[Effects of Different Water Stratification on the Vertical Distribution of Nitrogen in Sediment Interstitial Waters: A Case Study of the Three Gorges Reservoir and Xiaowan Reservoir].

To determine the reasons for the variation in the vertical distribution of nitrogen in sediment interstitial waters between different stratified reservoirs, the characteristics of overlying water-interstitial water in Xiangxi Bay, Yangtze River mainstream, and Xiaowan Reservoir were monitored. The vertical distribution of nitrogen in sediment interstitial waters in these different stratified waters were then analyzed, and the reasons for the variation in this distribution were assessed. The results showed:① the ρ(TN) in the sediment interstitial waters of the Yangtze River mainstream and Xiangxi Bay gradually increased with depth, while that of Xiaowan Reservoir reached its maximum at 12 cm and the bottom layer presented a "C" distribution. The ρ(NH4+) in the sediment interstitial waters of the Yangtze River mainstream and Xiangxi Bay exhibited an increasing trend with depth, while that of Xiaowan Reservoir was slightly higher in the bottom layer than in the surface layer, although the change with depth was not significant. Overall, the ρ(NH4+) in the sediment interstitial waters of the Yangtze River mainstream and Xiangxi Bay was higher than that of Xiaowan Reservoir, and the concentration ranges were as follows:0.512-8.289 mg·L-1, 0.968-9.307 mg·L-1, and 0.950-1.450 mg·L-1. The vertical distribution of the ρ(NO3-) in the sediment interstitial waters of all three waterbodies were opposite to that of ρ(NH4+). Moreover, the ρ(NO3-) in the sediment interstitial waters of Xiangxi Bay and the Yangtze River mainstream was higher than that of Xiaowan Reservoir. The concentration ranges were as follows:0.143-0.674 mg·L-1, 0.107-0.647 mg·L-1, and 0.050-0.051 mg·L-1. ② There were also significant differences in the vertical distribution of physical and chemical indices in the three water bodies. There was no significant change in the vertical distribution of the water temperature in the Yangtze River mainstream and the N2 value was <5×10-5 s-2; hence, the water was well mixed, and the vertical range of the dissolved oxygen content was 6.180-6.318 mg·L-1. The water temperature in the upper and middle reaches of Xiangxi Bay decreased vertically, while the water temperature in the lower reach presented a ladder-like distribution and the N2 values were all>5×10-5 s-2; thus, the water was in a stable stratified state and the dissolved oxygen content presented a "C" distribution. There was obvious stratification at the depths of 5-15 m and 54-70 m in Xiaowan Reservoir. The dissolved oxygen content decreased significantly at higher water temperature gradients, and there was no significant change along the water depth below 80 m. ③ The main reasons for the variation in the vertical distribution of nitrogen in the sediment interstitial waters of the three waterbodies were the differences in the overlying water hydrodynamics, dissolved oxygen distribution, and sediment environment. The ρ(NH4+) and ρ(NO3-) were higher in Xiangxi Bay, which may have increased the denitrification rate and subsequently have helped to remove nitrogen and reduce the nitrogen load in these waters.

[Relationship Between the Vertical Distribution of Nutrients and Bacterial Community Structures in Sediment Interstitial Waters of Stratified Reservoirs with Different Water Temperatures].

In order to study the relationship between the vertical distribution of nutrients and bacterial community structures in sediment interstitial waters of stratified reservoirs with different water temperatures, MiSeq high-throughput sequencing was used to analyze and compare the structural characteristics of sediment bacterial communities after reservoirs were built. Additionally, redundancy analysis (RDA) was used to assess the bacterial communities and environmental factors with Cannoco software. The results showed that the temperature difference between the surface and bottom layer of the Xiaowan Reservoir was 3.3℃, and the maximum thermal gradient was 0.2℃·m-1; thus, it was a typical stratified reservoir. The temperature difference between the surface and bottom layer of the Manwan Reservoir was 0.1℃; thus, it was a typical mixed reservoir. The average concentrations of NH4+-N and NO3--N in sediment interstitial waters of the Xiaowan Reservoir were 2.233 mg·L-1 and 0.030 mg·L-1, while those of Manwan were 2.569 mg·L-1 and 0.016 mg·L-1, respectively. In the different reservoirs, the concentrations of NH4+-N showed upward trends, and while variation of NO3--N was not obvious, the content of NO3--N reached a minimum value in the deep layer. In comparisons between reservoirs, only NO3--N showed a significant difference, in which Xiaowan had obviously higher concentrations than Manwan. The bacterial community structures in the Xiaowan and Manwan reservoir sediments had the same dominant bacteria at the phylum, class, and genus levels. The differences of water temperature stratification had no significant effect on nutrients and microorganisms in the sediments. Under the influence of other factors, the denitrifying bacteria in the Manwan Reservoir sediments were more abundant than those in the Xiaowan Reservoir, and the nitrifying bacteria and anammox bacteria in the Xiaowan Reservoir sediments were more abundant than those in the Manwan Reservoir. In the same reservoir, the denitrifying bacteria in the bottom of the sediments were more abundant, and the organic degradation bacteria, nitrifying bacteria, anammox bacteria, and phosphate-solubilizing bacteria were less abundant in this zone. These trends contributed to the differences of nutrients vertically in the different reservoirs.

