Study On The Relationship Between Algae And Carbon During The Bloom Of Pengxi River In The Three Gorges Reservoir Area

Since the Three Gorges Reservoir began to store water,major changes have occurred in the hydrological characteristics of the main tributaries in the reservoir.Water flow velocity slowed down and the retention time became longer,which caused a large amount of nutrient retention,and there were many reports of bloom blooms（main streams and tributaries such as Pengxi River）.Currently,the studies mostly have focused on the succession of algae species during blooms,the correlation between algae and environmental factors such as nutrients,dissolved oxygen,prediction and control of blooms,etc.There are few studies on the relationship between phytoplankton and greenhouse gas flux and algae photosynthetic carbon sequestration during blooms.To this end,this paper conducts in-situ monitoring experiments in the backwater area of the Pengxi River to analyze the changes in the phytoplankton biomass and different forms of carbon indicators in the water during the bloom period.Then according to the field monitoring situation,carrying out in-situ controlled experiments in the backwater area of Pengxi River and using continuous flow reactors to simulate the bloom process indoor,in order to analyze the bloom biomass and the characteristics of carbon source and sink under different nutrient and light conditions.Finally,the photosynthetic carbon sequestration intensity of algae during field blooms was statistically analyzed according to the water cycle theory and ecological dynamics model.The main conclusions of this study are as follows:fitted by the classic Monod equation.Finally,the photosynthetic carbon sequestration intensity of algae during field（1）During the continuous bloom monitoring of the backwater area of the Pengxi River in the Three Gorges Reservoir,it was found that:（1）The water temperature,DO,p H and alkalinity of the water body during each bloom were significantly affected by external meteorological conditions.The concentrations of TN and TP in the water body were far beyond the standard of eutrophication.Among them,NN and PO₄^3-were the main forms of N and P in the water,and AN was the inorganic nitrogen form preferentially absorbed and used by the phytoplankton in the water.Variations in nutrient concentration in the water during the study period were affected by various mechanisms such as regional input and output and biological effects.（2）There were significant differences in DIC,DOC,and POC concentrations under different operating conditions of the reservoir.Under the influence of algae-driven organisms and the operating status of the reservoir,the average CO₂ in April and May showed a"carbon sink"and it was a"carbon source"in June.CH₄ always showed a"source"characteristic,mainly related to the reservoir water level and algae bloom Photosynthesis is related.（2）The results of in situ controlled experiments during blooming period showed that nutritional salts were closely related to algae growth,and adding appropriate amounts of nitrogen and phosphorus nutrients would obviously promote the growth and reproduction of algae.The greater the light intensity,the higher the Chl.a concentration,which is the maximum when the light transmittance is 100%.During the experiment in May and May,as the concentration of nutrients in the device changed,a transition between carbon sources and sinks was formed,In April,the 0.05 mg·L^-1-P group reached the maximum peak-38.93 mmol·（m²·d）^-1,the maximum carbon sink value of-33.66 mmol·（m²·d）^-1was reached in the 0.5 mg·L^-1-N group on the 7th day in May.During the study period in June,the water body mainly appeared as the source of atmospheric CO₂,and the CO₂ flux released in the 0.15 mg·L^-1-N group device was much lower than that of the other N group,CH₄ always showed a"source"characteristic.Under different light conditions,the CO₂ flux in April and May showed a carbon sink,and in June it showed a carbon source,and the means were-10.12±19.05mmol·（m²·d）^-1,-16.68±14.99 mmol·（m²·d）^-1 and 22.55±29.38 mmol·（m²·d）^-1.And the larger the light intensity,the smaller the CO₂ flux.（3）In the indoor simulation experiment,proper nutrient addition and light promoted the increase of algae biomass.Under different nutrients and light conditions,the concentration of Chl.a increased first and then decreased.Among them,the algae biomass was highest in the reactor when N3-2.7 mg·L^-1,which promoted the algae growth rate the most.When the light intensity was 1000 lux,the Chl.a content in the reactor reached the highest value on the 4th day of the experiment,which was 274μg·L^-1.During the indoor simulation process,the CO₂ flux generally showed a carbon sink trend,and CH₄ was always used as the carbon source.The mean CO₂ flux was the largest in the N2-0.9 mg·L^-1group,which was-14.28±5.91 mmol·（m²·d）^-1.And when the light intensity was 2000lux,the carbon sink effect was the strongest,and its peak of CO₂ flux is-17.93 mmol·（m²·d）^-1.（4）Through the analysis of the classic marine growth kinetics model and the change process of the water carbon system,it found that:The addition of nutrients had a significant effect on the carbon cycle conversion in the effluent water body.When the concentration of P in April was 0.15 mg·L^-1,the carbon fixation strength reached a maximum of 1.62 g Cm^-2d^-1（in C）.And when the light transmittance was 100%under different light conditions,the changes in the components of the carbon system in water were most obvious.When the light transmittance was 100%in May,the carbon sequestration intensity was the largest,at 1.82 g Cm^-2d^-1.In the field-controlled simulation experiments:there are certain correlations between various parameters of the water carbon system,nutrients,and light intensity during different research periods.PO₄^3-showed a significant negative correlation with DIC and CO₂ flux,and a significant positive correlation with DOC and POC.The correlation between CH₄ flux and various nutrients was low,mainly because the release of CH₄ flux was jointly affected by methanogens and organic substrates in water,and its impact mechanism was more complicated.Negative correlation was showed between DIC and CO₂ flux and NO₃^-,NH₄⁺,NO₂^-and PO₄^3-.The light mainly showed a significant negative correlation with the DIC and CO₂ flux in the water in April and May,while the DOC and POC concentrations were positively correlated with the light intensity.In June,DIC was positively correlated with light,while DOC and CO₂ fluxes were negatively correlated with light.Combined with the indoor simulation equations,it was found that the multiple regression modelcould simulate the change of chlorophyll a during field blooms.The accuracy of the fitting effect is mainly related to the nutrients and external environmental conditions of algae growth.In addition,the Chl.a was fitted to estimate the carbon fixation strength of the algae.The results obtained were in good agreement with the measured values.The average determination coefficient r²was 0.8998.