similarly, TN and TP contents are also important conventional indicators for measuring water quality as they are associated with lake water eutrophication which can lead to cyanobacteria bloom and other consequences. The distinctness in release intensity of N and P could modify the N/P limitation in the lake, which affects algae growth and nutrient control [14
], such as normalized cyanobacteria outbreaks formed in lakes such as Lake Dian (Dianchi) [15
] and Lake Jian. Therefore, the accurate determination and analysis of TN and TP in lakes are of great significance for the management of lakes.
Lake Yangzong is the third deepest lake in the Yunnan province of southwestern China. It has a variety of social and economic functions, and its water quality is of high concern. Previous studies on Lake Yangzong mainly focused on the analysis of phytoplankton biomass and their population structure [12
], the dynamic characteristics and inflow capacity of TN and TP [18
], the source of arsenic pollution [20
], water quality change process [22
] and environmental risks caused by heavy metal and arsenic pollution [23
]. However, studies on water quality parameters were limited to monitoring data for a short time and on a small scale [24
]. Therefore, this study combined multi-season, high-frequency, large-scale and continuous water quality monitoring data to provide a scientific basis for the evaluation of the nutritional status of Lake Yangzong and water quality management.
2. Material and Methods
2.1. Physical Geographical Background of Lake Yangzong
Lake Yangzong is a freshwater lake located in Yiliang county, Yunnan Province (24°51′–24°58′ N, 102°55′–103°02′ E). Its average water depth is 20 m with a maximum depth of 31 m. The lake has an average altitude of about 1800 m above sea level, with a lake area of 31 km2
and a water volume of 6.04 × 108
. The catchment of Lake Yangzong belongs to the northern subtropical monsoon climate zone, covering an area of 192 km2
. The annual average temperature of Lake Yangzong is 15.2 °C, the average maximum temperature is 21.5 °C, and the average minimum temperature is 12.4 °C [25
], with an obvious difference in wet and dry seasons and strong evaporation in dry seasons [26
]. The main rivers flowing into the lake include the Yangzong River, Qixing River and Luxichong River in Chengjiang County, Baiyi River, and Tangchi River in Yiliang county [27
]. The annual average surface rainfall of Lake Yangzong is 824.7 mm. The rainy season is from May to October, during which precipitation accounts for 85% of the total annual rainfall, forming a typical low-latitude regional climate. The main water source of the lake is atmospheric precipitation. Rainfall is sufficient in summer and scarce in winter, forming two transition periods in spring and autumn [22
The basin economy is dominated by industry and supplemented by tourism. For a long time, the vulnerability of the lake’s ecosystem has been neglected, despite the effects of human activities. Chemical fertilizers have induced non-point pollution and long-distance water diversion into the lake, and combined with aquaculture, rural domestic sewage, solid wastes and soil and water loss caused by vegetation damage, they are the main sources of pollution of the lake [27
]. Lake Yangzong is the water supply for industry, agriculture and tourism.
In recent years, the water supply for real estate development and land replacement around the lake has depended mainly on water extraction. However, because of the catastrophic artificial arsenic pollution in 2008 and subsequent chemical treatments, the water quality of Lake Yangzong was seriously affected. Although the water quality has improved after treatment, the arsenic content was remained at a moderate level (in 2010, it fluctuated around 0.05 mg/L > 0.01 mg/L), and the nutrition level gradually rose [16
Based on the shape of the lake, three monitoring sites were set up in the southern (S1), middle (S2) and northern (S3) parts of Lake Yangzong (Figure 1
). In situ monitoring was performed in April and November of 2015, January, June and November of 2016, April, June, July and September of 2017, December 2020 and August 2021. The whole lake monitoring was carried out in May 2018 and 2019, December 2020 and August 2021 (in total, 17 points were sampled and measured). The sampling sites were marked and located by a GPS satellite navigator, and the water quality parameters, including water temperature (WT), dissolved oxygen (DO) concentration, Chlorophyll-a
) concentration, pH value and conductivity, were measured with a multi-parameter water quality monitoring instrument (YSI6600V2). A vertical line was set to monitor the water quality (including WT, DO, Chl-a, pH, conductivity and phycocyanin) at different depths at each site. The first data were measured about 0.5 m below the water surface, the last data were monitored 0.5 m above the bottom of the lake, and other data were collected at one-meter intervals. To ensure the accuracy of the data, each depth was measured six times.
