| Since the filling of the TGR, algal blooms have been detected in the DaNing River. Eutrophication and its important characteristics referred to algal bloom have became major problem of aquatic ecosystem degradation. Therefore, it is necessary to obtain in-depth information on the developmental mechanicsms of algae blooming and the regulating mechanisms of eutrophication. To elucidate the importance of regional and local factors controlling entrophication and algal bloom and design effective observation programs, the temporal coherence, the critical depth theory and the related monitoring data were carried out in the DaNing River. The results are mainly as follows:Hydrodynamic characteristics and its causing factors were investigated in the DaNing River. The results indicate that the hydrodynamics of the DaNing river could be generalized as a density-stratified flow. The upstream water were mainly flowed out the tributary by downslope-bottom density, meanwhile, the TGR mainstream water were flowed in the tributary by reverse density current. The difference of water temperature between the TGR mainstream and tributary is key regulatory factor for density current formation.The aim of this study was to understand the spatial and temporal variation of the entrophication and its regulating mechanisms in the DaNing River during experimental impounding period. The results indicated that most of the trophic states of mainstream and tributary were oligotropher to mesotropher, and the DaNing River has a comparatively high spatial and temporal heterogeneity in the entrophication. The degree of eutrophication in flood season was higher than other seasons, and the degree of eutrophication tended to decrease from mainstream to upstream. Eutrophic state of mainstream and tributary were dominated by suspended solids and organic pollutant under regional hydrological effects. Suspended solids and organic pollutant of mainstream and tributary exhibited strong response to hydrological condition, and significant high coherence was found among sites. By contrast, temporal coherence for total nitrogen, total phosphorus and chlorophyll-a in the DaNing River was lower due to weak hydrological response.To elucidate relationships between phytoplankton and related environmental factors, phytoplankton and environmental parameters of water quality in the Daning River were conducted using multivariate statistical analysis. Most of the investigated physico-chemical parameters (water temperature, total nitrogen, nitrate-N (NO 3—N),pH anddissolved oxygen concentration(DO)) in Daning River are significantly different among those sampling sites(ANOVA, P<0.05).The results indicate that the DaningRiver has a comparatively high spatial and temporal heterogeneity in the plankton community composition (ANOVA, p<0.05). The maximum cell density was recorded in DaChang. The minimum density, appearing at Wu Xia-kou sites.Multi-algal species blooms was observed in the same time and place with the maximum density appeared. The results of Redundancy Analysis (RDA) indicate that main soluble nutrients (TN, TP, NO 3—N and NO 2—N), suspended substance (SS) and transparency contributed significantly to phytopla-nkton community composition. Phytoplankton growth could enhance the pH value. The environmental characterizations and sample sites adjacency contributed significantly to phytoplankton community compsotition.Seasonal variation of primary productivity and its causing factors were investigated during the algae bloom period and dry season in the backwater area of the DaNing River. The results indicate that most of the investigated physico-chemical parameters (water temperature, DO concentration (saturation), SPC (specific conductivity), pH, TUR (Turbidity)) in the DaNing River are significantly different betweenthe algae bloom period and dry season (ANOVA, p<0.05). The results suggest that primary productivity and chlorophyll-a in algae bloom period are higher than those in the dry season (ANOVA, p<0.05). The vertical distribution of PG (Gross primary productivity), PN (net production) and Chl-a (chlorophyll-a) were not significantly different. The maximum of chlorophyll-a concentrations were obser-ved at 9 m underwater, and the maximun of PN,PG were observed from Om to 3m underwater. The results indicate that the DaNing River backwater area has a high spatial and temporal heterogeneity in the Gross primary productivity, net production and chlorophyll-a. The maximun of Gross primary productivity, net production and chlorophyll-a were observed from Om to 0.8m underwater. The results suggest that the water temperature and Turbidity are key regulatory factors for primary production. The changes of pH, DO concentration (saturation) and SPC may be the results of the change of the phytoplankton.Hydroligical condition contributed significantly to vertical distribution and accumulation of phytoplankton. According to observations, thermocline was influenced by increasing temperature in summer. The results show that a clear thermal stratification could cause the stratification of the physical and chemical parameters. The results indicate that vertical water column stability may not be prerequisite to spring blooms in the DaNing River.An enclosure experiment in the DaNing River was performed to examine the effect of mixing intensiy on the abundance, community composition and vertical distrubuton of phytoplankton. The results indicate that the mixing intensity is a key regulatory factor for phytoplankton community composition. The results show that the suitable mixing intensity is benefit for the growth rate and homogeneous distribution.
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