Succession Mechanism Of Phytoplankton Community Structure And Its Interaction With Carbon Cycle In Reservoirs

The sustainable development of humans and the global greenhouse effect promotes the increase in demand for clean energy.Hydropower reservoirs,as recognized clean energy,have been widely constructed in recent years.However,dams blocked the continuity of natural rivers and changed the ecological function and material circulation process of the river ecosystem,which eventually caused a series of ecological environment changes.The carbon cycle,as an important part of the function of the river ecosystem,will also be altered by the damming reservoir effect on its biogeochemical cycle.As the main primary producer in reservoir,phytoplankton can influence the reservoir carbon cycle by assimilating and absorbing inorganic carbon through photosynthesis to synthesize organic carbon,and are therefore considered to be the main driver of the reservoir carbon cycle;however,it is controversial whether changes in carbon cycle processes will turn to affect reservoir phytoplankton growth and community succession,and it is also poorly understood how carbon cycle processes change under different phytoplankton community succession patterns.To understand this interaction processes,we investigated phytoplankton community compositions,stable C isotope compositions,carbon cycle functional genes,and major environmental factors.By analyzing the changes in phytoplankton community structure due to dam interception,and the spatial and temporal succession patterns of phytoplankton communities and their environmental impact factors,stability of phytoplankton communities and its effect on dissolved CO₂ concentration and CO₂ exchange fluxes at the water-air interface in reservoirs,and stable carbon isotopes and functional genetic changes in the carbon cycle.Through the above research,systematically analyze the succession mechanism of phytoplankton community structure and its interaction with the carbon cycle in reservoir.The main results were as follows:（1）Damming reservoirs alter the structural composition,ecological niche,andαdiversity of phytoplankton communities.The structural composition and seasonal succession of phytoplankton communities in riverine and reservoir ecosystems are significantly different,the dominant species in river are mainly Bacillariophyta,such as Navicula sp.,Cyclotella sp.,Melosira granulate,Synedra sp.,Fragilaria sp.,and there is no significant spatial-temporal variation.However,the dominant species in reservoir belongs to Cyanophyta,Bacillariophyta or Pyrrophyta,such as Pseudanabaena sp.,Synedra sp.,Cyclotella sp.,Fragilaria sp.,Melosira granulate,Peridinium pusillum,and has obvious seasonal succession and vertical succession among different reservoirs.The phytoplankton was mainly distributed above 5 m depth,the phytoplankton abundance was highest in July and lowest in January and was gradually decreased with depth.Damming reservoirs increased the niche breadth of phytoplankton and the number of specialists species with narrower niche breadth.Both the species richness index and the Shannon diversity index are smaller in the river than in the reservoir,while the Pielou index has the opposite trend.（2）Environmental factors affecting the succession of phytoplankton communities in reservoirs and rivers are different and the assimilation and uptake of nutrients（C,N,P,Si）in reservoirs are controlled by the composition of phytoplankton communities.Strong phytoplankton photosynthesis drives significantly stronger interactions among environmental factors in the reservoir area than in the riverine area.The explained variance of environmental factors on phytoplankton community changes in the riverine area（29.36%）was significantly lower than that of the reservoir area（58.54%）.The main environmental factors affecting the changes in the phytoplankton community are T,NO₃^-,Si O₃^2-,TN,CO₂,which mainly affect the dominant species belonging to the non-diatom phylum.The phytoplankton growth in the reservoir area had significant retention efficiency for riverine nutrients,with the highest assimilate rate in July and the lowest in January.The concentrations of all major nutrients except ammonia nitrogen（NH₄⁺）showed lower concentrations in the reservoir area than in the river area.The annual average assimilates rate of CO₂,PO₄^3-,and Si O₃^2-were higher.The assimilate rate of the nutrients also increased with the increase of phytoplankton abundance and the dominant species shift to cyanobacteria or dinoflagellate.（3）The stability of the phytoplankton community in river and reservoir is different and affects the dissolved CO₂ concentration and the CO₂ exchange flux at the water-air interface（Flux（CO₂））.The stability of phytoplankton community in river was higher than that in reservoir and showed significant differences due to different dominant species compositions among reservoirs.Those reservoirs with cyanobacteria or dinoflagellate as the dominant species have lower community stability than with diatoms as the dominant species,and their Bray-Curtis community dissimilarity coefficients（BC）have a greater magnitude of variation and faster community succession rate at seasonal scales and reservoir profiles.Moreover,with the increase of the seasonal BC coefficient of the phytoplankton community in the reservoir,the concentration of CO₂ and Flux（CO₂）decreased significantly.An increase in the seasonal succession rate(K_Season)of the phytoplankton community promotes the absorption of dissolved CO₂ by phytoplankton,thereby reducing Flux（CO₂）,and as the biomass of the reservoir increases and the dominant species shifts from diatoms to cyanobacteria or dinoflagellate,the K_Season increases and was accompanied by an efficient fixed rate of dissolved CO₂ and low Flux（CO₂）.（4）Reservoir hydraulic load regulates the succession rate of the phytoplankton community and assimilate rate of nutrients.Low hydraulic load reservoirs with long water retention times and shallow water depths are conducive to rapid phytoplankton growth（especially non-diatom species such as cyanobacteria,chlorophyte,or dinoflagellate）and also can significantly increase the stratification intensity of environmental parameters such as dissolved oxygen（DO）,chlorophyll（Chl-a）,and p H in reservoir profiles,these reservoirs have faster succession rates of phytoplankton community and assimilate of nutrients.However,high hydraulic load reservoirs with short water retention time and deep water depth are not conducive to phytoplankton growth and reproduction,resulting in a slower community succession rate and assimilate rate of nutrients in these reservoirs.（5）Stable carbon isotope composition and carbon cycle functional genes confirm the synergistic change of phytoplankton community succession and carbon cycle in reservoir,and reveal the fundamental mechanisms of phytoplankton community succession based on the species specificity of phytoplankton carbon concentration mechanisms（CCMs）and their effects on carbon isotope fractionation.The stable isotopic composition of dissolved inorganic carbon(δ¹³C-DIC)and particulate inorganic carbon(δ¹³C-POC),as well as functional genes related to carbon cycle varied significantly with the phytoplankton community succession.Theδ¹³C-DIC andδ¹³C-POC of reservoirs with diatoms as the dominant species are generally lower than those of reservoirs with cyanobacteria or dinoflagellates as the dominant species,and the strong C fixation in the euphotic layer also resulted in greatδ¹³C-DIC and CO₂stratification in the reservoir profile.Moreover,cyanobacteria,green algae or dinoflagellate as dominant species in aquatic ecosystems have a higher abundance of carbon fixation and carbon degradation functional genes than diatoms as dominant species.The abundance of carbon fixation genes is higher than that of carbon degradation genes,especially dominated by cyanobacteria species.The results reveal that the phytoplankton community succession is triggered by the increase of water temperature,the phytoplankton photosynthesis changes the p H and thus regulates the dissolved inorganic carbon（CO₂ and HCO₃^-）availability.In turn,changes in CO₂ and HCO₃^-availability can regulate the direction of phytoplankton community succession due to the species specificity of CCMs,as CO₂ concentration decreases,the dominant phytoplankton shift to those species such as cyanobacteria or dinoflagellate that have efficient CCMs mechanisms and can continuously absorb HCO₃^-to maintain growth.