Similarly, TN and TP samples from the vertical section of the lake water were collected in 1 L brown polyethylene bottles in September, October, November and December 2016, June, July, September and October 2017, March, May, June, August and October 2018, May 2019, December 2020 and August 2021. The samples were sent to a laboratory and placed in a refrigerator at 4 °C for chemical analysis within 2 h after sample collection.
2.3. Analysis Methods
The TP and TN levels were measured using an ultraviolet spectrophotometer (UV-2600) to determine the absorbance at the wavelengths 200 nm, 275 nm (TN) and 700 nm (TP). Then, the absorbance of the standard sample was corrected to obtain the specific TN and TP contents.
2.4. Data Processing
Microsoft Excel 2016 was used to assess the recorded data. The vertical profile diagram, column chart and correlation analysis diagram of each parameter were drawn using Origin 2020b (Origin Lab, Ltd., Northampton, MA, USA). In the column chart, the ratio of the total concentration of Chl-a to phycocyanin at the vertical section of each monitoring site was calculated using Microsoft Excel 2016.
3.1. Seasonal Variation of Water Temperature
The temperature of deep water plateau lakes is affected by changes in air temperature [8
], with changes in temperature leading to the thermal stratification of lakes. Increasing depths of water lakes cause a slow decrease in water temperature at the epilimnion and hypolimnion, leading to a sharp decrease in the thermocline. Similar to other deep-water plateau lakes (high mountains), the water temperature of Lake Yangzong presents a stratification and mixing phenomenon in the vertical profile. The lake is layered in spring, summer and autumn, and mixed in winter (Figure 2
Our findings showed that the temperature change pattern could be divided into four seasons, which included April 2015 and 2017 in spring (between 14.2 °C and 19.8 °C); June 2016, June, July and September 2017 and August 2021 in summer (between 14.1 °C and 26.6 °C); November 2015 and 2016 in autumn (between 15 °C and 19 °C); and January 2016 and December 2020 in winter (between 13.3 °C and 15.1 °C). These grouping modes represented the division of the four seasons. In winter, the lake belonged to the mixed period. Except for slight changes in the epilimnion from 0 m to 2 m (January), the other water depths were evenly mixed. In spring, epilimnion, thermocline and hypolimnion appeared but were not stable and significant. In summer, the water temperature increased dramatically in the epilimnion (water depth: 0 m to 6 m), while the variation in hypolimnion (water depth: ~18 m) was not obvious. There was a significant temperature gradient and a wide range in the thermocline (water depth: 6 m to 18 m). The hierarchical structure was stable during this time. The water temperature dropped obviously from surface to bottom in autumn. Temperature changes dropped into the deep waters. Moreover, a relatively higher temperature formed in the hypolimnion before the end of summer, which lasted until autumn. The thermocline (water depth: 16 m to 20 m) was in a lower depth and had a smaller range, indicating the vanishing stage of thermal stratification. As soon as the temperature of epilimnion dropped to the temperature of hypolimnion, the lake became mixed. Among the four seasons, the surface temperature range was 14.49 °C to 25.58 °C, the maximum temperature difference was 11.09 °C, the bottom temperature range was 13.31 °C to 16.38 °C and the maximum temperature difference was 3.07 °C. The whole year’s maximum temperature and maximum temperature difference appeared in summer.
The difference among the three monitoring sites was not significant horizontally. In April 2015, the water temperature in the northern part of the lake was higher than the others. In July 2017, the water temperature in the southern part of the lake was higher than the others. In November 2015, according to the strict definition of thermocline [10
], the thermocline in the south part of the lake disappeared, and the mixing period started.
3.2. Seasonal Variation Characteristics of Water Quality Profile
3.2.1. Dissolved Oxygen (DO)
There were obvious seasonal stratification and mixing phenomena in the variation of the DO concentration in Lake Yangzong (Figure 3
Similar to the variation trend of water temperature, the higher the temperature differences are, the stronger the stratification stability and DO distribution are. The curve of dissolved oxygen concentration could also be divided into four seasons. The highest value of surface DO was recorded (about 10.69 mg/L) in July 2017. The DO in the vertical direction was similar in winter. The DO stratification phenomenon began in spring, and the concentration of DO decreased with the water depth. Even during this time, the stratification was not stable. The stratification phenomenon was obvious in the middle and north parts of the lake. In summer, an obvious gradient of DO formed within the range of 4 m to 12 m below the water surface, where the content of DO decreased sharply. In this period, the stable stratification of DO was formed, leading to a lack of DO in deeper waters. In autumn, the DO stratification phenomenon began to vanish, and the water above 15 m was evenly mixed. In the horizontal direction, the seasonal changes in the central and northern regions were more obvious. The change trends of DO in all groups were consistent with the trend of temperature change.
