Spatiotemporal Variation Characteristics Of Water Chemistry And Its Controlling Factors In Mitika Wetland,Tibet

The Mitika wetland is the water source of the Lhasa River,it is the only wetland in Tibet Autonomous Region that integrates the honor of internationally important wetlands and national nature reserves.Mitika wetland plays an important roles in regulating the melting water and surface runoff in the mountainous areas,controlling soil erosion,and maintaining the water level and water balance for the downstream.Its water environment quality of the wetland is not only the guarantee for the healthy development of the local ecosystem,but also has a direct impact on the water safety in the downstream.Water is the main component of wetland to play its ecological function and the changes of wetland water quality directly affects the ecological process of wetland.There is lack of report on the study of the water chemistry characteristics of Mitika wetland.In this study,the main water systems in Mitika wetland were taken as the research object.From 2018 to 2019,The physiochemical parameters,water chemistry composition and its control factors of the wetland water bodies were studied for the first time in warm season（July）and cold season（October）.The results showed that:（1）effected by the shallow water depth in the wetland,the major water system in Mitika wetland showed an oxidable condition and,the water temperature displayed a seasonal change.The average temperature in warm season is 12.9℃,which is significantly higher than that in the cold season（5.8℃）.Affected by the supply mode,the wetland water body was alkaline（average pH value is8.84）,and EC,TDS and Sal.were lower than that in the main stream of Lhasa River in the lower reaches;（2）The cations of Mitika wetland were mainly Ca²⁺and Mg²⁺,while the anions were mainly HCO₃^-and SO₄^2-.The main water chemistry type of wetland could be concluded as HCO₃·SO₄-Ca·Mg type;（3）The main ions in the water body of Mitika wetland were mainly controlled by the weathering of carbonate and silicate rocks in this basin.Among them,the weathering of carbonate rock was the main source of water ions,and the weathering of silicate rock was relatively weak.During the weathering process of carbonate rocks,sulfuric acid was also participated.Over all,the water environment of Mitika wetland was relatively decent,but a future study should be strengthened on the environmental effects of heavy metals on the natural wetland of plateau.