Integrated Study On Effecting Factors And Management Strategies Of Phytoplankton Carbon Sequestration Capacity In Lakes

Climate change has become one of the major challenges hindering the sustainable development of human society.While the“Carbon emission reduction”schedule relied on the traditional industries encounters a bottleneck,the“Nature based solutions”would become a favorable supplement to deal with the climate change.“Blue carbon”refers to the carbon sequestration in the aquatic ecosystem,and its concept has been extending from the ocean area to inland waters.Wetland ecosystems,such as lakes,reservoirs and ponds,account for only 5%-8%of the global area,which has a great impact on the regional carbon cycles.However,the study related to“blue carbon”in inland waters,especially in lakes,is lagging behind.The 2020 projects guidelines of the major research program,the mechanism of hydrosphere microorganisms driving the earth’s element cycles,propose that it is necessary to clarify the interaction mechanism relationship between lake community and environment in typical lake and wetland habitats,and to carry out relevant research by using the concepts of calculation and simulation.The main subjects of this study are to assess the carbon sequestration potential capacity of lakes,identify the water quality and land use change index that affect the carbon sequestration of lake phytoplankton,clarify the cross-scale correlation mechanism of lake carbon sequestration,and put forward some policy recommendations for the lake protection and blue carbon utilization.Further,this study is consisted of three parts:Part One Description Analysis:estimation of primary productivity and carbon sequestration of phytoplankton in lakes,Part Two Casual Relationship Analysis:identification of water and land environmental factors affecting carbon sequestration of phytoplankton in lakes,and Part Three Management Suggestion:multi-agent simulation of carbon sequestration of phytoplankton.Firstly,the primary productivity of phytoplankton and carbon sequestration were estimated.Based on the monitoring data of physical,chemical and biological indicators of 63 lakes in Wuhan city,an inland city in Central China,the vertical generalized production model（VGPM）was used to calculate the primary productivity of phytoplankton and the carbon sequestration of lakes.The main results were as follows:1)the total carbon sequestration of 63 lakes in Wuhan city in 2018 was 0.56×10⁶t C·a^-1,and the primary productivity was 870178.25 mg C·m^-2·a^-1.Although the total carbon sequestration of lakes in the study area was much lower than that in coastal waters of China(range from 8.62×10⁶t C·a^-1to 416.64×10⁶t C·a^-1),the carbon sequestration per unit area was slightly higher than that in the coastal water of China(range from 112000 mg C·m^-2·a^-1to 408000mg C·m^-2·a^-1).2)The carbon sequestration of each lake was between 14.91 mg C·m^-2·a^-1and 29305.08 mg C·m^-2·a^-1,and there was a great difference between lakes.The lakes with higher carbon sequestration were mainly distributed in the south-east of Wuhan city.There were strong positive relationships between carbon sequestration,phytoplankton primary productivity,lake water area,and carbon sequestration time.Among the studied 63 lakes,the Daoshi lake has the lowest annual carbon sequestration,which has an annual carbon sequestration of 19.70 t C·a^-1,a primary productivity of 98495.23 mg C·m^-2·a^-1,and a lake area of 0.20 km².The annual carbon sequestration of the Liangzihu lake was the highest,its annual carbon sequestration was 69191.96 t C·a^-1,its primary productivity was247114.13 mg C·m^-2·a^-1,its lake area was 280 km².Secondly,in order to clarify the water and land environmental impact and interaction mechanism of phytoplankton carbon sequestration in lakes,the nested-structure data was used to present the relationship between the water quality and land environmental factors across scales,and the hierarchical linear mode was introduced to identify the water and land environmental factors affecting phytoplankton carbon sequestration in studying area.The study proposed three sub-hypothesis frameworks in schedule.1)In the micro-directly-hypothesis sub-framework,the water transparency,water temperature,and phytoplankton biomass directly affect phytoplankton carbon sequestration.2)In the micro-indirectly hypothesis sub-framework,the water temperature,total nitrogen,total phosphorus,nitrogen phosphorus ratio,Rotifera biomass,Cladocera biomass,and Copepoda biomass directly affect the biomass of phytoplankton biomass,and indirectly affect the phytoplankton carbon sequestration.3)In the macro-hypothesis sub-framework,the change of land use proportion around the lake directly affects the water environment and indirectly affects the phytoplankton carbon sequestration.In addition,hierarchical linear modes with primary productivity as output variable,hierarchical linear modes with phytoplankton biomass as output variable,and multiple linear models with land use type proportion as output variables were established.Through the integration of theoretical hypothesis frameworks and mathematical model fitting results,the study found that:1)at the micro level,total phosphorus concentration had the strongest impact on phytoplankton biomass,followed by total nitrogen,Rotifera biomass and water temperature.2)Macro land use change and micro water environment change would have a synergistic effect on phytoplankton community,and then affect the carbon sequestration.The specific performance is as follows:（1）The increase of cultivated land area was accompanied by the increase of water transparency and water temperature,which promoted the primary productivity of phytoplankton,and had a positive impact on carbon sequestration of phytoplankton.（2）the increase of forest land area was accompanied by the increase of water temperature,which synergistically improved the biomass of phytoplankton,and further promoted the carbon sequestration.（3）the increase of grassland area was accompanied by the decrease of water transparency and water temperature,which synergistically reduced the primary productivity and biomass of phytoplankton,and weakens the carbon sequestration of phytoplankton.