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

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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.