[Analysis of Total Organic Carbon Source Differences Between New and Old Cascade Reservoirs using Carbon and Nitrogen Isotopes].

The ecological problems due to reservoir construction are causing unprecedented concern. To reveal the differences in organic carbon distribution characteristics and sediment sources of total organic carbon (TOC) between the old and new reservoirs, water samples, and sediment samples from reservoirs constructed in the three different periods of Miaowei, Gongguoqiao, and Dachaoshan were collected in November 2017. The temperature (T), dissolved oxygen (DO), TOC, redox potential (ORP), total nitrogen (TN), and total phosphorus (TP) of the water samples were measured. The isotopes 15N and 13C were used as indicators with IsoSource software to analyze the contributions of TOC sources and their source materials to the corresponding reservoir sediments, in order to explore the carbon cycle mechanism and evolution mode of reservoir. The results showed that the average concentrations of organic carbon in the waters of the Miaowei, Gongguoqiao, and Dachaoshan Reservoirs were 0.95 mg·L-1, 1.97 mg·L-1, and 4.64 mg·L-1, respectively. The range of organic carbon content in the corresponding sediments was 4.41-81.63 g·kg-1, 18.30-28.42 g·kg-1, and 9.16-14.46 g·kg-1, respectively. The cascade construction of the reservoirs resulted in a difference between the sediment sources of the new and old reservoirs and the surrounding recharge area, meaning that the TOC of the new and old reservoirs were significantly different. For the TOC of waterbodies, the difference between the thermodynamic state of water and dissolved oxygen indirectly affects the distribution trend of TOC. The sediments mainly reflect the influence of source elements, that is, the ability of the sedimentary environment to preserve organic matter was the main cause of the vertical distribution of DCS, MV, and GGQ sediments. In the evolution mode of cascade reservoir, the research shows that it can be preliminarily set as three stages. Firstly, due to the short age of MV, it is in the first stage and mainly accumulates the TOC from the upstream. GGQ is longer than the age of MV, and it is mainly used to decompose the upstream TOC, so it is defined as in the second stage. Finally, as an old reservoir, DCS mainly accumulates TOC sources around the reservoir, which can be regarded as the third stage.

[Influence of Rainfall on the in situ Growth of Dominant Algae Species in Xiangxi River].

To understand the influence of rainfall on the in situ growth (in a culture cage) of dominant algae species in the Xiangxi River tributary of the Three Gorges Reservoir, culture experiments were carried out to measure the biomass of Microcystis aeruginosa, Chlorella vulgaris, and Chlorella aeruginosa before and after rainfall. The results showed that ① during the study period (October 4-18, 2017), there were significant differences in hydrodynamic conditions between the rainfall period and the non-rainfall period (ANOVA, P<0.05). Total Chl-a and the specific growth rate of the three main algae during rainfall period were significantly lower than during the non-rainfall period, which inhibited algae growth to some extent. The results of correlation analysis showed that the four hydrodynamic parameters characterizing vertical mixing had a highly significant negative correlation with the specific growth rate of the three dominant algae species. Meanwhile, the changes of shear force τ, the vertical turbulent viscosity coefficient Vr, and the vertical turbulent diffusion coefficient Vt were the key factors leading to the rapid decline of algae; ② the depth of the mixed layer was lower (1-2 m) before rainfall, but increased (>5 m) markedly after rainfall (October 10-18). At the same time, the concentration of Chl-a during the rainfall period was significantly lower than that during the non-rainfall period. The results of the correlation analysis showed that there was a highly significant positive correlation between the ratio of eutrophic depth to mixing depth (Zeu/Zmix) and the specific growth rate of the three dominant algae species. This indicated that the vertical disturbance of water was enhanced by rainfall, and mixing layer expanded continuously, which reduced the water temperature stratification and thus inhibited the growth and proliferation of algae; ③ there were significant differences in rainfall, water temperature, light intensity, total nitrogen, and dissolved total nitrogen between the rainfall period and non-rainfall period (ANOVA, P<0.05). Correlation analysis showed that changes in rainfall, water temperature, light intensity, total nitrogen, and dissolved total nitrogen caused by rainfall were the key environmental parameters affecting the in situ growth rate of three dominant algae.

