[High-Frequency Dynamics of Water Quality and Phytoplankton Community in Inflowing River Mouth of Xin'anjiang Reservoir, China].

The tail of the reservoir is the unstable zone regarding water quality and phytoplankton community. Therefore, it is the crucial zone in aquatic ecosystem transitions. To understand the transition characteristics and driving mechanisms of water environment dynamics, high-frequency monitoring of the water environment and phytoplankton community in the tail of a deep and large reservoir, the Xin'anjiang Reservoir in southeast of China, was conducted using a water quality monitoring buoy and three-day interval water sampling during 18 months. Results show clear seasonal thermal and oxygen stratification in the river mouth of the reservoir. The nutrient and chlorophyll-a concentrations also show stratifying phenomena during the thermal stratification period. Heavy rain and inflow quickly consume the stratification. Nutrient concentrations were highly dynamic in the river mouth. The total phosphorus ranges from 0.011 mg·L-1 to 0.188 mg·L-1, and total nitrogen ranges from 0.75 mg·L-1 to 2.76 mg·L-1. Dissolved phosphorus comprised 56% of total phosphorus, and dissolved nitrogen occupied 88% of total nitrogen, respectively. Nutrient concentrations were influenced strongly by rainfall intensity and inflow rate. Total phosphorus and nitrogen concentrations were significantly related to the three-day accumulated rainfall. Nutrient concentrations in the flood season (March to June) were significantly higher than in the non-flood season (P<0.001). Seasonal phytoplankton proliferation also significantly influenced by total phosphorus concentration. The phytoplankton community changes significantly with seasons and flood events. Bacillariophytea was generally dominant throughout the year, with the predominant genus of Fragilaria spp., Cyclotella spp., Synedra spp., and Melosira spp. Cyanophyta biomass peaked in July, August, and September, with the dominant genus of Aphanizomenon spp., Microcystis spp., and Oscillatoria spp. Apart from the high temperature, storm inflow events also triggered Cyanophyta proliferation. The proliferation of Chlorophyta was similar to Cyanophyta, with the predominant genus of Pediastrum spp. and Closterium spp.. While the Cryptophyta biomass peaked during March to May, with the predominant genus of Cryptomonas spp.. Redundancy analysis shows that the influence factors of phytoplankton community dynamics include the inflow rate, temperature, water level, water transparency, total nitrogen, total phosphorus, and nitrogen to phosphorus ratio. The meteorological and hydrological factors were major factors for phytoplankton dynamics during later autumn and winter, while the nutrient will be the co-driving factors of phytoplankton community dynamics during summer and early autumn. The research confirmed the huge influence of the intensity rainfall event on the water environment in reservoirs and described the key environmental conditions for phytoplankton community dynamics. The research is useful for the design of the monitoring and forecasting system for water safety in drinking water source reservoirs.

[Spatial-temporal Distribution of Suspended Solids and Its Sedimentation Flux and Nutrients Effects in Xin'anjiang Reservoir, China].

To analyze the spatial-temporal distribution and sedimentation characteristics of suspended solids in reservoirs, high-frequency monitoring of a sediment trap and buoy, combined with three-dimensional water sampling, was conducted and analyzed in Xin'anjiang Reservoir for a year. The results showed that the turbidity data of the buoy has significant correlation with rainfall, inflow, and suspended solids (SS), particularly for SS (P<0.01, R2=0.86). There is an obvious spatial difference in SS between spring and summer, when the rainfall season occurs (river area > transition area > lake area). However, there is little difference in SS concentration between autumn and winter. There is a spatial trend of river area > transition area > lake area (with rates of 27.82, 4.34, and 0.26 g·(m2·d)-1, respectively), and a temporal trend of spring and summer > autumn and winter. The sedimentation flux of the whole lake is 2.57×106 t·a-1 combined with the investigation of the four-season SS at 60 points across the whole lake, and the settlement flux in spring and summer is higher than that in autumn and winter. The contents of particulate nitrogen (PN) in JK, XJS, and DB were 6812, 15886, and 21986 mg·kg-1, and the particulate phosphorus (PP) contents were 2545, 3269, and 3077 mg·kg-1, respectively. Statistical analysis shows that there is a good exponential relationship between moderate rainfall and turbidity growth rate in the river area of the reservoir (R2=0.81). Moreover, the continuous heavy rainfall affects turbidity in river area, but has little effect on the transition area. The concentration of SS has a good exponential decay with distance from the river to the dam (R2=0.84), especially in spring and summer. Research shows that the average annual deposition rate in Xin'anjiang Reservoir is 0.07%, lower than other large reservoirs in the country; however, there are certain risks in front of the dam because the nutrient sediments are high. The results suggest that reservoir managers should pay attention to water and soil conservation in the watershed to reduce the impact of rainfall on reservoir water quality. Meanwhile, the potential nutrient internal release risk in the downstream area before the dam should be considered.

