Variation And Export Of Riverine Dissolved Organic Carbon In The Three Rivers' Headwater Regions

The Three Rivers'headwater region on the Qinghai-Tibet Plateau is the headwater region for Yangtze River,the Yellow River,and the Lancang River.There are numerous rivers in this region,and the total annual runoff is about 50 billion cubic meters.The Three Rivers'headwater region belongs to the alpine area,and its ecosystem is extremely fragile and sensitive to climate change.Changes in the ecological environment in this area may threaten the supply of water resources in China and Southeast Asian countries.Research on the water quality characteristics of river bodies in this area can provide important data for multi-objective decision-making for the utilization of the southwestern river water resources and ecological environment protection.Rivers are the only channels connecting terrestrial ecosystems and aquatic ecosystems.The transport of dissolved organic carbon?DOC?is not only an important component of the global carbon cycle,but also an important part of the food web of aquatic ecosystems.As a source of energy for many organisms,the DOC is a hot issue in the research of biogeochemistry under global warming scenarios.The Three Rivers'headwater region occupies an extent area of permafrost.Due to the protection of low temperature and permafrost,organic matter decomposition rate is slow and high stock of organic matter has been accumulated over a long period of time.This accumulation mechanism leads to a higher soil organic carbon content in permafrost than that of non-permafrost Area.During the past decades,permafrost has experienced widespread degradation,which is indicated by the increase of active layers and the disappearance of permafrost.Permafrost degradation can thaw previously frozen organic matter,which can result in decomposition or trans-regional export along with runoff.At the same time,the permafrost degradation on the Qinghai-Tibet Plateau generally leads to vegetation degradation,which not only changes the formation and accumulation process of organic carbon,but also affects surface runoff,thus affect the transport of dissolved organic carbon.Therefore,under the background of global changes and the increasing intensity of hydropower development in southwest China,it is important to understand the river water quality characteristics and its changing patterns in this region.So far,little is known about the water quality in this area.This study invesigated the spatial-temporal variation of DOC in the rivers of the Three Rivers'headwater region,examined the variation of the DOC content,characteristics of the river,and their influencing factors.The results can provide the scientific basis for the environmental protection of the Three Rivers'headwater region and the hydropower development of the southwestern rivers in China.Throughout a hydrological year,river water samples were collected from 47 small rivers and 2 large rivers in the Three Rivers'headwater regions.These samples were analyzed in laboratory,and the results were also discussed using spatial analysis.The water physico-chemical characteristics,such as DOC concentrations,total suspended solids?TSS?concentrations,and SUVA₂₅₄ values,conductivity and pH were measured under typical land cover types,including alpine swamp meadow?ASM?,alpine meadow?AM?,alpine steppe?AS?and alpine desert?AD?.The relationships among watershed areas,discharges and river DOC concentrations were exmamined.The seasonal variation patterns of the DOC concentrations and DOC fluxes of 47 rivers were analyzed to examine their responses to air temperature and precipitation.In addiiton,samples collected from 12 rivers under typical vegetation types were incubated in laboratory,and the relationships among the land cover types,the compositions of DOC,permafrost areas,biodegradability of dissolved organic carbon?BDOC?and the biodegradation kinetics were examined.The main conclusions of this study are as follows:?1?During summer,the riverine DOC concentrations ranged from 2.89 mg/L⁵.98 mg/L,with an average value of 3.89 mg/L in the Three Rivers'headwater regions.The highest DOC concentrations were recorded in the rivers within the catchment of ASM and AM,followed by meadow.These rivers also had the lowest TSS concentrations.The lowest DOC concentrations were recorded in rivers in the AS and AD.The riverine DOC concentrations were significantly positively correlated with the proportions of ASM in the catchments,but significantly negatively correlated with the proportions of AS and AD respectively.The SUVA₂₅₄ values were significantly negatively correlated with DOC concentrations.The rivers within AS and AD had the highest total suspended solids concentrations.Riverine DOC concentrations were also negatively correlated with pH,the catchment areas and the discharges.?2?The average monthly DOC concentrations of 47 rivers in the Three Rivers'headwater regions ranged from 2.92 mg/L to 6.82 mg/L,with an average of 4.39 mg/L.From the spring to the beginning of summer,DOC concentrations increased sharply.During this period,the monthly mean temperature rised from-11?to 2?,and reached the maximum value.Then,DOC concentrations decreased rapidly as the temperature rising the highest temperature from 2?to 13?.From summer to winter,the average DOC concentrations decreased gradually during the temperatures decreasing from 13?to-11?,and the lowest value was recorded in November.There were a great variance in the seasonal DOC export fluxes,ranging from 0.406 kg/?km²·d?to 11.022 kg/?km²·d?among the rivers under different land cover types.DOC export fluxes were positively correlated with average runoffs in the watersheds.The highest DOC export was recorded in the spring snowmelt period and the rainy summer.?3?The land cover types are the main controlling factors for the BDOC concentrations in the permafrost rivers of the Three Rivers'headwater regions.BDOC concentrations were higher in the rivers within the catchments of AM and ASM.The rivers within AS and BL had much lower BDOC concentrations.The values of the proportion of degradability of DOC?BDOC%?under different land cover types varied from 11.72%to 23.66%.The BDOC%were consistent with the changes of DOC and BDOC concentrations.The riverine DOC aromaticity increased along with the vegetation cover,while the biodegradation and degradation rates decreased gradually.During the incubation,the reaction of BDOC was in accordance with the first-order kinetics equation.Furthermore,the BDOC in continuous permafrost regions of river was greater than that in the non-continuous permafrost regions.The BDOC in higher discharges were lower than those with lower discharges.Taken together,the results suggest that the climate warming,which may accelerate permafrost and vegetation degradation,can lead to the decrease of riverine DOC concentration,but increase the DOC export and the soil erosion processes.At the same time,the riverine BDOC contents in the Three Rivers'headwater regions are higher than the soil organi carbon in the catchments.These high contents of BDOC will be experienced rapid decomposition during its transport and release into the atmosphere as greenhouse gas.Under global warming scenarios,the sediment contents in the rivers will increase,while the DOC contents will decrease.The increased sediment exports to the lower reaches of the rivers should be taken into consideration into the development of hydropower and environmental protection in the future.