[Wind Field Influences on the Spatial Distribution of Cyanobacterial Blooms and Nutrients in Meiliang Bay of Lake Taihu, China].

Wind field is a very important physical factor controlling the formation of cyanobacteria blooms. A surface particle tracking drift experiment was carried out to study the influence of wind field on the surface current in Meiliang Bay of Lake Taihu during the algal bloom season. For this, chlorophyll-a, nitrogen, phosphorus, the permanganate index, dissolved organic carbon (DOC), and dissolved oxygen (DO) were measured in surface, middle, and bottom waters of the Meiliang Bay during the cyanobacteria bloom period to test how wind field affects the temporal and spatial distribution of cyanobacterial blooms and biomass stock in the water column. The results showed that the average drift velocities of surface particles were 3.0 cm·s-1 and 5.0 cm·s-1 when wind speed averaged 1.9 m·s-1 and 2.3 m·s-1, respectively. The wind field determined the spatial distribution of cyanobacterial blooms in surface waters and led to a high spatial heterogeneity of cyanobacterial blooms. The spatial redistribution of cyanobacterial blooms exerted an important influence on water quality indexes such as particulate nitrogen, phosphorus, organic matter, and dissolved oxygen. The concentrations of particulate nitrogen, phosphorus, the permanganate index, and chlorophyll-a showed a similar vertical distribution pattern. Cyanobacterial blooms were less influenced by the distribution of dissolved nitrogen and dissolved organic carbon from external pollution, while long-term legacy loading played a more important role. This meant that the spatial distributions of dissolved nitrogen and dissolved organic carbon were different from that of chlorophyll-a. Because the redistribution of cyanobacterial blooms, as affected by wind fields, has a complex effect on the dissolved oxygen in the water column, the dissolved oxygen concentration decreased with depth, which may affect the release of soluble nutrients from the sediment. The cyanobacterial biomass stock in the surface water was estimated according to the survey of high-density sites. The dry matter of cyanobacteria in the surface 20 cm of Meiliang Bay was approximately 396 tons on the day of sampling. The results from the present study indicated that the factors influencing cyanobacterial blooms should be considered in sampling methods and the analysis of lake water quality due to the significant influence of wind fields on bloom drift. The collection of cyanobacteria has limited effect on the removal of the algal bloom biomass in whole lake, only being effective at prevention of the event of black spots in lake shore.

[Effects of Cyanobacterial Blooms in Eutrophic Lakes on Water Quality of Connected Rivers].

The location at which the Liangxi River meets Meiliang Bay of Lake Taihu was selected to explore the influence and controlling factors of algal blooms on the connected rivers. The cyanobacterial particulates and water quality parameters including nitrogen, phosphorus, and dissolved oxygen were monitored daily during the summer bloom season. The spatial variation in water quality parameters along the river were also surveyed, and data from hydrological and meteorological factors such as discharge, temperature, and wind field characteristics over the same period were collected to help investigate the effects of cyanobacterial blooms on the water quality of connected rivers. The results showed that the total fresh biomass of algal blooms entering Liangxi River for three months during the summer cyanobacterial bloom period in Meiliang Bay was 9733 t, which was similar to the amount harvested from the entire lake throughout the year. The flux of water bloom particles to the river varied widely, ranging from 75-496 t·d-1, with an average of 105 t·d-1. The flux was primarily controlled by hydrological and meteorological conditions. Water volume, temperature, and wind direction were the most important influencing factors. Lake water with a large amount of cyanobacterial particles entering the river can significantly improve the dissolved oxygen, ammonia-nitrogen, and other qualities of the river water in the city. It also significantly increased the concentrations of nutrients such as particulate nitrogen and phosphorus in the river, while it had a relatively weak effect on dissolved nitrogen and phosphorus. Spatially, the chlorophyll a concentration rapidly decreased with increasing distance from the lake, and the concentration of nitrogen and phosphorus exist as particulate cyanobacteria also declined. Upon being discharged into the canal 7 km away, the cyanobacterial particles were largely decomposed, and chlorophyll a content from the phytoplankton collected from the river by a 67 µm net decreased from 152.93 µg·L-1 to 1.99 µg·L-1. The results indicated that lakes plagued with cyanobacterial blooms have a great impact on the water quality of surrounding rivers. Although the connection between rivers and lakes can effectively relieve black spots and solve the black and odorous phenomenon in urban rivers, it had a great impact on nutrients concentration in the rivers. Depending on the different protection targets of rivers and lakes, lake water blooms and hydro-meteorological factors should be considered during water transfer management to optimize the water ecological services of lakes and rivers.

[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.