[Influence of Rainfall Intensity on the Nutrient Loading from an Inflowing River in the Plain River Network of the Taihu Catchment].

To reveal the law of external pollution in the plain river network of the Taihu catchment, we investigated nitrogen, phosphorus, and dissolved organic carbon daily, as well as automatic recording data for flow rate and rainfall in Dapu River from 1st March, 2017 to 28th February, 2018. Dapu River, a typical inflowing river of Lake Taihu, usually has a reciprocating flow. A reciprocating flow is a common condition in the plain river network located in the Yangtze Delta. The response flow rate in the river was relatively slow to different intensities of rainfall. Flow rate significantly increased only when the rainfall intensity reached heavy rain (>25 mm·d-1). The concentrations were statistically non-significant under different rainfall intensities, and the concentrations of total nitrogen averaged 3.00±0.58, 3.34±0.93, 3.55±1.05, and 3.37±1.14 mg·L-1 under heavy rain (>25 mm·d-1), medium rain (10 mm·d-1-25 mm·d-1), light rain (<10 mm·d-1), and no rain. The concentrations of total phosphorus averaged 0.228±0.068, 0.258±0.121, 0.219±0.083, and 0.225±0.121 mg·L-1, respectively. The concentration of dissolved organic carbon and nitrogen in the Dapu River changed little during the 3 typical rainfall processes, but the concentration of phosphorus increased significantly after heavy rain when the duration was 2 days. The concentration of total phosphorus and particulate phosphorus increased significantly after medium rain when the duration was 1 day. The concentration of phosphorus did not show an obvious change after light rain. The concentration of total nitrogen was the highest under light rain, and the concentration of total phosphorus was the highest under medium rain. The daily loadings of total nitrogen from the Dapu River to Lake Taihu were 7.64, 3.19, 3.21, and 2.62 t·d-1 under conditions of heavy rain, medium rain, light rain, and no rain, respectively. The daily loadings of total phosphorus were 0.59, 0.26, 0.22, and 0.20 t·d-1 under conditions of heavy rain, medium rain, light rain, and no rain, respectively. Although the daily loading of nutrients under heavy rain conditions was the highest, the contribution of annual nutrient loading was small during heavy rain days. The reason was that the percentage of heavy rain during the entire year was small. The total nitrogen loading in heavy rain days was 61.11 tons in the observatory year, which was only 5.6% of annual loading. The total phosphorus loading in heavy rain days was 4.72 tons, which was only 5.8% of the annual loading. These low percentages of nutrient loading under heavy rainfall are quite different from situations in mountain catchments. This research showed that the transport processes of nutrients in the plain river network was complicated and its influence on nutrient concentrations and loadings in connected rivers was slow and indirect. The amount of inflow water was an important source of external nutrient loading in the plain river network. These conclusions have reference value for estimating the external loading and deciding non-point control policy of lakes in plain river network of the Taihu catchment.

[Occurrence and Influencing Factors of Odorous Compounds in Reservoirs Used as Drinking Water Resources in Jiangsu Province].