[Spatial Distribution Characteristics of Nutrients and Chlorophyll A in the Lancang River Basin Under Cascade Reservoirs].

Construction of cascade reservoirs in Lancang River had a certain influence on the ecological water environment of the basin, dividing the water into natural reaches and reservoir reaches. To explore the impact of the cascade reservoirs on the ecological environment, the nutrient and the chlorophyll a distribution data were measured in the natural reaches, reservoir reaches, and tributaries in June 2017. The results showed that the TN ranged from 0.37 to 1.22 mg·L-1, with an average of 0.70 mg·L-1, and the TP ranged from 0.01 to 0.19 mg·L-1, with an average of 0.04 mg·L-1. One-way ANOVA analysis showed significant spatial differences in the TN, with the TN concentration following the order tributaries > reservior reaches > natural reaches. However, the TP concentration did not show a significant trend. The chlorophyll a concentration varied from 2.6 to 10.2 µg·L-1, with a mean of 5.8 µg·L-1. Pearson correlation analysis showed significant relationships between the chlorophyll a concentration and the TP, water temperature, and Zeu/Zmix. The relationship between chlorophyll a and Zeu/Zmix was particularly significant, which indicated the Zeu/Zmix caused by thermal stratification may be the key factor influencing the growth of phytoplankton.

[Speciation and Transformation of Phosphorus in Sediments During the Redox Cycle].

To study the mechanism of phosphorus cycling in sediment during the redox cycle, changes in physicochemical properties of overlying water and various forms of phosphorus in sediments were investigated as a way to quantify the redistribution of phosphorus. Additionally, the effect of the release flux of phosphate from sediments under controlled redox conditions was analyzed. The results showed that the redox potential Eh and the pH system, sulfur system, carbon system, and iron-related changes exhibited periodicity and played an important role in explaining the migration and transformation mechanism in the interface phosphorus of the sediment-water phase. During the redox cycle, the phosphorus content of each species varied with the redox conditions and time. Because of this, quantitative analysis based on changes in water-sediment phosphorus could be obtained. Reducible phosphorus (BD-P) and iron-aluminum-bound phosphorus (NaOH-rP) were reversibly redistributed into weakly adsorbed phosphorus (NH4Cl-P), polyphosphorus/organophosphorous (NaOH-nrP), residual phosphorus (Rest-P), and interstitial water-soluble active phosphorus (SRP). Additionally, 93.7% of phosphorus in the sediment was not released into the water phase during the reduction reaction. The 92% of change in the overlying water total phosphorus (TP) was the SRP of overlying water, which showed that the exchange of the sediment-water phase were mainly soluble active phosphorus in this cycle. According to Fick's First Law, the maximum phosphorus flux was 0.58 mg·(m2·d)-1 during reduction and 0.16-0.22 mg·(m2·d)-1 on day seven of the oxidation phase. In the oxidation stage, the diffusion flux decreased with time, while the opposite trend occurred in the reduction reaction. This indicated that the anaerobic state accelerated the diffusion of phosphorus in sediments, and that oxygen exposure decreased the phosphorus flux in sediments.

[Distribution Characteristics and Release Fluxes of Phosphorus Forms in Xiangxi Bay Sediments in the Three Gorges Reservoir Before and After Impoundment].

The sediment storage environment in tributaries has been altered by impoundment of water in the Three Gorges Reservoir area, affecting the distribution of phosphorus forms in sediment and processes at the sediment-water interface. Through collection of sediment and overlying water samples in Xiangxi Bay in August 2016 (before impoundment) and October (after impoundment), the distribution characteristics of sedimentary phosphorus and the environmental conditions of storage before and after impoundment were analyzed. Fluxes of PO43--P at the sediment-water interface were also estimated. Results show that pH increased, alkalinity and reducibility were enhanced, and Eh in sediments decreased after impoundment. The relative content of phosphorus in sediments changed as follows:NaOH-P > HCl-P > OP to HCl-P > OP > NaOH-P; this could be attributed to changes in the depositional environment. Compared to pre-impoundment values, TP values after impoundment in sediment, overlying water ρ(PO43--P), and interstitial water ρ (PO43--P) were 1.3 times, 3.7 times, and 8.3 times higher, increasing the risk of nutrient release in sediments of Xiangxi Bay. The manifestation of PO43--P in sendiments of Xiangxi River generally is "source" pre-impoundment and post-impoundment, but the PO43--P diffusive flux increased from -0.0029-0.0059 mg·(m2·d)-1 pre-impoundment to 0.0067-0.1071 mg·(m2·d)-1 post-impoundment. The release of phosphorus from sediments at the bottom of Xiangxi Bay increased after impoundment.

