Transformation And Fate Of Carbon Caused By Algae Decay In The Yulin River,Three Gorges Reservoir Area

The transformation and fate of carbon in rivers has an important impact on global greenhouse gas emissions.Influenced by river damming,the frequency of algal blooms in the tributaries of the Three Gorges Reservoir increases,and the dominant algae species succession from fluvial algal blooms?Bacillariophyta and Pyrrophyta?to lacustrine algal blooms?Cyanophyta and Chlorophyta?occurs.Algae blooms provide fresh organic matter to rivers,and different dominant algae species contain different algal organic matter?AOM?,which can cause the change of carbon transformation and fate in rivers.In order to clarify the role of algae in changing the carbon transformation of rivers,the Yulin River,a tributary of the Three Gorges Reservoir,was selected as sampling site,and the detailed research works can be divided into three parts:First,the spatial distribution of carbon in the Yulin River,as well as the influences of human activities and environmental factors on carbon distribution were investigated in situ.Then,through laboratory experiment,the natural and rapid decay of Microcystis aeruginosa were simulated,the key factors and mechanisms affecting carbon transformation and fate during algae decay were explored,and the differences and causes of carbon release in different algae decay patterns were compared.Finally,the simulation experiment of the decay of fluvial algal blooms?Cyclotella sp.?and lacustrine algal blooms?Chlorella pyrenoidosa and Microcystis aeruginosa?was conducted,and the differences and causes of carbon transformation and fate in dominant algae species succession caused by damming were explored.From the estuary to Donghe and Xihe sections of the Yulin River,carbon in water phase was mainly composed of inorganic carbon,and was higher in the downstream than in the upstream.While carbon in sedimentary facies was mainly composed of organic carbon,varied greatly along the river,and showed the characteristics of accumulation in front of the dam and decrease behind the dam.The coastal human activities significantly increased the concentration of carbon in water phase and shaped the community structure of planktonic bacteria,but had no significant impact on carbon in sedimentary facies and sedimentary bacteria community.On the contrary,the interception of river damming both significantly reduced the content of downstream sedimentary carbon and caused the accumulation of sedimentary carbon in front of the dam,but had no significant effect on carbon in water phase in the short term.Environmental factors mainly affected the spatial distribution of carbon in water phase.Further,the linear correlation between chlorophyll a?Chl-a?and particulate organic carbon?POC?was the most obvious,accordingly the contribution of algae-derived biomass to carbon in the Yulin River could not be ignored,and might affect the release of methane?CH₄?and carbon dioxide?CO₂?.Based on the results of in-situ investigation,the decay experiments of Microcystis aeruginosa,a dominant species of algal blooms in the Yulin River,was conducted.It was found that the decay of Microcystis aeruginosa was of stages,and that the decay had a significant promoting effect on the release of CH₄ and CO₂ in rivers.The cumulative release of CH₄ was 22.80 times and 37.72 times of the control group,respectively,and the cumulative release of CO₂ was 5.36 times and 4.03 times of the control group,respectively.The main mechanism of algae decay promoting carbon release was that algae released a large amount of fresh AOM during decay.The biodegradable components,mainly aromatic proteins and soluble microbial metabolites,provided substrates for microbial metabolism,and created suitable anaerobic environment for methanogens at the same time,promoting the production of CH₄ and CO₂.The total carbon released by Microcystis aeruginosa in different decay modes was not significantly different,but the rapid release of AOM during rapid decay significantly changed the fate of algae-derived carbon,making CH₄ account for 27.49%of the total carbon released by rapid decay,which was 1.86 times that of natural decay.Furthermore,the decay experiments of fluvial and lacustrine dominant algae showed that the carbon transformation and fate of different dominant algae species was different during the decay process.As to the carbon in water phase,the degradation rate of particulate total carbon?PTC?of Cyclotella sp.?83.3%?was lower than that of Chlorella pyrenoidosa?95.3%?and Microcystis aeruginosa?94.5%?.For sedimentary carbon,Cyclotella sp.was more likely to settle on the surface of sediments because of its larger sedimentation coefficient and the lack of movement structure.As a result,the contribution of algae-derived carbon of Cyclotella sp.to surface STC?39.4%?was larger than that of Chlorella pyrenoidosa?17.4%?and Microcystis aeruginosa?17.3%?,leading to a higher abundance of sedimentary microorganisms at the same time.In terms of carbon release,the decay of Cyclotella sp.,Chlorella pyrenoidosa and Microcystis aeruginosa could significantly promote the release of CH₄ and CO₂,but the fate of carbon were different.Cyclotella sp.released more CO₂ per unit carbon than Chlorella pyrenoidosa and Microcystis aeruginosa,while Chlorella pyrenoidosa and Microcystis aeruginosa released more CH₄ per unit carbon.This was mainly due to the less biodegradable AOM provided by Cyclotella sp.blooms,resulting in relatively low abundance of methanogens,and the result of methane oxidizing bacteria transforming CH₄ into CO₂ in aerobic environment.It was concluded that the succession of dominant algae species in tributaries from fluvial algal blooms to lacustrine algal blooms would reduce the carbon deposition of rivers and increase CH₄ release,which would aggravate the greenhouse effect of rivers,and then increased the frequency of Cyanobacteria blooms,forming malignant positive feedback.In summary,the contributions of algae to the carbon of the Yulin River could not be ignored.AOM released by the decay of algae could significantly promote the release of CH₄ and CO₂ by providing metabolic substrates and creating anaerobic environment.Algae succession and algal blooms caused by river damming had a positive feedback effect on the greenhouse effect.In view of this,future research should further investigate the impact of algae and damming on river ecosystem,and explore measures to deal with adverse effects,so that dam could create both socioeconomic benefits and ecological benefits at the same time.