（4）the water transparency and total nitrogen concentration increased with the increase of water area,which enhanced phytoplankton biomass and primary productivity,and then promoted phytoplankton carbon sequestration.Finally,the study proposed that lake social-ecological management system was a complex adaptive system with multi-objective hierarchy and emergence.The emergence of lakes was manifested in the changes of micro physicochemical factors,phytoplankton and zooplankton in their respective behaviors and interactions.And there would be changes in lake carbon sequestration and water environment quality in the system level.Four subsystems were defined in this study,such as phytoplankton behavior subsystem,zooplankton behavior subsystem,physicochemical factor environment subsystem and manager decision subsystem.The behind three subsystems were the main systems.The consumption,growth,reproduction,movement,death,predation and predation in phytoplankton and zooplankton subsystems,and the diffusion,consumption,and release of nitrogen and phosphorus nutrients in environmental subsystem,and the exchange rules of biological carbon,nitrogen and phosphorus between subsystems were quantified.As a boundary sub-system,the manager decision-making subsystem adjusted the state of the main systems by making corresponding lake management policies.Then the mathematical function was transformed into computer simulation language,and the model was constructed and adjusted by using Net Logo computer simulation tool.The temporal and spatial dynamics of carbon sequestration related to lake,phytoplankton,zooplankton,and bioclast were simulated under different water quality scenario,temperature scenario,pollutant discharge scenario and fish swarm release scenario.The main findings were as follows:1)In the water quality scenario,based on the water temperature was set at 20℃and the light condition were sufficient.（1）the change of nitrogen and phosphorus nutrient concentration in the water would have a nonlinear effect on the carbon sequestration.With the increase of nitrogen and phosphorus concentration in the water,the promotion effect of nitrogen and phosphorus on carbon sequestration would gradually weaken.（2）the effect of phosphorus on carbon sequestration was stronger than that of nitrogen,which was shown as follows:the nitrogen concentration decreased from the standard limit value of Class III water（total nitrogen:1.0 mg/L）to the standard limit value of Class V water（total nitrogen:2.0 mg/L）,the carbon sequestration amount increases by 34000 units.However,the phosphorus concentration decreased from the limit value of class III water（total phosphorus:0.05 mg/L）to the limit value of class V water（total phosphorus:0.2 mg/L）,and the carbon sequestration increased by 197000 units.（3）The nitrogen and the phosphorus not only affect carbon sequestration,but also have effects on the relative biomass accumulation of phytoplankton and zooplankton in the water environment.The biomass of phytoplankton is higher than that of zooplankton in the water environment with low concentrations of nitrogen an phosphorus（such as SNIIIPIII,total nitrogen:1.0mg/L,total phosphorus:0.05mg/L）.The biomass of zooplankton was higher than that of phytoplankton in the water environment with high concentration of nitrogen and phosphorus（SNVPV,total nitrogen:2.0mg/L,total phosphorus:0.2mg/L）.2)Under the temperature scenario,the concentration of nitrogen and phosphorus in the water body was set as the standard limit of ClassⅣwater body（total nitrogen:1.5 mg/L,total phosphorus:0.10 mg/L）.There was a nonlinear relationship between water temperature and carbon sequestration.With the increase of temperature,the promotion effect of temperature on carbon sequestration of the lake would gradually increase.3)Under the pollutant emission scenario,the concentration of nitrogen and phosphorus in the water body is set as the standard limit of class IV water body,temperature can promote carbon sequestration,phytoplankton and zooplankton biomass,and the promotion effect on phytoplankton biomass is stronger than zooplankton.The change of temperature will change the driving mechanism of carbon sequestration in lake ecosystem.For example,at low temperature（such as 0℃and 10℃）,carbon sequestration mainly depends on zooplankton and phytoplankton.At medium and high temperatures（such as 20℃and 30℃）,the contribution of debris to carbon sequestration increases gradually.3)Under the pollutant emission scenario,the water temperature was set to 20℃,and the concentration of nitrogen and phosphorus in water body was set as the standard limit of ClassⅣwater body,（1）Compared with the First-level A emission scenario（total nitrogen:15 mg/L,total phosphorus:0.5 mg/L）,the carbon sequestration of the First-level B emission scenario（total nitrogen:20mg/L,total phosphorus:1.0mg/L）increased by 293000 units.（2）Although total amount of nitrogen and phosphorus in the system increased under the First-level B emission scenario,it did not have a significant impact on the water quality.Because this part of the increased nitrogen and phosphorus entered the phytoplankton and zooplankton through feeding.4)Under the fish swarm release scenario,the water temperature was set at 20℃,the initial concentration of nitrogen and phosphorus in water was set as the standard limit value of ClassⅣwater body,and the nitrogen and phosphorus emission concentration were set as First-level A.Compared with the strategy of feeding only on phytoplankton or omnivorous fish,the strategy of feeding only on zooplankton was the best.Under this situation,the amount of carbon sequestration could be greatly improved.At the same time,the quality of water environment could also be improved.In conclusion,there is a certain carbon sequestration capacity and potential in lakes of Wuhan city in China.The carbon sequestration capacity of lakes is affected not only by water temperature,nitrogen,phosphorus,and zooplankton biomass from the micro level,but also by land use cover and change from the macro level.The concept of"social ecological management system of Lake"is put forward in this paper.Combined with the constrained management objective of"water environmental quality can only develop for the better",a system management decision-making framework of combining the elastic index of"carbon sequestration"with the constrained index of"water environmental quality"is further proposed.