[Dynamic Replenishment Process of Nutrients in Tributary of Channel Reservoir].

The process of phytoplankton succession and change in primary productivity are directly determined by the seasonal variations in nutrients in tributary bays of channel reservoirs. This study aimed at analyzing the effects of the main nutrient sources on seasonal distribution of nutrients in Xiangxi Bay based on the field data collected in 2010 and 2011. The tracing inorganic ions were Cl- and Na+ that showed significant differences (P<0.01) between the mainstream and the tributary. The nutrient contribution rates of different sources were calculated by the binary linear model. The results showed that the nutrient contribution rate of the mainstream was more than 75% during the dry season and the impoundment period. The intrusion of the mainstream was the main source of nutrients in Xiangxi Bay. The difference in the contribution rates between the mainstream and the upstream decreased during the pre-flood drawdown period and the flood season. The concentration of total phosphorus was higher in the upstream, while the concentration of total nitrogen was higher in the mainstream. The patterns of density currents were different and the nutrient contribution rates of different sources changed accordingly. Therefore, the nutrients of Xiangxi Bay changed regularly in different operation periods of the Three Gorges Reservoir.

[Effect of the Rainfall on Extinction of Cyanobacteria Bloom and Its Mechanism Analysis].

There were three rainfall events with different intensity in the Xiangxi Bay (XXB) from May 24 to June 2 in 2016. The factors such as hydrodynamics, water temperature, optical properties, and chlorophyll a concentrations during the rainfall events were analyzed. During the May 27 moderate rain period, the upstream flow of the reservoir bay increased by 1.9 times and the average mixing layer depth in the whole reservoir increased 8.2 m, compared to those before the rainfall event. During the June 1 light rain period, the average mixing layer depth in the whole reservoir increased 1.6 m and the average chlorophyll concentration reduced 2.02 µg·L-1, compared with those before the rainfall event. During the June 2 heavy rain period, the upstream flow of the reservoir bay increased by 4 times, the average mixing layer depth in the whole reservoir increased 7.9 m and the average chlorophyll concentration reduced 14.64 µg·L-1, compared with those before the rainfall event. The algae moved from the upstream to the downstream with water that reduced the concentration of algae in the XXB. The water temperature stratification weakened during the rain event and the average mixing layer depth in the whole reservoir increased, destroying the algal growth environment. After the rainfall, under suitable light and temperature conditions for 2-3 d, the water temperature stratification of the reservoir was recovered and rapid growth and reproduction of algae occurred. As a result, the chlorophyll concentrations in the reservoir increased. Rainfall has a periodic inhibitory effect on the outbreak of algal blooms; however, it cannot fundamentally solve the problem of tribal bay blooms.

[Characteristics of Nitrogen Release at the Sediment-Water Interface in the Typical Tributaries of the Three Gorges Reservoir During the Sensitive Period in Spring].

Overlying water and sediment interstitial water samples were acquired to study the nitrogen release between sediments and water interfaces in Xiangxi Bay in April 2016 during the Sensitive Period in spring. The spatial distribution of different forms of nitrogen in the sediment was analyzed, the diffusion fluxes of different forms of nitrogen in the sediments and water systems were also measured, and a correlation analysis with environmental factors was conducted. The results show that overlying water and sediment interstitial water ρ(TN) ranges from 1.10 to 6.90 mg·L-1 and 6.19 to 32.57 mg·L-1 respectively; indicating the nitrogen concentrations in the overlying and interstitial water of sediments have a certain variation along the process and vertically. The interstitial water nitrogen concentrations in the upstream area are higher than those in the downstream area. The interstitial water ρ(NH4+-N) in the sediment is significantly larger than that in the overlying water, but the interstitial water ρ(NO3--N) in the sediment is slightly smaller than that in the overlying water. Xiangxi Bay sediment acts as a source of NH4+-N; however, for NO3--N it is a sink. The diffusive fluxes of NH4+-N range from 2.70 to 4.72 mg·(m2·d)-1; and the diffusive fluxes of NO3--N range from -1.61 to -0.62 mg·(m2·d)-1. Nitrogen is mainly present in the form of ammonium nitrogen in the sediment of Xiangxi Bay. The ρ(NH4+-N) in the sediment ranges from 69.97-1185.97 mg·kg-1, ρ(NO3--N) ranges from 2.78-38.17 mg·kg-1, and the ρ(NH4+-N) in sediments in the surface at 0-8 cm changes with the same trend.

