[Driving Factors Analyze of Phytoplankton Community by Comparison of Population and Functional Groups and Water Quality Evaluation in Dongting Lake].

Phytoplankton is the most important component of water ecosystems, which could indicate the state of the water environment owing to its sensitivity to water environment variation. However, its response to the environment is influenced by classification methods. To understand the phytoplankton population(phyla and genera) and functional groups(FG) for driving response characteristics and applicability to the environment in Dongting Lake, a total of four samples were collected from the lake from March to December 2019, and the distribution characteristics of the phytoplankton population and functional groups and their responses to environmental factors were compared and analyzed. Meanwhile, the applicability of the TLI index, Shannon-Wiener index, and Q index was compared in Dongting Lake. The results showed that a total of 61 genera belonging to six phyla of phytoplankton were detected in Dongting Lake, which could be divided into 23 functional groups and nine dominant functional groups. The succession trend of functional groups was P/MP/D(March)→MP/P/J(June)→MP/H1(September)→Y/P/MP(December). The results of hierarchical segmentation showed that the population distribution and change in phytoplankton were driven by environmental factors more than the area in Dongting Lake. The main environmental factors affecting phytoplankton population and functional groups were water temperature(WT), permanganate index, dissolved oxygen(DO), conductivity(Cond), water level(WL), and total phosphorus(TP). RDA analysis showed that phytoplankton functional groups identified phytoplankton response to environmental factors better than phytoplankton population. It was shown that using the Q index to evaluate water quality had better applicability in Dongting Lake.

Effects of Summer and Autumn Drought on Eutrophication and the Phytoplankton Community in Dongting Lake in 2022.

Since July 2022, the Yangtze River basin has experienced the most severe hydro-meteorological drought since record collection started in 1961, which has greatly affected the ecological environment of the Dongting Lake (DTL) basin. To investigate the effects of drought events on the eutrophication and phytoplankton community structure of DTL, the lake was sampled twice in August and September 2022 based on the water level fluctuations resulting in 47 samples. Furthermore, we combined the comprehensive trophic level index (TLI) and phytoplankton Shannon-Wiener diversity index (H) to characterize and evaluate the eutrophication status. The key influencing factors of the phytoplankton community were identified using redundancy analysis (RDA), hierarchical partitioning, and the Jaccard similarity index (J). Our results showed that the TLI of DTL changed from light-moderate eutrophication status (August) to mesotrophic status (September), whereas the H changed from light or no pollution to medium pollution. The phytoplankton abundance in August (122.06 × 104 cells/L) was less than that in September (351.18 × 104 cells/L) in DTL. A trend in phytoplankton community succession from Bacillariophyta to Chlorophyta and Cyanophyta was shown. The combination of physiochemical and ecological assessment more accurately characterized the true eutrophic status of the aquatic ecosystem. The RDA showed that the key influencing factors in the phytoplankton community were water temperature (WT), pH, nitrogen and phosphorus nutrients, and the permanganate index (CODMn) in August, while dissolved oxygen (DO) and redox potential (ORP) were the key factors in September. Hierarchical partitioning further indicated that temporal and spatial variations had a greater impact on the phytoplankton community. And the J of each region was slightly similar and very dissimilar, from August to September, which indicated a decreased hydrological connectivity of DTL during drought. These analyses indicated that the risk to the water ecology of DTL intensified during the summer-autumn drought in 2022. Safeguarding hydrological connectivity in the DTL region is a prerequisite for promoting energy flow, material cycle, and water ecosystem health.

[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.