Impacts Of Small Dam Construction On Wetland Vegetation And Carbon Flux Patterns In Subtropical Rivers During The Early Stages

While climate change and human activities have had a serious impact on wetland ecosystems,they have also significantly altered the carbon cycle characteristics of inland waters,affecting wetland carbon pools and carbon fluxes.Dams are an infrastructure for economic and social development,contributing significantly to human economic development and environmental improvements.The large number of subtropical rivers and the intensive construction of small dams,which affect the hydrological processes of rivers and also alter the carbon cycle of riverine wetlands,have often been overlooked,especially in the early days after the construction of small dams,a critical turning point.In this context,the present study focuses on the habitat evolution of the geomorphologically complex areas such as riverbanks,rivers,drawdown areas and drawdown area water,including changes in plant diversity and CO2 and CH4 fluxes,in the Guanshan River Wetland,Ningxiang City,Changsha,Hunan Province,which was constructed in 2019.The results of this study show that the ecological and carbon fluxes of the small barrage were not only affected by the construction of the barrage,but also by the changes in plant diversity and CO2 and CH4 fluxes.The main findings of this study are as follows:(1)Plant species diversity decreased significantly at the early years of the dam,and the ratio of perennials to annuals changed.The number of species in the early years of the dam was significantly smaller in summer than in winter(19 in summer<32 in winter),while the number of species in the riparian zone did not change significantly(26 in summer and 22 in winter).The spatial differences in vegetation diversity between the riparian zone and the riparian zone were large(Bray-curtis coefficient of variation:0.836),so that the plant species diversity in the pre-dam riparian zone was significantly reduced after the dam was built and during the high water period.The number and abundance of annual plant species was significantly higher in winter than in summer,with a small increase in perennial plant species and no significant change in abundance.(2)Dam construction significantly changed the spatial and temporal patterns of CO2 and CH4 fluxes.CO2 fluxes were significantly greater in summer than in winter,showing a strong CO2 source in summer and a weak CO2 source in winter.In terms of different habitats,CO2 fluxes were significantly higher in riparian and drawdown areas than in water body areas.Water bodies only show CO2 uptake in winter and are a weak sink for CO2.The study area is a weak source of CH4 emissions in both summer and winter,with CH4 fluxes from riparian and drawdown areas being significantly lower than water body CH4 fluxes,and CH4 emissions and uptake being largely neutral.CH4 emissions from water bodies are a strong source and significantly higher than those from riparian and drawdown areas.CO2 and CH4 fluxes at the soil-air interface in the study area are influenced by a number of environmental factors,of which moisture and temperature are the most important environmental factors limiting CO2 and CH4 fluxes.Increasing temperature promotes CO2 emissions and moisture limits CH4 emissions.Water-gas interface CO2 fluxes and diffusive fluxes are limited by dissolved oxygen,water temperature and pH,and water-gas interface CH4 fluxes and diffusive fluxes are limited by dissolved oxygen,water temperature,electrical conductivity and total organic carbon.(3)The integrated habitat changes the spatial and temporal patterns of CO2 and CH4 fluxes,and the study illustrates that the Early years of the dam significantly increased the emissions of CH4 and CO2 in the floodplain.By influencing the exchange of material and energy between upstream and downstream rivers and the exchange of material and energy between land and water,the construction of dams promotes the emission of CO2 and CH4 in the basin,resulting in significantly higher fluxes in the basin during both the rich and dry periods than in the absence of dams,thus increasing CH4 and CO2 emissions in the early stages of dam construction.A cursory examination of the drawdown area shows that changes in the water level of the wetland after the construction of a small dam will increase the intensity of the carbon source and result in more carbon being emitted,so this should be taken into account when assessing the impact of dam construction on carbon emissions from riverine wetlands.The study of the impact of small dams on carbon emissions can provide data to support subtropical carbon estimates and provide recommendations for water level management.