[Evolution Characteristics and Driving Factors of Denitrification Community Based on Network Analysis in the Process of Spring Thermal Layer Formation in Zhoucun Reservoir].

Combined with on-site water quality investigation and nirS gene high-throughput sequencing technology, the evolution characteristics and influencing factors of the denitrification community during the formation of spring thermal stratification in Zhoucun Reservoir were analyzed. The results show that the water body stratification gradually formed during this period, and the environmental factors (NO3-, NH4+, TN, TOC, BOD5, permanganate index, TP, Fe, and Mn) showed significant differences (P<0.01); nitrogen showed a significant decline process. High-throughput sequencing provided 8703 OTU, which were divided into three phyla and eight major genera, proteobacteria accounted for the largest proportion with 45.27%-78.90%. The α-diversity except for the Simpson index showed that the ACE index, Chao index, Shannon index, and coverage index showed significant differences (P<0.05). The principal coordinate analysis showed the denitrification community exhibited significant differences in the spring, which was consistent with adonis result (P<0.001); network analysis (OTU-OTU) showed that there were seven main modules in this period, including 316 edges of 131 nodes, and the proportion of positive correlation edges was 95.25%. Network analysis (OTU-environmental factors) showed that there were five modules in this period, including 329 edges of 140 nodes, and the proportion of positive correlation edges was 51.98%. Sixty-two indicator OTU and 28 keystone OTU were obtained based on the indicator OTU analysis and network analysis. RDA and mantel test analysis indicated that T, DO, NO3-, TN, TOC, BOD5, and TP were the main environmental factors driving the denitrifying bacterial community structure and the key denitrifying OTU evolution in spring. Our results will provide technical support for the migration and transformation of nitrogen in reservoir water and pollution control.

[Response of the Water Quality of a Stratified Reservoir to an Extreme El Niño Event During Summer].

Global warming can intensify the El Niño phenomenon that recurs every 2-7 years, which will lead to a great interannual variability of climate and may induce the deterioration of the water quality of reservoirs. To study the influence of the extreme El Niño events on the water quality of stratified reservoirs during summer, field surveys were conducted in Zhoucun Reservoir and its inflow rivers from May to August in a normal year (2012) and a strong El Niño year (2015). Temporal variations of physical and chemical index were investigated during monitoring. The results showed that the Zhoucun Reservoir was stratified during the study period. The precipitation in the summer of the normal year was significantly higher than that in the El Niño year at the same period. In the summer of the normal year, the water level increased from 124.26 m to 127.14 m and the hypolimnion thickness increased by 3.1 m. However, in 2015, the rapid decrease of the water level from May to August (from 121.65 m to 119.46 m) led to the decrease of the hypolimnion thickness (by 3.2 m). The inflow rivers belonged to surface current and its nutrients concentrations were obviously higher than those in the epilimnion. The inflow nutrients loads increased significantly in the summer of the normal year, as a result, total nitrogen increased from 1.00 mg·L-1 to 2.06 mg·L-1, nitrate increased from 0.19 mg·L-1 to 1.28 mg·L-1, and total phosphorus increased from 0.023 mg·L-1 to 0.088 mg·L-1 in the lacustrine zone of the reservoir. In contrast, the nutrients concentrations changed little in the summer of the El Niño year due to the decrease in runoff. Nonetheless, the reducing pollutants concentrations of the hypolimnion in the El Niño year were significantly higher than those in the normal year, which may be due to the temporal variations of hypolimnion thicknesses. The maximum concentrations of iron, manganese, ammonium and sulfide in the summer of the El Niño year were 0.38, 1.36, 2.36 and 1.67 mg·L-1, respectively. All these index exceeded the standards for surface water Class Ⅲ. We conclude that the extreme El Niño event has an apparent influence on the nutrients concentrations in the epilimnion and the pollutants concentrations in the hypolimnion in Zhoucun Reservoir.

[Seasonal Stratification and the Response of Water Quality of a Temperate Reservoir--Zhoucun Reservoir in North of China].

In order to reveal the seasonal stratification and the response of water quality of a temperate reservoir Zhoucun Reservoir in North of China. The physical, chemical and phytoplankton indexes were continuously monitored from July 2014 to June 2015. The results showed that the thermal stratification was monomictic, which lasted from April to October. The thermal stratification played an important role in the change of water environment. The hypoxia in the bottom water caused hy thermal stratification led to the release of nutrients and reductants from sediment. During the stratification period, the mean concentrations of total nitrogen, total phosphorus, manganese and sulfide were 1.18, 0.11, 0.47 and 0.48 mg · L⁻¹, respectively. The vertical distribution of phytoplankton was significantly influenced by water temperature stratification. During the thermal stratification period, phytoplankton abundance was higher and the average phytoplankton ahundance was 16.35 x 106 cells · L⁻¹ in the upper water of the reservoir, while remained at low levels at the bottom.

[Analysis of Distribution Characteristics and Source of Dissolved Organic Matter from Zhoucun Reservoir in Summer Based on Fluorescence Spectroscopy and PARAFAC].

The fluorescent components were examined using excitation emission matrix fluorescence spectroscopy-parallel factor analysis technique for samples collected in August, 2015 from Zhouncun Reservoir. Principal component analysis was used to study the main factors and their relative contributions to DOM. Three fluorescent components were identified by PARAFAC, including fulvic-like component(C1:260,350/420 nm), protein-like(C2:280/360 nm) and humic-like (C3:270,390/530 nm) which showed the same source. The even spatial distribution of each component, higher total fluorescence intensity in storage port, high fluorescence index, high biological index, low humification index and the freshness index which was close to one showed that the DOM had a strong autochthonous contribution. The results of PCA showed that the autochthonous contribution reached 70.86%, and those three components of Zhoucun Reservoir could be connected with aph(440) by nonlinear multiple regression which means we can use the three-dimensional fluorescence spectrum results of DOM to control the pollution sources and indicate the eutrophication degree of Zhoucun Reservoir.

[Optical Characteristics of Dissolved Organic Matter from Two Different Full Mixed Reservoirs in Winter Based on UV-vis and EEMs].

The differences in the resource and characteristics of DOM between Zhoucun Reservoir and Jinpen Reservoir were studied by fluorescence ultraviolet-visible (UV-vis) and excitation-emission matrix spectra combined with parallel analysis (EEMs-PARAFAC). The results showed that three fluorescent components were identified by PARAFAC model in Zhoucun Reservoir and Jinpen Reservoir, including long wave humus-like component (C1: 350/460 nm), visible fulvic-like (C2: 335/410 nm) and protein-like (C3: 260,285/360 nm). However, the fluorescence intensity and the relative proportions of DOM exhibited significant difference (P<0.01) in two reservoirs. Moreover, the concentration of protein-like component in Zhoucun Reservoir was higher than that in Jinpen Reservoir, while the concentration of humus-like component presented the opposite trend. Based on the comparison of fluorescence index, biological index, humification index and the freshness index, the DOM of Zhoucun Reservoir where the land-use type was cultivated, livestock and residential, the internal pollution of sediments was serious, indicating a strong autochthonous component and aquatic bacterial origin, whereas the DOM of Jinpen Reservoir dominated by forest system had a higher terrigenous contribution in winter. From all the results, hydrological conditions of the reservoir and the characteristics of coastal ecological environment were important factors to influence the DOM sources and characteristics.