[Characteristics of Eukaryotic Phytoplankton Community Structure and Its Relationship with Environmental Factors in Danjiangkou Reservoir].

Eukaryotic phytoplankton play crucial roles in ecosystem processes, and understanding their community composition and abundance is of great importance to protect the balance of aquatic ecosystems. Danjiangkou Reservoir is a canyon lake in central China that acts as the water source of the Mid-route of South-to-North Water Diversion Project. In this study, the composition of planktonic microeukaryotes from water samples with a depth ranging from 0.5 to 20 m in Danjiangkou Reservoir were investigated using high-throughput 18S rDNA gene sequencing. The environmental factors including pH, water temperature (WT), nitrate (NO3--N), ammonia (NH4+-N), total nitrogen (TN), conductivity (Cond.), water depth, and dissolved oxygen (DO) in the reservoir areas were measured, and their correlations with the abundance and diversity of eukaryotic phytoplankton were analyzed. The results of high-throughput sequencing showed that a total of 68 genera of phytoplankton belonging to 12 phyla were detected, and the phytoplankton communities were mainly composed of Dinophyceae, Bacillariophyta, and Chlorophyta. In particular, significant differences in the diversity of phytoplankton communities on a vertical distribution were found. The diversity and abundance of the phytoplankton communities in Songgang (SG), Tumen (TM), and Dangzikou (DZK) decreased with the increase in water depth, and the relationship between phytoplankton and water depth in other sampling sites was not obvious. The results of RDA showed that NO3--N, water depth, DO, pH, and WT could influence the vertical distribution of the phytoplankton community in the Danjiangkou Reservoir. Among these phytoplankton types, Dinophyceae was negatively correlated with NO3--N and water depth and positively correlated with other environmental factors. Meanwhile, Bacillariophyta was positively correlated with NO3--N and water depth and negatively correlated with other environmental factors. In addition, Chlorophyta was negatively correlated with WT, pH, and DO and not significantly correlated with water depth.

[Characteristics of Macrobenthos Community Structure and Their Relationships with Environmental Factors in Rivers of Beijing in Spring].

Macrobenthos from 20 sites of 17 rivers in Beijing were investigated in the spring of 2019, and the relationship between community structure characteristics and environmental factors was analyzed. A total of 64 classification units (species) of macrobenthos from 3 phyla, 6 classes, and 32 families were identified. We found that there were large spatial differences between the density composition and dominant species of macrobenthos. Mountainous river area had the highest density of aquatic insects (33.95 ind.·m-2), and the main dominant species were Simuliidae sp., Hydropsyche sp., and Paraciadius alpicola. In suburban areas, Chironomidae and Oligochaeta showed the largest average density, which was 82.58 ind.·m-2 and 36.21 ind.·m-2, respectively, and Polypedilum nubeculosum and Chironomus pallidivittatus were the dominant species in this area. Furthermore, macrobenthos in urban rivers were mainly Gastropods (88.75 ind.·m-2), and the dominant species were Bellamya aeruginosa and Bellamya purificata. Fourteen more species were found in the urban and suburban river areas than in mountain areas. According to the results of the canonical correspondence analysis, there were many factors affecting the community structure of macrobenthos, including water temperature, TN, Chl-a, pH, and human activity. Nutrient concentration was the main factor affecting the community structure of macrobenthos in the suburban river area, while human activities and water temperature were the major determinants of the macrobenthos community structure in mountainous and urban rivers.

[Analysis of River Zooplankton Community Characteristics in Autumn in Beijing].

To clarify the spatial distribution characteristics of zooplankton communities and the relationship between zooplankton and water environmental factors, 81 samples from 28 rivers and lakes in Beijing were collected in September 2017. A total of 68 species of zooplankton were identified, including 17 species of protozoa (25%), 36 species of rotifers (53%), 13 species of cladocera (19%), and copepods (3%, which were mainly copepodids and nauplii). Among 15 dominant species, 12 were copepods and rotifers. Zooplankton communities varied between the different rivers:The zooplankton in rivers in mountainous areas were mainly protozoa and copepods; in suburban areas, communities were dominated by copepods; and in urban areas, rotifers were dominant. A comprehensive water quality index, the Shannon-Wiener index, the Pielou evenness index, and the Margalef richness index were also calculated. Results showed that the water quality of the rivers in mountainous areas was generally better than in urban areas, which was better than in suburban areas. TN was found to be the main form of pollution using the single factor water quality index. Principal component analysis (PCA) and canonical correspondence analysis (CCA) identified that NH4+-N, BOD5, and COD had a significant effect on zooplankton community characteristics.

Morphological and physiological responses of Heteropogon contortus to drought stress in a dry-hot valley.

BACKGROUND: Heteropogon contortus is a valuable pasture species that is widely used for vegetation restoration in dry-hot valleys of China. However, to date, its morphological and physiological responses to drought, and the underlying mechanisms are not well understood. This study was aimed to investigate the morphological and physiological changes of H. contortus under drought stress during the dry-hot season. Heteropogon contortus was planted in pots and subjected to four levels of soil water treatments: above 85 % (control), 70-75 % (light stress), 55-60 % (moderate stress) or 35-40 % (severe stress) of field capacity. RESULTS: Within the total stress period (0-29 days), H. contortus grew rapidly in the light stress, whereas severe stress had a negative impact on growth. Aboveground biomass decreased together with increasing drought stress, whereas root biomass increased. Consequently, the root/shoot ratio of the severe stress treatment increased by 80 % compared to that of the control treatment. The ratio of bound water/free water (BW/FW) was the most sensitive parameter to drought and showed a value under severe stress that was 152.83 % more than that in the control treatment. Although leaf water potential (LWP) and leaf relative water content (RWC) decreased with progressive water stress, H. contortus managed to maintain a relatively high RWC (nearly 70 %) in the severe stress condition. We also detected a significant reduction (below 0.6) in the ratio of variable fluorescence/maximum fluorescence (Fv/Fm) in the severe stress treatment. CONCLUSIONS: Our results show that H. contortus adapts to drought mainly by avoidance mechanisms, and its morphological and physiological characteristics are inhibited under severe stress, but can recover at a certain time after re-watering. These findings might help limited water resources to be fully used for vegetation management in the studied region.