Study On CO₂ Concentration Mechanism And Its Influencing Factors Of Dominant Algae In The Pengxi River Of The Three Gorges Reservoir

In recent years,the emission of greenhouse gases such as CO₂ and CH₄ in large reservoirs has attracted much attention.In the existing studies,people have discussed the greenhouse gas production and accumulation in reservoir at the ecosystem level.There are more studies on greenhouse gas production and accumulation in lake and reservoir related to algae.In the existing research on greenhouse gases in the Three Gorges Reservoir,there are many literatures on greenhouse gases flux monitoring and water bloom caused by reservoir operation.There are few studies on important carbon fixation methods and related enzyme activity changes of dominant algae during the water bloom.According to research,the tributaries of the Three Gorges Reservoir area usually break out of large-scale cyanobacteria and green algae blooms at the turn of spring and summer,so this paper selects the Gaoyang backwater area of the Pengxi River,a tributary of the Three Gorges Reservoir area,as the research area.In situ monitoring of the blooms that broke out in April,May and June.Designed the outdoor and indoor experimental devices for water nutrient and light intensity control experiments,and the algae habitat characteristics,algae physiological indicators and so on were analyzed.Through the C++computer program,the Fridlyand model was used to simulate the CO₂ concentration mechanism（CCM）of dominant algal blooms,and the HCO₃^-and CO₂ concentrations,transport rates and their response characteristics to changes in nutrients and light intensity within the algal cells were analyzed.It aims to provide a method for reference for the future carbon cycle model simulation research of the Three Gorges Reservoir ecosystem.The specific results obtained by the paper are as follows:（1）Pengxi River was limited by phosphorus in April and June,and between nitrogen limit and phosphorus limit in May.The fluctuation of the nitrogen-phosphorus mass concentration ratio（TN/TP）was mainly regulated by the phosphorus content.When the concentration of nitrogen in the water was too high,it would inhibit the release of TP.Different periods of rainfall have different effects on air temperature and air pressure,and the reduction in light intensity would also exacerbate the impact of rainfall on some water indicators,thereby changing the characteristics of nitrogen and phosphorus in water.When studying the change law of eutrophication characteristics of water,comprehensive consideration should be given to the nutrient concentration of the water,climate and water characteristics.（2）Through the water bloom tracking and monitoring,a total of 28 species of algae in 22 genus in 4 gates of Pengxi River in the Three Gorges Reservoir were identified.The total density of algal cells were significantly positively correlated with chlorophyll a.The dominant algae in April were mainly cyanobacteria,green algae and diatoms,the dominant algae in May were mainly cyanobacteria,green algae,dinoflagellates and diatoms,while the dominant algae in June were cyanobacteria,green algae and dinoflagellates.The activity of extracellular carbonic anhydrase（CA）in April,May and June were 55.16±40.97×10^-6EU·cell^-1,23.52±15.55×10^-6EU·cell^-1,65.79±26.78×10^-6EU·cell^-1.The extracellular CA activity was extremely unstable,and increases with increasing nitrogen concentration,and too low and high light intensity were not conducive to the expression of extracellular CA activity,at the same time affected by the concentration of metal ions,TN/TP,algae cell density,water temperature,concentration of CO₂ and HCO₃^-in water.（3）The concentration of HCO₃^-in water in April,May and June were1798.29umol·L^-1～2077.68umol·L^-1,1038.61umol·L^-1～1792.54 umol·L^-1,1046.87 umol·L^-1～2177.03umol·L^-1,HCO₃^-was the main form of carbon source used by algal cells.The concentration of CO₂ in water were 2.66 umol·L^-1～10.41 umol·L^-1,1.24 umol·L^-1～9.40umol·L^-1,4.67 umol·L^-1～122.00 umol·L^-1.Excessive increase of nitrogen concentration would lead to the increase of CO₂ concentration in water.TN/TP and light intensity were important factors that affect the CO₂ concentration in natural water.However,algae cells could maintain CO₂ steady state in carboxylates through CCM without being affected by the external environment,supporting their photosynthesis to maintain a competitive advantage.Only in unstable environments such as high concentration of carbon and rainfall,the steady state of CO₂ inside algae cells may be destroyed.Algae’s energy synthesis and distribution tend to preferentially protect its own growth and reproduction to achieve stable survival.（4）Whether the transport rate of CO₂ in water by algae is positive or negative depended on whether the CO₂ concentration in the water exceeds the critical value,which in turn affected the transformation of carbon source and carbon sink phenomena.When the CO₂ concentration in the water was lower than 12.40umol·L^-1,it was negative transport,that is,CO₂ diffused from the algae cells to the water body;when the CO₂concentration in the water was higher than 13.83 umol·L^-1,it was positive transport,that is,the algae cells absorbed CO₂ in water.Therefore,the critical value was in the range of12.40 umol·L^-1～13.83 umol·L^-1.When the CO₂ concentration in the water was lower than the critical value,the carbon used by algae mainly comes from the HCO₃^-in water or atmospheric CO₂,which may cause the occurrence of carbon sinks.When the CO₂concentration in the water exceed the critical value,the carbon used by algae mainly comes from HCO₃^-and CO₂ in the water.At the same time,the CO₂ discharged by respiration into the water may eventually lead to the occurrence of carbon source phenomenon.Rainfall caused the CO₂ concentration in the water to exceed the above-mentioned critical value of the CO₂ concentration in the water,and strengthen the influence of nutrient concentration changes on the rate of carbon fixation by algae.Therefore,the change of rainfall intensity in different periods may have an important effect on the transition between carbon sources and sinks during the water bloom of the Three Gorges Reservoir.Changes in light intensity also change the rate of carbon fixation and CCM efficiency of algae.High light intensity would accelerate the operation of CCM in algae cell.