In order to recognize the risk of odorous compounds and its driving mechanisms in water source reservoirs, the water quality, plankton, and odorous compounds of 17 provincial water source reservoirs in Jiangsu Province were investigated during a high-risk period of odorous compounds. A high eutrophication status, such as high algal biomass and low transparency, were widely observed in our study reservoirs. In addition, 2-methylisoborneol (MIB) exceeded the standard in some water layers of one-third of the reservoirs, of which the average concentration was (13.7±20.7) ng·L-1. Geosmin (GSM) was also detected in several reservoirs, although the maximum concentration of 4.6 ng·L-1 did not exceed the drinking water quality standard. With respect to the relationships between odorous compounds and environmental conditions, significant correlation (P<0.05) was noted between the MIB concentration and eutrophication indicators, including chlorophyll-a, Secchi depth, suspended solids, and comprehensive nutrition state index (TLI), particularly for chlorophyll-a and TLI (P<0.01). These results indicate that the risk of odorous compounds in water source reservoirs depend largely on the eutrophic status. Therefore, nutrient reduction, improvement in vegetation coverage of the reservoir basin, reasonable fishing practices are considered as effective strategies to avoid the risk of the odorous compounds in reservoirs.

[Effects of Hydrological and Meteorological Conditions on Diatom Proliferation in Reservoirs].

The proliferation of diatoms is an ecological disaster that harms the water quality of many reservoirs in China. In order to reveal the driving factors of abnormal algal blooms in reservoirs, phytoplankton community structure and the associated environmental factors from 2009 to 2016 from Shahe Reservoir in Tianmuhu were analyzed. Results showed that diatoms and there dominant genus were predominately driven by temperature, and the relationship between hydrology and nutrition was insignificant. Distinct relationships were recorded between different diatom genera and temperature over the past eight years. The optimum growth temperature of diatoms and Synedra were both 27℃, while that for Cyclotella and Achnanthes were both 19℃. Low temperature was suitable for the growth of Melosira, and its growth was compromised with increasing temperature. In a multivariable statistical analysis of the dominant diatom genus and the associated environmental factors, we found that the dominant diatom genus responded differently to the associated environmental factors. The total biomass of the diatom and the biomass of Synedra and Achnanthes were significantly and positively correlated with rainfall (P<0.05). The biomass of Cyclotella was significantly and positively correlated with total phosphorus, rainfall, and water level (P<0.05) and significantly and negatively correlated with water exchange rate (P<0.05). The biomass of Melosira was significantly and positively correlated with total phosphorus. The annual peak biomass of diatoms can be predicted by a multiple regression model with independent variables, including rainfall intensity, dissolved total phosphorus, and the accumulated temperature in winter and spring. Our results indicated that the proliferation of diatoms responds significantly to hydrological and meteorological factors while insignificantly to nitrogen and phosphorus loading. External load reduction is needed to maintain a good water quality in the reservoir. In addition, extreme weather conditions should be given attention to provide an early warning for diatom proliferation.

[Thermal Stratification and Its Impacts on Water Quality in Shahe Reservoir, Liyang, China].

Based on the one-year hourly water temperature profiles and the associated environmental drivers during the past eight years in Shahe Reservoir, Jiangsu Province, China from 2009 to 2016, the factors underlying the seasonal variation of thermal stratification and water quality response were investigated. It was shown that the thermal stratification was a typical subtropical one-cycle mixing model, lasting from May to September. The thermal stratification appeared and disappeared when the surface water temperature was 21 ℃ in the late spring and 19 ℃ in the middle of autumn. The difference between the water temperature at the epilimnion and hypolimnion increased with increasing solar radiation. When the air temperature was above 30 ℃, the stability of the thermal stratification increased. Heavy storms reduced the temperature of the surface water and weakened the temperature stratification of the column above a 5 m depth but had limited impact on the stratification of the hypolimnion deeper than 5 m. The thermal stratification greatly impacted the water quality of the lake. Hypoxia in the bottom water occurred by thermal stratification, leading to increased NH4+-N. The concentrations of dissolved oxygen, total phosphorus, and suspended solids in the hypolimnion increased after the disappearance of thermal stratification. Our results indicated that the thermal stratification was mainly controlled by solar radiation and the thermal stratification favored the growth of cyanobacteria and led to the release of nutrients from the sediment, threatening the water quality. Attention should be paid to thermal stratification to prevent algal blooms and related water quality deterioration.