Carbon Limitation Of Biological Carbon Pump Effect And Its Significance For Eutrophication Management

The carbon cycle has always been the focus of global change research.The carbonate weathering carbon sink coupled with aquatic photosynthesis is an important part of the carbon cycle,and the biological carbon pump is the key mechanism to stabilize the carbonate weathering carbon sink.Aquatic photosynthetic organisms convert inorganic carbon in water bodies into organic matter through biological carbon pumps,and then bury them to form stable carbon sinks.The biological carbon pump effect will be limited by carbon（carbon fertilization）.At present,the carbon limitation mechanism of the biological carbon pump is still unclear,and the response mechanism of planktonic algae and submerged plants to changes in water DIC concentration is rarely reported.In addition,the research on carbon limitation of biological carbon pump only stays in test sites and small watersheds,while in large natural karst lakes,the carbon limitation of biological carbon pump needs to be further studied.Based on the above problems,this study took Dali Erhai Lake as the research object,and conducted field monitoring and field sampling from July 2020 to April 2021.The community structure and carbon isotopes have been systematically studied,and the following new understandings have been obtained:（1）The hydrochemistry and carbon isotopes of Erhai Lake and the rivers flowing into the lake have obvious temporal and spatial changes,and the biological carbon pump effect is the main reason for the changes in hydrochemistry and carbon isotopes.In terms of time,the water temperature（T）,electrical conductivity（EC）,[Ca²⁺],dissolved oxygen saturation（DO%）,dissolved carbon dioxide(CO_2（aq）)and carbon dioxide partial pressure（p CO₂）of lake water are high in summer and low in winter The characteristics of p H and[HCO₃^-]are high in winter and low in summer,which indicates that the biological carbon pump effect is mainly affected by temperature and light,and the biological carbon pump is the strongest in summer.At the same time,when the aquatic photosynthetic organisms use DIC as the inorganic carbon source for photosynthesis,they will preferentially use the lighter ¹²C,resulting in theδ¹³C_DIC value of Erhai Lake showing the characteristics of positive in summer and negative in winter.Spatially,the average values of water temperature,p H,EC,[HCO₃^-],and Chl-a in Erhai Lake were higher than those of the rivers entering the lake,while TN,TP,DO（%）,CO_2（aq）and p CO₂ were lower than those of the rivers entering the lake.this is mainly because the hydrodynamic conditions between rivers and lakes are different,Erhai Lake is more suitable for the growth of aquatic photosynthetic organisms,that is,Erhai Lake has a stronger biological carbon pump effect.（2）The biomass and population structure of planktonic algae have obvious seasonal changes.The abundance of planktonic algae is the highest in summer and the lowest in spring,while the algal biomass is the largest in autumn,and there is no significant difference in other seasons.And diatoms are the main groups,and in other seasons,diatoms and green algae are the dominant groups.The growth of planktonic algae is mainly carbon-limited in summer and phosphorus-limited in winter.At the same time,the change of algal community structure is regulated by the concentration of CO_2（aq）in the water body.When the concentration of CO_2（aq）is higher than 10μmol/L,the planktonic algae in the system are mainly diatoms and green algae,and when the concentration of CO_2（aq）is lower than 10μmol/L,cyanobacteria will become the dominant group,which indicates that in the water body in the early stage of eutrophication,the growth of diatoms and green algae can be promoted by increasing the concentration of CO_2（aq）,thereby avoiding the outbreak of cyanobacteria.In addition,blooming diatoms are also beneficial for increasing carbon sinks.（3）The biomass and carbon isotopes of the four submerged plants have obvious seasonal changes.The biomass of the four submerged plants in summer was much higher than that in spring;theδ¹³C_plantof Vallisneria natans,Ceratophyllum demersum and Myriophyllum were all positive in summer and negative in spring,indicating that the changes ofδ¹³C_plant of submerged plants were affected by the effect of biological carbon pump.Impact.The growth of Vallisneria natans,Ceratophyllum demersum and Potamogeton malaianus was mainly regulated by the concentration of inorganic carbon in the water,while the growth of Myriophyllum was mainly controlled by nitrogen.The inorganic carbon utilization mechanism of submerged plants is controlled by the CO_2（aq）concentration in the water body.When the CO_2（aq）concentration is too low,the submerged plants will activate carbonic anhydrase and directly utilize[HCO₃^-]as an inorganic carbon source.