| Hejiang is a tributary of Xijiang River system belong to Pearl River basin, is also the main source of drinking water of Hezhou city of Guangxi province, the safety of water quality is of great significance for social stability and economic development of Hezhou. Dinoflagellates and cyanobacteria blooms, occurring in spring and summer in the two large and medium-sized reservoirs(Guishi Reservoirs and Hemian-shi Reservoirs) respectively in 2014 of Hejiang River, had a great threat to the residents for water supply security. In order to explore the key factors influencing algal blooms in the two reservoirs, the community structure of phytoplankton and water quality were investigated and analyzed in wet and dry seasons in 2013, then analyzed the many factors contributes to blooms. Such methods like trophic state index(TLI),the Shannon-Wiener Index H’ and the Pielou evenness index J’ methods were used to analyse and evaluate the related research contents, using multivariate statistical analysis to analyze the impact of environmental factors on phytoplankton community composition. The object was to identify the key environmental factors of algae bloom in two reservoirs, and provide basis for algae bloom prevention and water quality management in Hejiang River. The main conclusions are as follows:1) A total of 130 species belonging to seven phyla of phytoplankton was identified. Including 63 species of seven phyla in wet season and 103 species of five phyla in dry season, phytoplankton community structure in Hejiang River was represented by Chlorophyta, Bacillariophyta and Cyanophyta. Chlorophyta was the dominant division in wet season, followed by Bacillariophyta. The mean value of phytoplankton density was 2.13×106 cells/L. Bacillariophyta dominated the phytoplankton in dry season, and the mean value of phytoplankton density was 3.71×106cells/L. The dominant species were Microcystis flos-aquae, Anabaena circinalis, Oscillatoria tenuis, Cryptomonas erosa, Navicula exigua, Aulacoseira varians, Aulacoseira granulate, Scenedesmus quadricauda, Actinastrum hantzschii, Chlamydomonas microsphaera, Chlorella valgaris.2) Based on RDA results, nitrogen and phosphorus were important environmental factors affecting the distribution of phytoplankton in Hejiang River. Hejiang River was classified as mesotrophic according to comprehensive trophic state index. The algal cell density content in Guishi reservoir is higher, up to 107 orders of magnitude,having higher potential risk of algal blooms.3) There are 66 species phytoplankton in Guishi Reservoir during cyanobacteria bloom, mainly in Chlorophyta, Bacillariophyta and Cyanophyta. The phytoplankton species in Xiakou(G3) sample point is single, others are more. The average of total cell density was 1.56×108cells/L, G3 sample point was highest, reached 5.36×108cells/L in blooms early. The cell density of Microcystis account for 98.09%, the corresponding chlorophyll a concentration is 74.48μg/L. on June 16 to June 19, the cells density of total algal range of 1.31×107~3.90×108cells/L, the Microcystis cell density ratio is 52.17%~96.22%. On June 26, cell density of total algal range of 4.76×106~9.60×106cells/L, Microcystis density range of 2.09×106 ~ 4.32×106cells/L, accounted for 43.78% ~ 49.92%.The dominant species Microcystis wesenbergii showed a gradual decreasing trend over time, and the cell density distributes at the surface and underwater 5 meters, showing a decreasing trend with depth.4) Guishi Reservoir was seriously polluted by exogenous pollution for its natural characteristics. The pollutants mainly from large-scale farming, agricultural non-point source pollution and rural domestic pollution. Therefore, we must strengthen the control of reservoir surrounding pollution, reduce the pollution caused by exogenous substances, thereby reducing the level of reservoir nutrition and preventing the occurrence of water algae blooms.5) Dinoflagellate bloom in Hemian-shi Reservoir was found in the reservoir dam only in the initial stage, serious bloom broken out in reservoir dam(1#) and Butou Dock(2#) on March 8-15, The cell density of Peridiniopsis niei was 6.64×107cells/L 和 2.39×107cells/L, 97.96% and 92.69% of total cell density, respectively, the corresponding chlorophyll a concentrations reached 833μg/L and 251μg/L. Blooms fade at the end of March, dinoflagellates cell density range of 3.32×105cells/L ~ 1.62×106cells/L, chlorophyll a concentration varied from 0.93 ~12.4μg/L. Xinzhong Dock(3#) found no water blooms, total cell density of algal range of 1.13×105~5.83×105cells/L, chlorophyll a concentration range of 0.06 ~1.97mg/L. Peridiniopsis niei was distributed at the surface to five meters underwater, showed a trend of decrease with increasing water depth.6) RDA analysis shows that the Peridiniopsis niei was significantly positive correlated with the pH, permanganate index and nitrogen and phosphorus, have positive correlation with hydraulic retention time(HRT). Rainfall is a major cause of leading to the blooms fade, thus hydrodynamic factors were essential for the algae blooms fading in the nutritional water, we can prevent the occurrence of algae bloom by hydrodynamic control. |
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