Changes and driving factors of psychrophilic diatom community structure of algal mats in Huanglong Scenic Area

Huanglong Scenic Area, located in Songpan county, Sichuan Province, China, is listed as the World Natural Heritage, and is covered with travertine landscape at an altitude of about 3,145-3,588 m. With the plateau temperate sub-frigid monsoon climate, the melt water from snow mountains, the atmospheric precipitation and the underground karst water are the main water sources of Huanglong Scenic Area. In addition, covered by ice and snow at extremely low temperatures, Huanglong Scenic Area has a half-year-long freezing period with an annual average temperature of only 1.1℃. A large number of algae, mainly psychrophilic diatoms, grow in the water coverage of the travertine deposition area in the scenic area. The diatom community, an indicator of water quality change, is very sensitive to environmental changes in natural rivers. At present, there are still few studies related to this alpine karst area. In order to explore the structure of algal mats as well as the relationship between the community structure of psychrophilic diatoms in algal mats and environmental variables in Huanglong Scenic Area, we analyzed water samples and algae mats from 8 typical scenic spots in October 2021. The water temperature (T), pH and conductivity (Ec) as well as the concentrations of ${\rm{HCO}}_3^{-}$ and ${\rm{CO}}_3^{2-}$ of the sampling points were measured on site. At the same time, the geographic data of the sampling points were recorded. The concentrations of dissolved silicon (Dsi), cations (Ca²⁺, Mg²⁺, Al³⁺, K⁺), anions (Cl⁻, ${\rm{NO}}_3^{-}$, ${\rm{SO}}_4^{2-}$), TN and TP were determined in the laboratory. The structural compositions of the algal mats and the characteristics of the diatom community structure were analyzed. The main driving factors affecting the water of Huanglong Scenic Area were judged by Principal Component Analysis (PCA), and one of the two environmental variables with greater correlation was eliminated by Spearman Correlation Analysis. After Detrend Correspondence Analysis (DCA) of diatom species abundance in algal mats, Redundancy Analysis (RDA) was selected to analyze the relationship between environmental variables and diatom communities. The results show that the concentrations of Ca²⁺ and ${\rm{HCO}}_3^{-}$ in the waterbody is high, and water mainly belongs to the HCO₃-Ca type, followed by the HCO₃-Ca-Mg type. The algal mats are mainly composed of travertine particles, filamentous algae and diatoms. The study area is an alpine karst micro-ecosystem with travertine as the base and filamentous algae as the skeleton, inhabiting a large number of diatoms. 72 species of diatoms from 25 genera are identified from algal mats, among which 16 species have a total relative abundance greater than 2%. The genus Cymbella (14 species), Diatoma. (8 species), Cymbopleura (7 species), Caloneis (4 species) and Synedra (4 species) contribute the most. There are 13 dominant species of diatoms in Huanglong Scenic Area. 4 species from the genus Cymbella have absolute advantage (Y_total = 0.132) at the genus level. At the species level, there is little difference in the dominance index of each dominant species (0.023≤Y≤0.053), among which the dominance index of Diatoma vulgaris is the largest, and that of Cymbopleura linearis is the smallest. In terms of the diversity index, there are some differences in the Shannon-Weiner index, Simpson diversity index and Pielou index at various sampling points. As the altitude decreases, the diversity index from #1 to #8 fluctuates, rising and then falling repeatedly. This may be related to the water circulation system of Huanglong Scenic Area. The analyses of the relationship between environmental variables and diatom communities by RDA show that the eigenvalues of the first two axes are 0.43 and 0.24, respectively, explaining 67.68% of the cumulative variance of data about diatom community species. The correlation between environmental variables and diatom community structure in algal mats is T>DSi>pCO₂>TN> Mg²⁺>Ca²⁺>altitude, and the three environmental variables of Cl⁻, ${\rm{NO}}_3^{-}$ and ${\rm{SO}}_4^{2-}$ are excluded because of the low correlations. The significance test of the influence degree of each factor by Monte Carlo test finds that T (F=2.8, P=0.02) and DSi (F=2.3, P=0.04) are environmental variables that significantly affect the algae community structure of the algal mats. In addition, although TN is not the most significant driving factor affecting diatoms of the algal mats in Huanglong Scenic Area, there is a trend of eutrophication in the waterbody of the scenic spot, which will affect the algal community structure, so TN may be a potential driving factor. The above conclusions can provide a basis for the establishment of the diatom species bank in Huanglong Scenic Area and the monitoring and management of the river water environment in the plateau karst area.