Transport And Transformation Mechanisms Of Organic Carbon Affected By Reservoirs

As a result of increasing global climate change and the massive consumption of fossil fuels,hydroelectricity is attracting much attention as an essential renewable resource and reservoirs are being built on a global scale.While reservoirs have facilitated social development,they have also brought about significant environmental impacts,such as the retention of nutrients from land-based sources,increased fragmentation of rivers,greenhouse gas emissions and increased eutrophication of aquatic systems.In recent years,more and more attention has been paid to the environmental effects of reservoirs from the world,but most of the existing studies focus on greenhouse gas emissions and materials interception,while the critical processes of carbon cycling in river-reservoir systems are still unclear.Furthermore,an investigation of the key processes will help us to further understand the vital role of reservoirs plays in global climate change.In our research,we exampled cascade reservoirs Chaishitan（CST）,Longtan（LT）,Yantan（YT）and Dahua（DH）reservoirs from Pearl river in different seasons,and combined concentration,stable carbon isotopes,optical spectroscopy and ultra-high-resolution mass spectrometry with the corresponding hydrological information to investigate the transport and transformation of different-forms carbon in the river-reservoir system from molecular to bulk perspective.We aim to answer the transport and transformation of different forms of carbon in the river-reservoir system,the influence of the transformation process on the carbon cycle,and the impact on the river-reservoir system.The main results of this study are as follows:（1）Basic hydrochemical parameters such as temperature,p H and dissolved oxygen showed significant spatial and temporal heterogeneity distribution in the river-reservoir system.Consequently,different forms of carbon such as dissolved inorganic carbon（DIC）,dissolved organic carbon（DOC）and particulate organic carbon（POC）show a significant temporal distribution in the river-reservoir system accordingly.Among them,reservoirs have significant seasonal impacts on dissolved organic matter（DOM）molecules in rivers.Compared to CHO compounds,CHON compounds are more easily degraded in the reservoir effect.The reservoir effect significantly enhances the signal of oxygen-poor molecular compounds in riverine DOM,contributing to the corresponding DOM that is hard to participate in downstream biogeochemical cycles.In addition,a large number of aerosol-sourced DOM molecules in the reservoir area due to the reservoir interception effect suggests that atmospheric deposition may be an important source of dissolved black carbon（DBC）-like compounds in the reservoir.Overall,the apparent anthropogenic and phytoplankton source DOM signals are also significantly lower in the reservoir area,suggesting a higher modification of organic matter by reservoirs compared to river systems.（2）The ultimate impact of large reservoirs（e.g.,LT reservoirs）on riverine DIC showed depletion in all seasons.In contrast,the ultimate impact of LT reservoirs on riverine DOC showed addition and depletion in seasons with high primary productivity（summer and autumn）and low primary productivity（spring and winter）,respectively.In addition,the effect of LT reservoirs on riverine POC in all seasons is an interception effect,i.e.,a large amount of terrestrial POC is deposited in the river-reservoir system,while reservoir POC shows a clear phytoplankton source signal and is significantly depleted in the reservoir area.The transformation intensity of different-forms carbon such as DIC,DOC and POC in the river-reservoir system shows prominent seasonal characteristics,while their absolute variation percent are mainly influenced by hydrological regulation,suggesting that the environmental impacts generated by the reservoir effect can be further weakened through anthropogenic regulation.（3）A corresponding cumulative effect of the cascade reservoir was observed,mainly exhibited as degradation,in which the cascade reservoir showed a selective process on DOM molecular composition,i.e.,the abundance of highly unsaturated CHO compounds increased gradually after the cascade reservoir process,while the abundance of unsaturated CHO compounds decreases gradually after the cascade reservoir process.The abundance of highly aromatic CHO compounds gradually decreases after the cascade reservoir process.Unlike CHO molecules,the abundance of highly unsaturated CHON compounds also decreases after the cascade reservoir process,possibly due to their lower energy molecular bonds or specific microbial community distribution.The abundance of CHOS molecules of anthropogenic sources also decreases after the cascade reservoir process,suggesting that anthropogenic activities may also be directly or indirectly involved in the biogeochemical processes in the reservoir,particularly in the cascade reservoir.In addition,the high proportion of aerosol sources of DBC in the cascade reservoir also suggests that the influence of atmospheric inputs on reservoirs should not be neglected in the future.（4）The Pearl River is characterized by high mean annual runoff,making small and medium-sized reservoirs with shorter water retention times and weaker stratification,thus suppressing the reservoir effect.However,compared to rivers,reservoir effects can still be observed and result in different levels of response for different-forms carbon.The DOC in inflow is often a distinctly terrestrial signal and is accompanied by more positiveδ¹³C values,while the increased activity of planktonic algae in the reservoir area leads to higher DOC concentrations and more negativeδ¹³C values.Although POC had similar C/N values to freshwater planktonic algae at an overall level,the reservoir area had more negativeδ¹³C values than the inflow,particularly in the production layer（0-15 m）of each reservoir.On the other hand,the DIC has relatively more positiveδ¹³C values in the production layer of the reservoir,which are more pronounced during the high productivity season and when the reservoir has a certain water retention time.This phenomenon is weaker during high flow periods,e.g.,DOC,POC and DIC in YT and DH reservoirs have a clear terrestrial signal in summer.Nevertheless,DOC degradation in the DH reservoir was observed during the summer,which is consistent with the molecular level observations.In general,reservoirs of different regulation types respond to seasonal and hydrological regulation to varying degrees.Reservoirs with short water retention times such as DH and YT are dominated by seasonal variability,while reservoirs with long water retention times such as LT and CST are dominated by hydrological regulation.Results from spectral characterization,molecular structure and isotopic signatures indicate rapid transformations between different carbon forms in the river-reservoir system,including between organic and inorganic carbon,and between particulate organic and dissolved organic carbon,accelerating the rate of carbon cycling.In summary,our results suggest that the main form of reservoir action on organic matter,both from the molecular and bulk perspective,is the degradation effect.Therefore,we propose a conceptual organic matter transport and transformation model in the river-to-reservoir system.At the same time,due to the complexity of the molecular composition of DOM,the reservoir effect on the treatment of DOM shows prominent spatial and temporal heterogeneity characteristics.The reservoir’s 0-15 m layer may be the central area of biogeochemical processes in the river-reservoir system,which suggests that the role played by small and medium-sized reservoirs in the global carbon cycle should be taken into account in future studies.Our results also provide scientific support for predicting how reservoirs respond to global climate change and anthropogenic activities and provide a theoretical basis for further attenuating the ecological effects of reservoir subsurface and promoting the conservation and sustainable development of reservoir ecosystems.