| Caohai is a representative grass-type lake in the karst plateau of southwest my country.Aquatic plants play a very important ecological function and role in its ecosystem.In the past two years,the exogenous pollution of Caohai Lake has been effectively controlled,but the biomass of Caohai aquatic plants has decreased year by year.In order to investigate the reasons,this study tried to investigate the occurrence of Caohai aquatic plants from the perspective of the temporal and spatial distribution of nutrients.The reason for the change is to explore the characteristics of the impact of changes in water bodies and sediments on the stoichiometry of plants,and to provide relevant theoretical support for the scientific management of grass-type lakes.In this study,two samples were taken of the surface sediments,aquatic plants and overlying water of Caohai Lake(November 2019 and November 2020),and the degree of eutrophication of the overlying water was evaluated according to the comprehensive trophic index(TLI)method;The single factor standard index method and the soil comprehensive nutrient index method(FF)evaluated the pollution degree of organic matter and nitrogen and phosphorus in surface sediments.Analyze the physical and chemical indicators of water bodies and the temporal and spatial distribution characteristics of carbon,nitrogen and phosphorus and their stoichiometric ratios in sediments,and analyze the carbon,nitrogen and phosphorus and stoichiometric ratios of different types of aquatic plants.According to the occurrence and temporal and spatial distribution characteristics of phosphorus in surface sediments,the effects of surface sediments at various sites on the occurrence of phosphorus under different environmental factors were compared,and the following conclusions were obtained through research:(1)The water quality of Caohai in October 2020 showed a trend of eutrophication compared with October 2019,most of which were at mesotrophic level,with mild eutrophication levels in the northeastern and central waters of the lake.(2)Within two years,the organic matter and total nitrogen of Caohai sediments were both Grade 4 pollution,which showed an increasing trend over time.The total phosphorus content is classified as second-level pollution in shallow water areas and third-level pollution in deep water areas.In addition,the sediments of Caohai have serious siltation of organic carbon,and the risk of phosphorus release from the sediments in the lake core area increases.The contents of S-TOC,S-TN,and S-TP in the surface sediments show a trend of increasing from the lakeside zone to the center of the lake.It is necessary to guard against the bio-sedimentation process of the Caohai surface sediments.(3)The impact of sediment stoichiometry on plants is greater than that of water bodies,and it profoundly affects the ecological stoichiometry of plants.In emergent plants,Scirpus validus Vahl and Typha orientalis Presl are sensitive to phosphorus,and Juncus effusus L.are weakly sensitive to phosphorus.The homeostasis of Potamogeton lucens Linn.in submerged plants is stronger than that of Ceratophyllum demersum L.and Myriophyllum verticillatum L.,and it is more tolerant to eutrophic water bodies,which may be the reason for the decrease of Ceratophyllum demersum L.and Myriophyllum verticillatum L.in Caohai.(4)In 2020,the average phosphorus form content of Caohai surface sediments is lower than that in 2019.Within two years,the phosphorus form is dominated by Fe/A1-P and Org-P,followed by calcium phosphorus Ca-P,and in 2020 BA-P in S-TP(61.51%)has a downward trend compared with 2019(62.85%). |
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