Spatio-Temporal Distribution And Environmental Drivers Of Lake-Water Chlorophyll-a Level And Diatom Communities In Yilong Lake

Lakes are an important part of inland water bodies and provide important ecosystem service.Due to the increasing intensity of human activities,lake ecosystems are significantly sensitive to processes such as climate change and watershed development.Algae is an important primary producer in freshwater ecosystems,with high species diversity and sensitive to changes in water environment.Their productivity level and community building model are reliable indicators to reflect the environmental gradient of lakes.Among them,the succession pattern and environmental condition identification of the dominant species of diatom are important contents in the evaluation of lake ecological health and environmental quality.Surveys of existing lakes show that the growth of freshwater phytoplankton is significantly affected by the seasonal fluctuations of nutrient salts and physical parameters of water.Compared with deep water lakes,shallow water lakes are more sensitive to changes in water environment.In the middle and high latitudes,the continuous input of nutrients can lead to the homogenization of environmental conditions in large shallow lakes,and reduce the spatial distribution difference and seasonal fluctuation amplitude of algae.In recent years,with the continuous development of the basin,the nine plateau lakes in Yunnan all showed a long-term feature of rising nutrient levels.In recent years,a large number of researches on lake algae and diatoms have been carried out in Yunnan,and the independent analysis of different algae indexes is mainly used in modern investigations.Taking Yilong Lake as the investigation object,this study intends to analyze the spatial heterogeneity and seasonal variation of algae indicators and water environmental factors in eutrophic shallow lakes in subtropical areas,and compare and analyze the spatio-temporal variation characteristics and influencing factors of different indicators dominated by algae,as well as the differences of driving factors of the same indicator in different seasonal variations.In this study,Yilong Lake,a large shallow lake in subtropical region,was selected as the research object,and a lake seasonal survey was carried out during2020-2022.The indexes of water quality,chlorophyll a（Chla）and diatoms community were comprehensively analyzed at 20 lake sampling sites,and the spatial and temporal patterns and main characteristics of algae distribution were identified.Further combined with nutrients(TN,NO₃^-,NO₂^-,NH₄⁺,Si O₃^2-,TP,PO₄^3-,N:P),physical indicators of water（water depth and temperature）and meteorological factors（wind speed,air temperature,precipitation）and other driving factors,quantitative evaluation of biological indicators was carried out.The main driving factors of spatial and temporal distribution of algae and their heterogeneity on different time scales were diagnosed.The main conclusions of this paper are as follows:（1）There are significant spatial changes and seasonal fluctuations in the water environment of Yilong Lake.During the two years of investigation,the overall nutrition level of the water body is high,the average trophic state index TSI（73.5）,the overall level of eutrophic.The nutrient salt in Yilong Lake was affected by the input of the basin,and there was a significant difference between the western lake and the central and eastern lake.There were significant seasonal fluctuations of nutrient salts and physical parameters in Yilong Lake（P<0.05）,reflecting that Yilong Lake is located in the subtropical monsoon area of low latitude and high altitude,with distinct dry and rainy seasons.（2）The chlorophyll a concentration in Yilong Lake showed significant spatial and seasonal variation characteristics.The concentration of chlorophyll a in the western lakes（111.48±80.08μg/L）was slightly higher than that in the central and eastern lakes（102.63±65.06μg/L）.In terms of seasonal fluctuation,time difference was significant（P<0.05）,the rainy season（150.26±65.95μg/L）was significantly higher than the dry season（58.55±30.6μg/L）.The spatial difference of chlorophyll a concentration in rainy season（standard deviation=65.95μg/L）was significantly higher than that in dry season（standard deviation=30.6μg/L）.（3）The analysis of diatoms in the water showed that 102 species of diatoms in23 genera were found in 160 samples,and there were significant spatial succession and seasonal changes.The seasonal succession of planktic diatoms and benthic diatoms was obvious.The community structure of diatoms was dominated by mesotrophic tolerant species（83.5±17.33%）of plankton type,in which Aulacoseira ambigua（66.01±18.68%）was the dominant species,and Aulacoseira granulata,11.21±9%)was the secondary dominant species,and benthic diatom species（such as Fragilaria）increased significantly and became the secondary dominant species（28.1±17.77%）in the four seasons from June 2021 to March 2022.ANOSIM analysis showed that there were significant spatial differences in diatom community composition between the western lakes and the central and eastern lakes（P<0.05）.There was significant inter-annual succession of plankton-benthic diatom species,and the plankton-benthic diatom species was dominant from June 2020 to March 2021（95.1±3.97%）,and the number of species increased significantly from June 2021 to the end of March 2022（28.1±17.8%）.（4）The results of statistical analysis showed that the spatial and temporal changes of chlorophyllin a and diatom community structure were significantly affected by nutrients,as well as by physical parameters of water and meteorological factors.The variance results showed that nutrients were the main environmental factors driving the spatial changes of algae biomass（21.78±19.04%）and diatom community structure（22.33±18.42%）,and the explanatory quantity of water physical index was relatively low（8.78±10.61%;0.91±1.99%）.RDA analysis,correlation analysis,variance decomposition analysis and multiple regression analysis all showed that nutrient salts,physical parameters of water and meteorological factors had different effects on the changes of chlorophyllin a and diatom communities.In terms of seasonal variation,the independent interpretation of chlorophyll a by meteorological factors was the highest（3.4%,1.6%）.However,at the eight seasonal（interannual）scales,the independent explanations of nutrients,physical conditions and meteorological factors increased significantly,and the interpretation of nutrients for chlorophyll a was the highest（32%）.The independent explanations of nutrient for seasonal variation of diatom community were the highest in both season（18.6%,8.9%）and interyear（16.6%）.（5）The seasonal succession of planktonic and benthic species in diatom community is mainly affected by hydrodynamic conditions and nutrient salts.Multiple regression and correlation analysis showed that the succession of planktic and benthic diatoms was significantly correlated with TN,wind speed,WT and water depth,plankton diatoms were positively correlated with TN（r=0.37,P<0.001）and wind speed（r=0.32,P<0.001）and Aulacoseira spp was significantly superior when the wind speed was higher than 2 m/s.Planktic diatoms and WT（r=-0.36,P<0.001）,water depth（r=0.24,P=0.002）were negatively correlated,while the correlation with TP was low.In conclusion,the spatial heterogeneity of nutrients in water(TN,TP,Si O₃^2-,N:P)is an important factor affecting the spatial distribution of algae.In terms of seasonal variation,nutrients and meteorological factors（precipitation,wind speed,air temperature）had significant effects on algae changes in water bodies.TN and wind speed are also important factors driving planktic and benthic diatom succession.At the same time,there were seasonal differences in the variance explanations of nutrient salts,physical parameters of water and meteorological factors on the seasonal fluctuations of algae biomass and diatom community.This study revealed that nitrogen and phosphorus and dissolved silicates were the main factors driving the spatial changes of phytoplankton biomass and community structure in Yilong Lake.In terms of seasonal and interannual variation,the seasonal fluctuation of algae primary productivity and diatom community structure was significantly affected by nitrogen and phosphorus nutrient elements and meteorological factors,and the effect of temperature on algae biomass was significantly enhanced.Therefore,in the prevention and control of algae and lake pollution in large eutrophic shallow lakes,attention should be paid to the input of nitrogen and phosphorus nutrients and seasonal changes,and the continuous influence of climate warming and water temperature fluctuations should be addressed.