3.2.2. pH Values and Their Variations
The water in Lake Yangzong was alkaline, and the change in pH is obvious (Figure 4
In spring, summer and autumn, with the stratification of temperature and DO, the pH was also stratified. The same four seasons were analyzed according to the grouping mode of temperature and DO. In winter, the variation range of pH was small, with some differences found at the surface. In January 2016, in the range of 0 m to 2 m, the pH decreased sharply in the southern part of the lake (from 8.91 to 8.07). Conversely, in the northern and central parts of the lake, the pH increased within the top 2 m. In spring, an increase in variation range was observed, with the difference between surface and bottom indicating the stratification of pH. In April 2015, there was a dramatically changing layer in the vertical profile, but in April 2017, the pH gradient at each depth was similar. Since the depths differed in the three monitoring sites, sharp changes in the layers were located at different depths. In summer, except in June 2017, the pH decreased with depth and formed a stable pH stratification. During this period, the changes mainly appeared in the southern part of the lake. In autumn, a variation trend in pH identical to the trend of temperature and DO was observed. In the central part of the lake, the pH value at the epilimnion was relatively low.
Conductivity refers to the ability to transmit electricity. It is mainly affected by salinity, dissolved solids, temperature and water supply. In Lake Yangzong, seasonal variation in conductivity was obvious, i.e., the vertical change in conductivity in spring and winter was not significant, while it was clearly stratified in summer and autumn (Figure 5
Similar to the temperature, DO and pH classification, the changes in conductivity could also be divided into four seasons. In April 2015, January 2016 and April 2017, except for the difference in value, the vertical and horizontal conductivity variation was insignificant. In spring, conductivity slightly increased with depth in the vertical direction, but with a very small amount. The changes at the three monitoring sites were similar. The conductivity in April 2017 (0.458 mS/cm) was higher than in April 2015 (0.447 mS/cm) and had no stratification. In summer, the stable stratification of conductivity began to form. At this stage, there was a sharp increase in the middle part of the vertical profile. In June 2017, the conductivity showed an abnormally high value (up to 0.532 mS/cm). The horizontal distribution difference was small in the same month. Except in July 2017, the surface conductivity of the northern part was higher than the southern and central parts. Among them, the highest conductivity was recorded in June 2017. In autumn, the conductivity was higher than that of summer. Vertically, the conductivity increased sharply within the range of 16 m to 20 m. Horizontally, there was no significant difference among the three monitoring sites.
3.2.4. Chlorophyll-a (Chl-a)
Compared with other parameters, the variation in Chl-a
demonstrated distinct characteristics, and the concentration of Chl-a
was higher in autumn and winter (Figure 6
Significant changes could be found in different seasons. There were obvious differences in both Chl-a value and variation trends in winter and early spring. In spring, the difference in the vertical and horizontal directions was obvious. The concentration of Chl-a was significantly high in the northern part of the lake in April 2015, and there was a clear peak within the 4 m to 8 m water depth. In the central and northern parts of the lake, there were lower values between the 4 m and 6 m water depths, respectively. In April 2017, the concentration of Chl-a first increased and then decreased with an increase in depth. The concentration in the central and northern parts increased sharply within 4 m of the epilimnion, while in the southern part of the lake, the Chl-a concentration increased less within 6 m of the epilimnion and did not change much in other depth ranges. The Chl-a concentration in April 2015 was higher than that of April 2017. In spring, the changes in the lake were disordered, and the concentration of Chl-a was higher in the northern part of the lake. In summer, a stable peak formed in the vertical profile. In July 2017, a peak was recorded from 0 m to 5 m at a value of ~5 μg/L. In June 2016 and 2017, the peak and depth decreased more than in July 2017. The stable stratification structure formed at all monitoring sites and all months during this stage, except for the variation in June 2017 in the southern part. In autumn, the Chl-a concentration in November 2015 was extremely high and clustered in the range of 2 m to 18 m water depth. The stratification in this stage was obvious, and the variation trend of all monitoring sites was consistent. In winter, the concentration was higher than in early spring. In January 2016, the concentration of Chl-a increased within the top 4 m and remained similarly constant at ~6 μg/L below 4 m. In December 2020, Chl-a demonstrated little change at each depth.
