Study On The Effects Of Submerged Vegetation Reconstruction On Nitrogen And Phosphorus Loads And Microbial Communities In Lakes

With the rapid development of economy,dramatic growth of population and the acceleration of industrialization and urbanization in China,eutrophication and ecosystem degradation in shallow lakes are becoming increasingly serious,which pose a great threat to the sustainable development of social economy and human health.The reconstruction of submerged macrophyte community is an effective measure to restore the eutrophic and damaged lake ecosystems.As an important component of the lake ecosystem,submerged macrophytes not only have the functions of water purification and algae control,but also can change the microbial community structure and further influence the material cycling process.Currently,compared to the marine microbiomes,the lake microbiomics has been relatively little studied,and the understanding of microbially driven biogeochemical cycling of nitrogen and phosphorus in lakes is more limited.This paper takes Lake Jinhu,a typical shallow lake in Hubei province,as the research area.Both in situ investigations and controlled experiments were used to investigate the responses of internal nitrogen and phosphorus morphological characteristics and microbial community characteristics of the lake to submerged vegetation reconstruction.We used different forms of nitrogen and phosphorus extraction methods,thin film diffusion gradient technique and high-resolution pore water sampling technique to study the changes of nitrogen and phosphorus in overlying water and sediments during the reconstruction of submerged vegetation.In addition,the high-throughput sequencing technology was used to study the changes in microbial community composition and functions related to nitrogen and phosphorus cycling.The main findings are as follows:1.The physicochemical characteristics of submerged vegetation,overlying water and sediment were continuous observed during submerged vegetation reconstruction in the Jinhu Lake from February 2021 to February 2022.The results showed that the reconstructed submerged plant community successfully propagated and spread in Jinhu Lake,and the biomass and coverage of the submerged plant community showed a pattern of increasing and then decreasing,reaching the maximum value of 587.22 g/m~2 and 93.8%in September 2021,respectively.The successful restoration of submerged plants improved the water transparency and dissolved oxygen,and reduce the concentrations of total nitrogen,ammonia nitrogen,nitrate nitrogen,total phosphorus,total dissolved phosphorus and dissolved reactive phosphorus in the overlying water.Meanwhile,submerged plants can significantly reduce the concentrations of organic nitrogen,ammonia nitrogen,calcium phosphorus and iron phosphorus in the surface sediment(0-5 cm)during their life history cycle,resulting in a significant decrease in sediment nitrogen and phosphorus concentrations.2.At the end of July(mid-plant growth)and end of October(late plant growth)in 2021,through the study of the epiphytic bacterial community on the leaves of three submerged plants(Vallisneria natans,Hydrilla verticillata,Potamogeton maackianus)successfully restored in Jinhu Lake,the results showed that the community structure and function of epiphytic bacteria on the leaves of submerged plants differed significantly from those of planktonic bacteria in the surrounding water,and the alpha diversity of the epiphytic bacterial communities on the leaves of all three submerged plants was significantly higher than that of the planktonic bacterial communities.The structure and function of the epiphytic bacterial community showed significant differences among different growth periods and species of submerged plants,and there were specific microbial taxa enriched on the leaf biofilms of three submerged plants.In the two growth periods,the alpha diversity of the epiphytic bacterial community was significantly higher in October than in July.While among the three submerged macrophytes,the alpha diversity and the abundance of functions related to nitrogen and phosphorus transformation were higher in the epiphytic bacteria of P.maackianus.3.From December 2020 to May 2021,by constructing an in-situ enclosure simulation system in Jinhu Lake,we investigated the effects of winter-growing Potamogeton crispus communities on the physicochemical properties and the structure and function of microbial communities of sediments.The results showed that the growth of P.crispus changed the physicochemical properties of sediment such as moisture content,pH,oxidation-reduction potential and organic matter.In addition,the restoration of P.crispus community not only reduced the concentration of nitrogen in the form of organic nitrogen and ammonia nitrogen and the concentration of phosphorus in the form of OP,Ca-P and Fe-P in the sediment,but also reduced the effective state of nitrogen and phosphorus at the sediment-water interface.Meanwhile,the P.crispus community changed the structure and function of the sediment microbial community,and affected the transport and transformation of sediment nitrogen and phosphorus by regulating the abundance of nitrogen cycle functional genes such as nifK,nosZ and ureA and phosphorus cycle functional genes such as mdh,nudF and ugpB.4.From March to October 2021,by constructing an in-situ enclosure simulation system in Jinhu Lake,the response of overlying water and sediment nitrogen and phosphorus concentrations as well as the sediment microbial community structure and function to the submerged vegetation reestablishment were investigated after stable establishment of two submerged plant communities with different species richness.The results showed that the reconstruction of submerged macrophyte communities could control the release of nitrogen and phosphorus from the sediment,reduce the nitrogen and phosphorus concentrations in the overlying water and sediment,and change the community composition and nitrogen and phosphorus cycling functions of the sediment microbial community.Also,the restoration of submerged macrophytes could influence the transport and transformation of nitrogen and phosphorus in lake sediments by regulating functional genes related to nitrogen cycling pathways such as denitrification,nitrate assimilation reduction,nitrate dissimilation reduction,nitrification and nitrogen fixation and phosphorus cycling pathways such as inorganic phosphorus solubilization,phosphorus starvation response regulation and phosphorus uptake and transport.Furthermore,different submerged macrophyte communities had different effects on the sediment microbial community,and these differences were mainly related to the different life history of the submerged plants in the community.In summary,the reconstruction of submerged vegetation can effectively reduce the nitrogen and phosphorus loads to the overlying water and sediment,and its effect on the microbial community can also support the transport and transformation of internal nitrogen and phosphorus in the lake.This paper enriched the impact of submerged vegetation reconstruction on the restoration of degraded lake ecosystem and the mechanism of effect on internal nitrogen and phosphorus loads to the lake from the perspective of lake microbial community,which would provide a new theoretical support for submerged vegetation restoration of degraded lake ecosystem.