[Differences in Diffusive Fluxes of Nutrients from Sediment Between the Natural River Areas and Reservoirs in the Lancang River Basin].

A field survey was carried out to investigate the diffusive fluxes of nitrogen and phosphorus from sediment in the natural river areas and cascaded reservoirs in the Lancang River Basin from February through March in 2017. In the natural river areas, the mean value of total nitrogen (TN) concentrations in the sediment pore water was about 15.254 mg ·L-1, and the total phosphorus (TP) was only 0.654 mg ·L-1; while in the reservoirs, the mean value of TN and TP were 6.577 mg ·L-1 and 1.432 mg ·L-1, respectively. Both nutrients in the overlying water were less than that in the sediment pore water. The mean diffusive flux of total dissolved nitrogen (DTN) was almost 2.117 mg ·(m2 ·d)-1 in the natural river areas, and the mean value of total dissolved phosphorus (DTP) was 0.044 mg ·(m2 ·d)-1, while the mean value of DTN and DTP were 0.785 mg ·(m2 ·d)-1 and 0.053 mg ·(m2 ·d)-1 in the reservoirs, respectively. The TN content reaches the maximum in the overlying water-interstitial water vertical surface sediments. It was concluded that cascaded hydropower had greatly changed the nutrient exchange between the sediment and overlying water. Different hydrodynamics and varied chemical environments in the sediment could be additional causes.

[Nutrient Distribution Characteristics of the Sediment-water System in the Xiangxi River During the Impoundment of TGR].

This study characterizes the nutrient distribution of the sediment-water system in the Xiangxi River (XXR) during the impoundment of the Three Gorges Reservoir (TGR). In 2016, the research group obtained samples in the Xiangxi River, analyzed the distribution of nitrogen, phosphorus, and O.M. (organic matter) in the sediment-water system, explored the characteristics of "source-sink" between the interstitial water and overlying water, and carried out a cluster analysis for the sampling sites. The results showed that ρ(TN) in the sediments was higher than in the estuary, and the ρ(TN) was close to that in the middle and downstream samples of the river. The ρ(TP) in the upstream sample was the highest of all sites, ρ(O.M.) presented low distribution characteristics in the downstream, and the maximum value of ρ(O.M.) exceeded the critical point (1.5%) within a depth of 10 cm, and there was a certain risk of release. For the distribution in the overlying water, ρ(DTN) and ρ(DTP) were the largest in the estuary, ρ(DTN) decreased from the estuary to the upstream, and ρ(DTP) did not change along the river. During the study, DTN, NH4+-N, and DTP in all sites (except CJ point) were released as nutrients to the overlaying water as the "source," but some points for NO3--N and PO43--P presented as "sources," and some of them presented as "sinks." The "source-sink" process of nitrogen was stronger than that of phosphorus. This was due to the difference between the oxidation environment at the bottom of the XXR and the stream flow backward depth of the Xiangxi bay during impoundment. Based on the results of a cluster analysis, the characteristics of CJ, 1, and 2 indicated similar sediment-water systems, while the characteristics of 3 and 4 were similar.

[Bacterioplankton Community Structure in the Lancang River Basin and the Analysis of Its Driving Environmental Factors].

In order to explore the bacterioplankton diversity community structure in the Lancang River basin, as well as to study the relationship between the bacterial diversity and environmental factors, MiSeq high-throughput sequencing was used to analyze and compare the bacterial diversity and community composition of samples in February 2017. Furthermore, Pearson correlation analysis and redundancy analysis (RDA) were used to identify the key environmental factors for the change of bacterial community structure in the natural river and reservoirs of the Lancang river. The results show that the ACE index and Shannon index of the natural river are higher than those of the reservoir, and the main environmental factors causing the diversity of planktonic bacteria in natural river and reservoir segments are water temperature (WT), dissolved oxygen (DO), turbidity (Tur), permanganate index, pH, and total nitrogen (TN). Overall, 26772 optimized reads were obtained from samples based on the high-throughput sequencing of the V3 and V4 region of the 16S rRNA gene. The bacterial species detected in these samples cover 45 phyla and 965 genera. Taxonomic assignment analysis indicated that Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria are the abundant phyla. Among them, the Proteobacteria content is relatively rich, accounting for 36%-94% of the bacterial community. The predominant groups are α-Proteobacteria, ß-Proteobacteria, and γ-Proteobacteria, at the genus level, with Proteobacteria ratios of 0.39%-21.56%, 0.39%-55.80% and 31.09%-99.18%, respectively. The environmental factors affecting the community structure of planktonic bacteria are different in natural river and reservoir. DO and Tur are the key environmental factors affecting the community structure of planktonic bacteria in the natural river, while the community structure of planktonic bacteria in reservoirs is mainly affected by WT, permanganate index, and TN.

