Algal Blooms Monitoring And Risk Assessment Of Cytotoxicity Induced By Microcystins In Reservoirs

Eutrophication problems in aquatic ecosystems have deteriorated as the economy advances leading to algal blooms and microcystins (MCs) production that pose significant risks to drinking water security worldwide, then monitoring techniques of algal blooms and associated microcystin, and methods for aquatic health risk assessment have recently gained increasing attention, especially in China.This dissertation focuses on the pollution of algal blooms and associated MCs in reservoirs, recent advances on health risks caused by algal blooms and associated toxins, production mechanism of microcystins, and the relationship between environmental factors and microcystins were discussed in this dissertation. We selected Qingshan Lake Reservoir in Lin’an of western Hangzhou as the study region, to study the spatial and temporal distribution of microcystin, to explore major nutrients (NH4+and NO3-) and environmental factors regulating harmful algal growth and microcystin production. Furthermore, an original method of assessing cytotoxicity induced by microcystin pollution was established. We combined Landsat ETM+remote sensing images with field measurements of algal biomass (Chloropyll-a, Chl-a) to establish a quantitative multiple linear regression model with water quality inversion techniques. Moreover, three common freshwater diatoms (Nitzschia sp., Cyclotella meneghiniana, Gomphonema parvulum) in Taihu Lake watershed were selected, to determine the influence of different nitrogen species on diatom indicators, and its sensitivity difference of common freshwater diatoms under stress of different nitrogen source. The main conclusions are listed as follows:(1) The dominant algae and seasonal distribution in Qingshan Lake was identified and major environmental factors regulating algal dominance and microcystin distribution were determined in QingshanLake. Algal succession in Qingshan Lake was Diatom (winter)--green algae (winter-spring)--Cyanobacteria (summer, dominated by Microcystis, followed by Nostoc, Anabaena, and Oscillaloria);HPLC-MS and ELISA were used to measure MCs concentrations in surface water samples collected from Qingshan Lake, MCs concentrations ranged from0.00±0.00-1.28±0.05μg/L (MCLR); principal component analysis indicated that MCs production was mainly regulated by total phosphorus (TP), water temperature and cyanobacteria biovolume (CBV). The relative abundance of cyanobacteria positively related to TP (r=0.53, p<0.001) implies that the reduction of TP load is the key measure to control algal blooms, microcystin production and ensure drinking water safety.(2) A Chl-a concentrations prediction model based on typical month in the summer for Qingshan Lake was proposed. Landsat7ETM+images were combined with field monitoring data to establish multiple linear regressions, linear regressions model in May was Log(Chl-a)=1.65+0.87*(ETM+l/ETM+3)-3.39*(ETM+2/ETM+3)+0.89*(ETM+3/ETM+4)(R2=0.72, p<0.01), and linear regressions model in August was Log(Chl-a)=2.94-1.37*(ETM+l/ETM+3)+0.40*(ETM+2/ETM+3)-0.20*(ETM+3/ETM+4)(R2=0.92, p<0.001), and it turned out that the models effectively predicted Chl-a concentrations distribution and eutrophication pollution characteristics in Qingshan Lake. The regression models can help identifying and early warning of algal blooms.(3) An original method assessing cytotoxicity induced by MCs was established based on EC50values of fish lymphocyte apoptosis. Typical apoptosis in Grass carp lymphocyte was detected after exposure under lnmol/L MCLR or MCRR for2h, significant dose-effect was determined with increasing MCs concentrations. Similar typical apoptosis in Grass carp lymphocyte and significant dose-effect was also determined after exposure under lnmol/L MCLR or MCRR with1,5,10,50100nmol/L CHCl2Br or CHClBr2for2h. Joint effect coefficient analysis indicated that the joint effect between MCLR-CHCl2Br, MCRR-CHCl2Br, MCLR-CHClBr2and MCRR-CHClBr2were all additive, implying the joint pollution between microcystins and disinfection by-products will enhance the threat to drinking water safety caused by microcystins joint pollution.(4) The sensitivity difference of common freshwater diatoms under stress of different nitrogen source was confirmed. To explore the possibility of diatoms acting as competitive species against cyanobacteria, different sensitivity of three common freshwater diatoms(Cyclotella meneghiniana, Nitzschia sp. and Gomphonema parvulum) in Taihu watershed to different nitrogen species was studied, transmission electron microscopy (TEM) was used to characterize the ultrastructural changes of chloroplast. Response of photosynthetic and antioxidant enzyme systems indicated that Cyclotella meneghiniana was the most sensitive indicator under NH4+stress, Gomphonema parvulum and Nitzschia sp. has more tolerance under NH4+stress on the growth than C.meneghiniana. NH4+has direct stress on photosynthetic system of C.meneghiniana, and the stress can be alleviated by the coexistance of NO3, which may be caused by different transport system of NO3-. Transmission electron microscopy (TEM) results showed that a large number of electronic dense dark particles were identified in the chloroplast, but the integrity of the membraneof Nitzschia sp. may be due to a transitional adaptation mechanism under NH4+stress.