Phycocyanin and Chl-a
have the same function; they can be used to estimate the amount of plankton and the trophic state of the water lakes. The difference between them is that Chl-a
exists in almost all eukaryotes, while phycocyanin mainly exists in the cyanobacteria. The seasonal distribution of phycocyanin in Lake Yangzong had its own characteristics (Figure 7
Using the four seasonal classifications, the variation trend of phycocyanin was different from that of Chl-a. The phycocyanin concentration gradually decreased from spring, was lowest in summer and then gradually increased to its highest in winter. The concentration of phycocyanin was higher in spring than in autumn.
In winter and early spring, the phycocyanin concentration was at its highest level and was largely distributed at water depths of 4 m to 17 m. In January 2016, the phycocyanin concentration first increased, then stabilized. In April 2015, the phycocyanin concentration gradually increased from south to north, with little change in the horizontal direction in the remaining months. The concentration in April 2017 was higher than in April 2015. In summer, stratification stably developed, and compared to spring, the range was narrower. In June 2016 and June 2017, the phycocyanin concentration peaked between a water depth of 10 m to 12 m in the central and northern part of the lake, and stratification was obvious. In July 2017, the phycocyanin concentration increased sharply at water depths ranging from 3 m to 8 m and reached a peak at about 6 m water depth and then suddenly decreased. In September 2017, the phycocyanin concentration decreased sharply after reaching a peak at 5 m to 7 m water depth and was highest at water depths ranging from 0 m to 3 m compared to the other months. In autumn, the concentration increased and remained at a middle level. The stratification maintained a similar trend at different monitoring sites. In November 2015 and 2016, the phycocyanin concentration in the entire lake was relatively high and stable at water depths ranging from 4 m to 16 m water depth, with small vertical changes.
3.3. Seasonal Variation Characteristics of CRQI
The relative amount of cyanobacteria in Lake Yangzong was relatively high in spring, summer and winter (Figure 8
shows that from April 2015 to April 2017, the CRQI decreased first and then increased. In September 2017, the CRQI was the highest among the three monitoring loci, and the maximum value was recorded in the central part of the lake. Comparing April 2015 to April 2017, an increase in CRQI could be observed, especially in the central part of the lake. It also showed that in 2017, the number of cyanobacteria was higher than that of 2016, except in June 2017. The minimum value of the CRQI monitored was recorded in November 2015, with few differences among the three monitoring sites. Compared with November 2015, the CRQI increased in November 2016, most notably in the southern and central parts of the lake.
3.4. TN and TP Contents and Their Correlation with Other Indexes
The TN content in Lake Yangzong demonstrated a certain change in different seasons. The monitoring data showed that the TN content was lower in the summer and autumn of 2016, 2017 and 2018, and higher in winter and spring (Figure 9
), which was related to less water in winter and spring. The TN content in August 2021 increased compared to 2018 (about 28%). The TP content decreased from 2016 to 2018 (Figure 10
). The TP content in May 2019 did not change significantly from May 2018, but in August 2021 (about 0.04 mg/L), it was significantly higher than that of August 2018 (0.03 mg/L).
During May 2018 and May 2019, the TN and TP contents at the Hypolimnion of the lake were significantly higher than at the epilimnion. In May 2018 and 2019, TN and TP contents of epilimnion were relatively uniform, but in May 2019, TN and TP contents of hypolimnion were higher in the middle of the lake area. In December 2020, the TN and TP contents in the epilimnion and hypolimnion were extremely high. The TN content at the epilimnion was higher in the south and higher at the hypolimnion in the east. The TP content at the epilimnion was relatively uniform in the southeast and higher in the hypolimnion. In August 2021, the TN and TP contents at the epilimnion were relatively uniform (0.78 mg/L < TN < 1.21 mg/L, 0.02 mg/L < TP < 0.05 mg/L). The TN content at the hypolimnion was significantly higher in the north than in the south, and the TP at the bottom was higher in the northeast (about 0.14 mg/L).
In the vertical distribution, the TP and TN contents were significantly higher at 20 m depth than at 0 m, 2 m, 4 m and 10 m depths.