[Spatial Difference and Causes Analysis of the δ15 N of Suspended Particulate Matter in the Lancang River Basin].

The influence of cascade hydropower construction on the migration and transformation of raw material and ecological problems in the Lancang River Basin is of concern to scholars domestically and internationally. Based on stable isotope technology, this study analyzed the spatial distribution of nitrogen in the Lancang River Basin and explained the cause of this distribution. The results showed that the range of dissolved inorganic nitrogen (DIN) was 0.28-0.60mg·L-1, and that of DIN in the downstream area of the Lancang River was 0.39-1.15mg·L-1. The range of suspended particulate matter δ15 N in the upstream area of this river was 4.52‰-6.72‰, and that in the downstream reservoir section was heavier than that in the upstream reservoir, which varied between 2.3‰ and 11.8‰. This study used Isosource to analyze the source of suspended particulate matter. The results showed that industrial wastewater and sewage, soil organic matter, atmospheric sedimentation, and agrochemicals are the main sources of suspended particulate matter, and their respective total contribution rates are 42.43%, 22.38%, 18.16%, and 17.03%. At the same time, the δ15 N of suspended particulate matter in Xiaowan, Manwan, and Dachaosan dams downstream of the Lancang River were influenced by algal assimilation, it leads to a reduction in the number of δ15 N of suspended particles.

[Succession Pattern of Phytoplankton of Daning River in the Three Gorges Reservoir and Its Driving Factors].

To elucidate succession pattern of phytoplankton in the Daning River and its driving factors, multivariate statistical analysis was conducted. By using the monitoring data in different seasons of Daning river during April 2012 to January 2013, this paper analyzed the succession pattern of phytoplankton in the Daning River and its driving factors in typical tributaries of river-style reservoirs. According to the characteristics of water level, the operational period of the TGR was classified into following four stages:stage Ⅰ (pre-November-April), stage Ⅱ (May-July), stage Ⅲ (July-September) and stage Ⅳ (September-November). ① The results indicated that the values of Chlorophyll-a concentrations and algal density showed similar seasonal variations, with the highest values occurring in stage Ⅲ, followed by stages Ⅳ, Ⅱ and Ⅰ. Succession of C-R-S growth strategies was the same generally:CR-R type dominated in stage Ⅰ, CS, CR/CS and R-CR dominated in stage Ⅱ, Ⅲ and Ⅳ, respectively. ② The mean values of Margalef index and Pielou index in stage Ⅳ and Ⅲ were significantly greater than those in stage Ⅱ and Ⅰ; the value of Shannon-waver index showed that the highest value in stage Ⅲ, followed in a descending order by stage Ⅰ,Ⅱ and Ⅳ; the successional rate had the highest value in stage Ⅳ, followed in descending order by stage Ⅲ, Ⅰ and Ⅱ. ③ The results of Correlation analysis suggested that no significant relationships were observed between the environmental parameters and phytoplankton abundance in stage Ⅰ. The results indicated that relative water column stability(RWCS), index of feasible energy for phytoplankton (Et) and index of feasible energy (Ef*) were key regulatory factors for phytoplankton community in stage Ⅰ. The results indicated that Et, Ef* and total phosphorus (TP) were key regulatory factors for phytoplankton abundance in stage Ⅱ. The results of the redundancy analysis (RDA) suggested that RWCS, TP and the ratio of euphotic depth[Deu(λPAR)] to mixing depth (Dmix)[Deu(λPAR)/Dmix] were key regulatory factors for phytoplankton community composition in stage Ⅱ. The results indicated that Ef* and TP were key regulatory factors for phytoplankton abundance in stage Ⅲ. The results of the RDA suggested that[Deu(λPAR)/Dmix],Et, Ef* and TP were key regulatory factors for phytoplankton community composition in stage Ⅲ. The results indicated that TP was key regulatory factor for phytoplankton abundance in stage Ⅳ. The results of the RDA suggested that[Deu(λPAR)/Dmix] was key regulatory factor for phytoplankton community composition in stage Ⅳ.

[Purification Effect of Piggery Wastewater with Chlorella pyrenoidosa by Immobilized Biofilm-Attached Culture].

