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Shiyan Reservoir is located in a typical high-density built-up area in Shenzhen, and its water environment quality and water ecological health are key to the safety and security of Shenzhen's drinking water. The 13-way water quality and water ecological indicators of Shiyan Reservoir were monitored for 14 months. The chlorophyll a concentration in Shiyan Reservoir ranged from 21.25 to 88.77 μg/L. The temporal heterogeneity of the chlorophyll a concentration was obvious, and the variation was drastic; the highest chlorophyll a concentration was from April to July, which was maintained at about 85 μg/L, and the risk of algal bloom was high. The annual average water temperature of Shiyan Reservoir is 25.31℃, and the water temperature from May to September is about 30℃ for a long time, so the risk of algal bloom is high. The COD of the reservoir is 1.71~3.08 mg/L, and the TOC is 2.22~5.13 mg/L. The organic pollution of the reservoir is very low, and it reaches the environmental quality of surface water Ⅰ standard. Transparency is 0.73~1.16m, relative to 2010~2015, the transparency of Shiyan Reservoir increased significantly. Shiyan Reservoir TN pollution is serious, during the investigation period, TN is 1.66~2.49 mg/L, for Class V water quality. In recent years, Shenzhen has carried out a systematic water environment comprehensive treatment project, TN decreased significantly, but it is still the primary pollutant in Shiyan Reservoir. Shiyan Reservoir TP 0.01 ~ 0.043 mg / L, total phosphorus concentration is low, to achieve the environmental quality of surface water Ⅰ ~ Ⅱ class standards. The monthly average integrated trophic state of the reservoir is located between 43.13 and 53.81, with an annual average value of 50.49. The monthly average integrated trophic state of April and November 2021 and April and May 2022 is less than 50, which is in the state of light eutrophication, and the other months are in the state of moderate eutrophication. The correlation analysis showed that water temperature was the primary influence factor of phytoplankton in flood season, and the correlation coefficient was as high as 0.69. The main correlation factors of phytoplankton in flood season were WT, TOC, nitrate nitrogen, and SD, and the main correlation factors of phytoplankton in dry season were TOC, DO, and WT. Throughout the whole year, phytoplankton was highly positively correlated with WT, TOC, moderately positively correlated with COD, pH, and turbidity, and moderately negatively correlated with transparency (SD). transparency (SD) was moderately negatively correlated. Two-factor ANOVA showed that the variance of each environmental factor was better explained by the period and sampling site and the interaction effect between the two (R2 ranging from 23.7% to 78.1%). The main influence of environmental factors in Shiyan Reservoir was period, and the temporal heterogeneity of environmental factors was significantly higher than the spatial heterogeneity. A total of 21 genera of cyanobacteria, 24 genera of diatoms, 39 genera of chlorophyta, 3 genera of euglenophyta, 2 genera of pyrrophyta, 1 genus of chrysophyta and 2 genera of cryptobranchs were detected in Shiyan Reservoir. Cyanobacteria dominated from May to October, and diatoms dominated from November to February. There were 9 species of cyanobacteria were dominant in Shiyan Reservoir, including Raphidiopsis, Oscillatoria, Phormidium, Planktothrix, Anabaena, Merismopedia, Chroococcus, Microcystis and Snowella.
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