Correlation analysis of various indicators of Lake Yangzong, including water quality parameters, TN and TP contents in October and September 2016, June, July and September 2017, December 2020 and August 2021 (Figure 11
) revealed a high correlation coefficient between DO and Chl-a
= 0.83), and the correlation between temperature and DO, pH and Chl-a
was also relatively high (p
≥ 0.54). However, the correlation between TN and TP was relatively low (p
There were also differences in the TN and TP compositions at different sites and seasons (Figure 12
and Figure 13
The content of particulate nitrogen (PN) and dissolved total nitrogen (DTN) at S1 was significantly higher in December 2020 than in August 2021, and the opposite was found at S3. In December 2020, the nitrate nitrogen (NO3−) content changed very little with water depth, and in August 2021, the NO3− content gradually increased with depth.
At S1, in December 2020, the particulate phosphorus (PP) content was significantly higher than in August 2021 and fluctuated greatly with water depth. There was little difference between the two seasons in dissolved total phosphorus (DTP). The content of PP at S3 in August 2021 also fluctuated with water depth. The orthophosphate (PO43−) content of the whole lake in December 2020 was significantly higher than in August 2021.
4. Analysis Discussion
4.1. Mixing Type of Lake Yangzong
The seasonal temperature stratification and mixing characteristics of Lake Yangzong could be divided into six types [29
]. Since the lake was located in temperate, subtropical mountainous areas, was affected by ocean climate cycled once a year and had a minimum water temperature ≥4 °C, it was classified as a warm monomictic lake. Lake Yangzong maintained this form from spring to autumn, during which the thermocline switched between present and absent, forming a dynamic cyclical process from stratification to mixing.
4.2. Water Quality Parameters
Water temperature is an important factor in determining the primary productivity of lakes, affecting their physical properties, chemical reaction processes and biological activity. Variations in water temperature and the formation and disappearance of thermocline significantly influenced the levels of the chemical parameters [9
]. The environmental parameters follow the thermal structure and stratification in response to the climatological condition. According to the seasonal variation in water temperature, April, June, July, September and November could be classified as the stratification period, and January and December as the mixed period.
DO is the molecular oxygen dissolved in the water from the air. It is influenced by temperature, algae growth, biochemical reaction, etc. During the thermal stratification period, algal photosynthetic and atmosphere reaeration efficiency in epilimnion was high. With increasing water depth, photosynthesis weakened. Respiration of the upper aquatic organisms consumed the oxygen produced by photosynthesis, causing a lack of oxygen in the deeper waters. In addition, DO deletion in the hypolimnion is due to geochemical DO consumption during decomposition and stable stratification prevent mixing. Therefore, in this period, DO decreased from the surface to the bottom, especially in summer, DO in the middle layer of the lake decreased sharply and tended to be ~0 (0.34 mg/L) at the bottom layer. According to a study by Kalff Jacob et al. [10
], the solubility of DO in freshwater mainly depended on the water temperature. Under constant air pressure, a lower water temperature led to a higher concentration of DO. Therefore, compared with December 2020 and January 2016, the winter season, January had a higher DO (7.925 m/L) at lower temperatures (Figure 2
and Figure 3
The concentration of CO2 in water usually affects pH change, limiting the amount of phytoplankton at the surface and the decomposition of organic matter at the bottom. The photosynthesis of algae at the epilimnion consumed a large amount of CO2, causing a reduction in radical acid ions and an increase in pH value. The bottom layer displayed low denitrification, which ultimately reduced the pH. Temperature plays an important role in the growth of algae and the decomposition of organic matter. Therefore, pH level was mostly controlled by photosynthetic activity. The pH value was kept at a high level in the upper profile and a low level in the lower profile. The variation in pH value in Lake Yangzong was in line with the trend. In the mixing period at a lower temperature, the photosynthesis of planktonic algae was weak, the physical and chemical reaction in the lake was not significant, the acidity and alkalinity of the lake changed slightly in the vertical direction and the water was homogeneous in this period.