Piggery wastewater treatment with microalgae is a biological recycling technology. To evaluate the purification effect, this study investigated the treatment of piggery wastewater at different dilution ratios with Chlorella pyrenoidosa by attached cultivation and lipid production of algae cells and explored the tolerance of Chlorella pyrenoidosa to the piggery wastewater, which has high ammonia nitrogen. The piggery wastewater was diluted with purified water 1-, 2-, 5-, and 10-fold in culture media. The removal efficiencies of COD, ammonia nitrogen, total nitrogen, and total phosphorus and the enrichment effect of the heavy metals copper, zinc, and iron were measured. Meanwhile, we investigated the lipid production of Chlorella pyrenoidosa in variously diluted wastewater (1-, 2-, 5-, and 10-fold). It turned out that the purification effects of COD, ammonia nitrogen, total nitrogen, and total phosphorus were best when the piggery wastewater was diluted 5-fold, and the removal efficiencies were 86.8%, 94.1%, 85.2%, and 84.3%, respectively. Correspondingly, the lipid content was as high as 32.7%, and the removal efficiencies of the heavy metals copper, zinc, and iron were 72.9%, 70.0%, and 73.0%, respectively. The biomass productivity was 4.21 g·(m2·d)-1 at the end of the experiment. This research makes an effective connection between microalgae and piggery wastewater, which is difficult to purify deeply, and provides a theoretical basis for achieving algal biofuel production and decreasing the cost of wastewater treatment.

[Vertical Stratification Characteristics of Dissolved Oxygen and Phytoplankton in Thousand-Island Lake and Their Influencing Factors].

According to the data collected from the five monitoring sites in front of the dam in Thousand-island Lake in September 2015, the vertical distribution characteristics of dissolved oxygen (DO), water temperature, pH, turbidity, conductivity and phytoplankton were analyzed. The influencing factors of special stratification of DO and vertical distribution of phytoplankton were also discussed. The results showed that:① The vertical distribution of DO presented "the surface is higher than the underlying" mode. DO ranged from 1.95 mg·L-1 to 8.25 mg·L-1 and the average concentration was 5.10 mg·L-1. Low-oxygen zones appeared between 12 m and 20 m and the minimum concentration was 1.95 mg·L-1 at 17 m. The concentration of DO maintained at a high level between 0 m and 12 m and the vertical variance was small. Sudden drop of DO occurred between 12 m and 20 m and there was an anoxic zone (<4.0 mg·L-1). The concentration of DO returned to normal level between 20 m and 38 m. Then it reduced under 38m as the water depth increased. The vertical distribution of pH showed the same trend with that of DO and their sudden change regions appeared in the same zone. ② Vertical phytoplankton biomass displayed significant difference. The phytoplankton biomass of S1, S2 and S3 presented:mid-layer > surface > bottom. But S4, S5 presented:bottom > surface > mid-layer. In addition, the depth between 20 m and 30 m was the best for the growth of phytoplankton. ③ The correlation analysis showed that DO and water temperature in the thermocline were significantly correlative. The range and degree of low-oxygen zones in Thousand-island Lake was determined by physical processes like thermal stratification of water and direct or indirect effects of the plankton. Phytoplankton and DO, pH showed significant negative correlation. The phytoplankton was mainly influenced by turbulent mixing and DO stratification in the surface layer while it was mainly influenced by light intensity at the bottom.

[Comparison of Relationship Between Conduction and Algal Bloom in Pengxi River and Modao River in Three Gorges Reservoir].

Pengxi River is the largest tributary in the northern bank of Yangtze River in the Three Gorges Reservoir (TGR) region, and serious algal bloom has been reported since 2003 when the TGR dam started impounding water. On the other hand, Modao River, a tributary in TGR locating in the same county with Pengxi River, was rarely reported with algal bloom in the same time period. In this study, water samples were collected on the same day in both rivers in spring and summer, the most likely blooming seasons in TGR, to compare the water quality parameters and Chlorophyll-a(Chl-a) concentration dynamics in order to understand the mechanism of algal bloom in TGR. The results showed that the maximum value of Chl-a in Pengxi river reached 60.5 µg·L-1 in spring, and was only 7.8 µg·L-1 in summer, while in Modao river the Chl-a content was 2.92 µg·L-1 in spring and 7.48 µg·L-1in summer. Both rivers stratified during samplings, but having layers of thermocline and hypolimnion with the lack of epilimnion, and most of Chl-a content was located in thermocline. Also in both rivers, TN and TP exceeded the international threshold for eutrophication, with the average TN and TP values of 2.305 mg·L-1 and 0.053 mg·L-1in Pengxi River in spring, and 1.673 mg·L-1 and 0.097 mg·L-1 in summer, respectively, while in Modao River the values were 1.875 mg·L-1 and 0.075 mg·L-1 in spring, and 1.79 mg·L-1 and 0.054 mg·L-1 in summer respectively. Nutrients concentrations showed no significant correlation with Chl-a. On the other hand, conductivity value and trend were totally different between the two rivers:in Modao River in spring, the conductivity in upstream was only 75% of that in the main stream of the Yangtze River, and the backwater from the main stream reached to the middle in Modao river, where the highest Chl-a among all the river sampling sites was detected; summer conductivity distribution was similar with that in spring. Different from Modao River, the conductivity in upstream of Pengxi River in spring was 150% of that in the main stream of Yangtze, the backwater from the main stream reached area between sampling sites of PX04 and PX05 (upper than the middle reach); its upstream had significantly high content of Chl-a and conductivity, and these two factors were significantly positively correlated. The study showed that other than N and P, other ions in the Pengxi River played an important role in bursting "bloom", and need to be considered regarding bloom control.

