Study On Temporal And Spatial Characteristics Of Water Carbon And Methane Emission In Small Lakes And Reservoirs In Metropolitan Area

lake is an active carbon-reactor,which emits a large amount of methane（CH₄）into the atmosphere each year.Continuous changes in the watershed environment had intense impact on lake ecosystems,however,carbon in surface water and air-water interface CH₄ emissions from small metropolitan reservoirs have been rarely reported.In this study,71 small reservoirs was clarified by five subarea in the metropolitan area of Chongqing.Based on high spatio-temporal resolution investigation and analysis,the spatio-temporal characteristics of carbon in surface water and CH₄ flux from 71 small reservoirs with different anthropogenic effects were discussed.We also measured sediment organic carbon and CH₄ potential production rates,trying to find the key influencing factors of CH₄ production and emissions.The main results are as follows:（1）In order to clarify the general characteristics of surface water carbon in small metropolitan reservoirs,this study divided the small reservoirs’lake basin into five zones according to land use type.The results show that the carbon storage of small metropolitan reservoirs is high and shows the characteristics of light carbon enrichment.TIC accounts for about 3/4 of TC(32.94±12.15 mg·L^-1).The concentration of DOC accounts for 22.1%～28.9%of TC,but the average was relatively high,reach 7.39±2.62 mg·L^-1.TC and TIC were higher in autumn and winter than these in spring and summer;TOC was higher in winter and spring than this in summer and autumn;DOC was higher in winter than spring and summer,the lowest in autumn.the seasonal variations of surface water carbon were mainly driven by temperature,precipitation,and aquatic plant.With the deepening of urbanization and the concentration of N and P in small reservoirs increased,the environment became worse and organic pollution accumulated.From the forest zone to the urban-core zone,TC and TIC increased in a"cascade"and eventually reached highest concentration in the rapid-urban zone and urban-core zone.Organic carbons such as TOC,DOC,and POC showed an increase in fluctuation from the forest zone to the urban-core zone.With the deepening of urbanization,the total carbon and organic carbon in the surface water were gradually enriched.TC,TIC and POC in the small reservoirs within rapid-urban zone and the urban-core zone were even more than twice of the forest-zone.The input of biogenic substances such as C,N,and P lead to heterotrophic metabolism,and the impact of autochthonous carbon is another important factor.（2）In this study,the concentration of dissolved CH₄ and CH₄ emission from small reservoirs in the metropolitan area were analyzed by headspace equilibrium method and floating-box method.The results showed that during the study period,the dissolved CH₄ concentration in the surface water of these small reservoirs were all oversaturated and mainly distributed in 0.13～0.98μmol·L^-1 with an average of 0.79±1.20μmol·L^-1.The seasonal patterns were as follows:summer(0.94±1.33μmol·L^-1)was higher than spring(0.80±0.75μmol·L^-1)and winter(0.68±0.73μmol·L^-1),with autumn(0.54±0.48μmol·L^-1)lowest.The CH₄ flux ranges from 0.03 to 3.29 mmol·m^-2·d^-1,and all sampled small reservoirs were presented as CH₄ emission sources.The average CH₄ flux so high that reached 1.07±1.58 mmol·m^-2·d^-1.The seasonal variation of CH₄ emission flux was obvious:the highest in summer was 1.42±2.39 mmol·m^-2·d^-1,followed by autumn(1.16±1.51 mmol·m^-2·d^-1)and spring(1.05±1.14 mmol·m^-2·d^-1),and the lowest in winter(0.63±0.59 mmol·m^-2·d^-1).The seasonal variations of CH₄ concentration and CH₄ flux were impacted by temperature,precipitation and carbon input.（3）The characteristics of dissolved CH₄ concentration and CH₄ flux in small reservoirs from the metropolitan area are analyzed from land use,reservoir functions and environmental factors.Results showed that CH₄ concentration in small reservoirs from the different land use types varied:small reservoirs in zoneⅤwas higher than zoneⅢ、zoneⅣ,and significantly higher than zoneⅠand zoneⅡ.Spatial variation of mmol·m^-2·d^-1 fluxes was similar to CH₄ concentration:small reservoirs from ZoneⅢ(1.28±1.63 mmol·m^-2·d^-1),ZoneⅣ(1.28±1.52 mmol·m^-2·d^-1)and ZoneⅤ(1.21±1.02mmol·m^-2·d^-1)are strong sources of CH₄ emissions,their flux reaching twice that of zone II(0.56±0.87 mmol·m^-2·d^-1)and 4 times that of zone I(0.31±0.20 mmol·m^-2·d^-1).CH₄ flux from irrigated reservoirs(0.94±1.08 mmol·m^-2·d^-1),aquaculture reservoirs(1.20±1.10 mmol·m^-2·d^-1)and landscape reservoirs(1.16±1.08 mmol·m^-2·d^-1)were significantly higher than that of water-source reservoirs(0.49±0.68 mmol·m^-2·d^-1).The driving mechanism of CH₄ emission from different types of small reservoirs was complex and need further study.The CH₄ flux of small reservoirs were positively related to the environmental factors like water temperature,p H and electrical conductivity as basic metabolic factors.CH₄ flux was also significantly positively correlated with TN concentration,but was more closely correlated with TOC.TOC、water temperature and DIC were the best predictors of CH₄ flux,accounting for 12.6%of CH₄ emissions.With the deepening of urbanization and the change of land use,the anthropogenic effects on the reservoir were more and more severe,so the environment quality was worse and the input of C,N further promoted the production and emissions of small reservoirs’CH₄.（4）In order to clarify the variation characteristics of carbon in small reservoirs’sediments in the metropolitan area and the contribution of CH₄ potential production rates to the CH₄ flux,this study analyzed the organic carbon and CH₄ potential production rates of small reservoirs’sediments in five zones.The results showed that readily oxidizable carbon was the main component of total organic carbon（28%～81%）.The variation range of total organic carbon was large,reaching 0.77～33.82 mg·kg^-1,with an average of 6.99±5.36 g·kg^-1.The ranges of readily oxidizable carbon,dissolved organic carbon and microbiological carbon were 0.43～17.22 g·kg^-1,13.43～284.56mg·kg^-1,0.83～895.88 mg·kg^-1,respectively with averages of 3.80±2.78 g·kg^-1,108.44±37.28 mg·kg^-1 and 138.72±123.98 mg·kg^-1.There were no seasonal variations in total organic carbon,readily oxidizable carbon and dissolved organic carbon.（5）Total organic carbon,readily oxidizable carbon,dissolved organic carbon of small reservoirs’sediments were increased volatility from zoneⅠto zoneⅤ,suggesting that the anthropogenic effects have changed organic carbon content of small reservoirs’sediment.MBC from zoneⅠto zoneⅤshowed a significant cascade increase,means that small reservoirs in high urbanization level transformed organic carbon significantly faster.CH₄ potential production rates also showed the tendency of increase from zoneⅠto zoneⅤ.According to Pearson correlation analysis,the total organic carbon and readily oxidizable carbon in the sediments have a positive correlation with the CH₄potential production rates,and also were positively correlated with the CH₄ flux,indicating that the change of organic carbon in the sediments would affect the production and flux of CH₄ to some extent.The CH₄ potential production rates among small reservoirs with different functions varied:water-source reservoirs<irrigated reservoirs<landscape reservoirs<aquaculture reservoirs,which is highly consistent with the air-water interface CH₄ flux among reservoirs with different functions.Human activities in the metropolitan area have changed the quantity of total organic carbon and readily oxidizable carbon of the sediments,which had an important impact on the CH₄potential production rates.