The vegetation coverage in the catchment of Lake Yangzong was low and showed a steep slope. In the rainy seasons, a large amount of solid matter and industrial and agricultural pollutants were transported into the lake by rivers and surface runoff, which contained large amounts of nutrients. After entering the lake, a portion of the nutrients is consumed by aquatic organisms, with the rest sinking into the bottom of the lake. In addition, the nutrients in the surface sediments migrate upward and are released into the water when the bottom conditions, especially the redox status, change. As a result, the conductivity at the epilimnion was lower, and that of the middle and lower layers increased with depth. From November, Lake Yangzong entered the dry season until the middle of May the following year. During this period, the precipitation was very low, leading to greater evaporation of the lake water than the amount of precipitation, increasing the concentration of salt substances in the lake and increasing the conductivity [31
]. The conductivity remained at the same level in the vertical profile. In the study of Deng et al. [32
], it was found that the quantity of some cyanobacteria had a good correlation with the electrical conductivity, which was related to the K+
plasma brought by agricultural emissions. Large amounts of nutrients and pollutants due to domestic sewage discharge from residential areas, wastewater from factories and fertilizers entered the lake through seepage, increasing its conductivity. This also led to a higher CRQI value in April 2017 than in April 2015 (Figure 8
). The pH value and the conductivity were abnormally high in June 2017 (Figure 4
and Figure 5
) and might be attributed to the second phase of the arsenic pollution treatment project launched in June 2017. Through the Ferric Salt Coagulation method, a large amount of FeCl3
was injected into Lake Yangzong [33
], which might have led to an abnormally low phycocyanin concentration and CRQI.
The concentration of Chl-a and phycocyanin is used as indicators of phytoplankton biomass. The growth of algae was affected by temperature, light, nutrients and other factors. Epilimnion of Lake Yangzong has large hydrodynamic force and is not suitable for algal growth, so the Chl-a content is higher at 2.5–9 m. Algae growth positively correlated with temperature and DO in Lake Yangzong (p ≥ 0.60). During the thermal stratification of Lake Yangzong, the middle and upper water body temperature was suitable, and the DO was sufficient. At the same time, large amounts of precipitation in Lake Yangzong during rainy seasons with large amounts of pollutants provided sufficient nutrients for the algae to bloom. In the mixing period, the conductivity varied little at different water depths, indicating a homogeneous distribution of nutrients and algae at all depths.
4.3. Trophic Status and the Eutrophication
Eutrophication of water bodies is an aging phenomenon of water bodies. TN and TP contents are important indicators of eutrophication levels of lakes. However, the correlation between TN and TP in Lake Yangzong was generally low (p = 0.31).
The average content of TN in Lake Yangzong above 20 m depth was 0.79 mg/L, and that of TP was 0.04 mg/L, both of which were classified as Grade III water quality according to GB 3838-2002. However, the water nutrition below 20 m belonged to Grade IV (TN = 1.05 mg/L, TP = 0.06 mg/L). A sharp increase in TN and TP contents at the bottom of the lake indicated a significant release of nitrogen and phosphorus from sediments. The TN and TP contents at the hypolimnion were higher in the north than in other places in August 2021, and as the northern part was deeper than the southern part, the surface sediments released TN and TP much easier under anaerobic conditions. In the dry season, the lake water mainly comes from the rivers entering the lake. Therefore, in December 2020, the TN and TP contents at point S1 of Lake Yangzong were significantly high.
The TN and TP content in the water column showed no obvious stratification with changes in water depth, which was different from other parameters. The TN and TP content in August 2021 increased significantly compared to August 2018 (Figure 9
b and Figure 10
b), indicating the degree of nutrition in Lake Yangzong was still increasing.
Analysis of the vertical and horizontal spatial distribution characteristics of water quality parameters and nutrition clearly showed that Lake Yangzong undergoes complex seasonal changes.
Lake Yangzong was identified as a typical warm monomictic lake. Its water quality parameters showed obvious changes following stratification, excluding winter, indicating that the water quality parameters were strongly influenced by temperature variations. As the lake water temperature change almost followed the change in air temperature, the lake water quality parameters were also highly influenced by changes in air temperature.
In spring and autumn, the CRQI index was higher, indicating a higher risk of cyanobacterial bloom. Though the contents of TN and TP in Lake Yangzong were not high (the TN contents, especially, were still lower than 2.0 mg/L, the threshold value for algae blooming), it returned to higher values in December 2020 (TN = 1.3 mg/L, TP = 0.06 mg/L), causing a rise in the inter-annual variation. These findings suggest that Lake Yangzong is facing a serious algae blooming threat. The contents of different forms of nitrogen and phosphorus have increased at the bottom of the lake, showing that the nitrogen and phosphorus released from sediments were strengthened. Therefore, it is necessary to intensify lake water quality monitoring and control human activities and endogenous release to prevent further deterioration of the water of Lake Yangzong.