Effects Of Phytoplankton-bacterioplankton On Concentrations And Fluxes Of CH₄ And CO₂ In Typical Reservoirs In The Upper Reaches Of The Yangtze River

The rapid increase of greenhouse gases(GHGs)in the atmosphere is one of the main reason for global warming.Artificially constructed reservoirs are different in many ways from natural systems,which may increase greenhouse gas emissions from these systems.The microbial process in water determines the emission of methane and carbon dioxide.However,due to the construction of the reservoir makes the change of the trophic status and lead to the original ecological balance was broken,the number of microorganisms and structure as well as a variety of related environmental factors change,resulting in the reservoir in the actual methane and carbon dioxide emissions have greater uncertainty.Therefore,five typical reservoirs with different trophic status in the Yangtze River Basin,including Xiaoba II Reservoir,Xiluodu Reservoir,Xiangjiaba Reservoir,Three Gorges Reservoir and Shizitan Reservoir,were selected as the research objects in this paper.Flow cytometry and high-throughput sequencing were used to explore what reservoir characteristics cause CO₂ and CH₄ flux and concentration changes in typical reservoirs in the upper reaches of the Yangtze River;to reveal the relationship between phytoplankton-bacterioplankton and CO₂ and CH₄ in different trophic status;to analyzed the community diversity,community structure and network interaction of phytoplankton and bacterioplankton in different trophic states,and the coupling relationship between these community characteristics and CO₂,CH₄..Based on the preliminary understanding of CO₂ and CH₄ emissions in different trophic states of water,the effects of phytoplankton-bacterioplankton on CO₂ and CH₄ in different trophic states of water were further explored.The main conclusions of this paper are as follows:(1)By linear fitting of the GHG emission fluxes and reservoir ages of five typical reservoirs and reservoirs of other countries and climate types,this study found that there was no significant relationship between the reservoir age and CH₄ and CO₂greenhouse gas emission fluxes.The relationship between CH₄ and CO₂ emission and trophic status was more significant.The concentration of CH₄ increased exponentially with TLI index(R²=0.73,P<0.001).Meanwhile,the concentration of CO₂ was also significantly correlated with TLI index.As for the flux,CH₄ flux in the water-gas interface was the highest in eutrophic state,and CH₄ flux has a significant correlation with TLI index.However,no nonlinear increase relationship between CO₂ flux or concentration and TLI was observed in this study.Therefore,with the change of trophic from low trophic to high trophic,reservoir becomes CH₄ source,and eutrophication accelerates CH₄ emission.However,for CO₂,the effect of trophic on CO₂ emission is more complex than that of CH₄ emission.To some extent,reservoirs with high eutrophication may even become the"sink"of CO₂ emission.(2)The water body of the land use type(such as cultivated land,construction land,etc.)which is affected by human being is more likely to have eutrophic status.The different land use types around the reservoir may cause different changes in environmental factors in water.With the increase of forest area ratio around the reservoir,the concentration of DOC,TN,TP and NH₄-N will be significantly reduced,and the increase of these environmental factors has a significant correlation with the increase of CH₄ and CO₂ emissions.Therefore,the increase of forest area in the buffer zone around the reservoir may also have potential positive effect on reducing CH₄ and CO₂ emissions.(3)Based on flow cytometry,we found that the abundance of pico-phytoplankton was different in different trophic states.Under the condition of eutrophication,the abundance of phytoplankton is the highest.However,there was no significant difference in the abundance of planktonic heterotrophic bacteria under different trophic conditions.Based on the molecular biology method,we found that the diversity of phytoplankton community had a decreasing trend with the increasing of trophic status.Phytoplankton and bacterial community structure have different characteristics under different trophic conditions.The relative abundance of Bacillariophyta in low trophic state was the highest(P<0.0001).The abundance of Cyanobacteria and Chlorophyta was the highest in eutrophic state.The relative abundance of Acidobacteriota,Nitrospirota,Gemmatimonadota and Deinococcota in low trophic state were the highest.The phytoplankton and bacterioplankton networks were more complex than the medium and low trophic states under the eutrophic state,while there was more competition relationship between the phytoplankton species in the low trophic state.The bacterioplankton network in water was more responsive to the environment than the phytoplankton network,and was more susceptible to the disturbance of the external environment.(4)The emission of CH₄ and CO₂ in different trophic states may be related to the change of planktonic microbial community.Under the eutrophic state,the concentration of CH₄ increased exponentially with the increase of the abundance of phytoplankton(R²=0.66,P<0.001).The concentration of CO₂ was negatively correlated with the abundance of phytoplankton in three trophic status.The increase of CH₄ emission may be related to the increase of pico-phytoplankton in eutrophic water,while CO₂ emission is controlled by the photosynthesis and respiration of microorganisms respectively.In addition,the results showed that the Alpha diversity of phytoplankton and bacteria in different trophic status affected the CH₄ in water.With the increase of phytoplankton microbial diversity,the CH₄ concentration in water had an increasing trend.The reason for this phenomenon may be related to the carbon utilization rate of microorganisms.The results also showed that the increase of relative abundance of Cyanobacteria had a certain effect on CH₄ and CO₂ production.