[Temporal and Spatial Distribution of Environmental Factors and Phytoplankton During Algal Bloom Season in Pengxi River, Three Gorges Reservoir].

Planktonic algae and Related Environmental Factors were monitored during the period of algal blooms season in 2014 (Spring April 17 and summer 27 July) in Pengxi river, Three Gorges Reservoir. Mathematical statistical tools were used to analyze the community structure of planktonic algae in Pengxi River with its environmental factors. The results of sampling in spring showed that except the estuary site, S1 and the site close by, S2, the waters stratified, but without epilimnion, only had metalimnion and hypolimnion; the upstream sites had larger temperature difference between water layers than did the downstream ones; from all the 8 sampling sites from estuary to upstream, water depth, and the temperature, pH, conductivity, dissolved oxygen, chlorophyll a, TN and TP of the surface layer (0-0.5 m deep) were significantly different (ANOVA, P < 0.05). 25 species (genus) of planktonic algae were identified. The abundance of species ranged from 2.76 x 104 cell · L⁻¹ to 145.8 x 104 cell · L⁻¹. Ceratium hirundinella was the main dominant species, and Anabaena sp. was the sub-dominant species. Algal bloom in upstream sampling sites S7 (63.4 x 104cell · L⁻¹) and S8 (145.8 x 104cell · L⁻¹)were relatively serious in spring. Temperature of water, pH , conductivity, dissolved oxygen and NO3⁻ were the key regulatory factors for phytoplankton abundance based on redundancy analysis ( RDA). The results of sampling in summer showed similar stratification trends among sites; the depth of the same 8 sampling sites, and their surface layer's temperature of water, turbidity, redox potential, pH, water depth, conductivity, chlorophyll a, NH4⁺, NO3⁻, total nitrogen and total phosphorus were significantly different (ANOVA, P < 0.05). 46 species (genus) of phytoplanktonic algae were identified. The abundance ranged from 9.56 x 104 cell · L⁻¹ to 278.88 x 104 cell · L⁻¹. Phormidium sp. was the main dominant species, and Anabaena sp. was the sub- dominant algae. Algal bloom at the lower part of the river, S2 (216.44 x 104 cell · L⁻¹), S3 (278.88 x 104 cell · L⁻¹) and S4 (108.12 x 104 cell · L⁻¹) were relatively serious in summer. Turbidity, depth of water, TN, oxidation-reduction potential, conductivity and dissolved oxygen were the key regulatory factors for phytoplankton abundance based on RDA. Stratification had important effect on algal bloom formation.

[Influence of Periodic Temperature Disturbance on the Succession of Algal Community Structure].

In order to study the mechanism of the habitat disturbance of reservoir on algae blooms, some interior control experiments about the feature of algal diversity and the succession of community structure under different temperature disturbance cycle but the same amplitude condition were conducted, based on the intermediate disturbance hypothesis and combined with algal community habitat selection theory and ecological functional groups of algae. The results showed that:① the intermediate disturbance would accelerate the growth of algae and increase their diversity. Under the gradient of the intermediate disturbance group Δ22℃/48h, the diversity of phytoplankton was the highest, and the biomass reached the maximum, however, without absolute dominant algal species. While in the high frequency disturbance group Δ22℃/24h the biodiversity was relatively lower, but the algae biomass was reduced. ② the periodical change of temperature had obvious influence on the succession of the phytoplankton community, and the dominant species also presented certain differences. The succession of the algal advantageous function group followed the basic rule of X1(Chlorella)→J(Scenedesmus)→S1(Phormidium) or X2 (Chlamydomonas), and the community structure also presented a trend that the C/CR type algae took advantage and the superiority was gradually replaced by R type algae. When the high temperature disturbance was frequent, the R type algae (S1) was distinctly ascendant. During the experiment, the community structure was given priority to the C/R strategy algae with no or low disturbance. However, the coexistence of algae with different growth strategies was evident in group Δ22℃/48h. Meanwhile, the S strategy algae (L0) resistant to high temperature stress began to emerge.