Distribution And Migration Characteristics Of N,P,and TDS In The Water And Sediment In The Lake Ulansuhai

Water and sediment are important ways to regulate and control material exchange and transport in lake aquatic ecosystem.Lakes in cold regions with a long ice-sealing period have some unique characteristics in the distribution and migration of pollutants.The Lake Ulansuhai has an ice-sealing period of up to five months each year averagely,The subglacial water constructs a new medium environment for pollutants migration and transport at the interface between the subglacial water and sediment during the period.The lake water ecological environment therefore could be impacted through the processes of biological assimilation,physical sedimentation,and diffusion.Nitrogen,phosphorus and salinity are the major pollutants in the water environment of the Lake Ulansuhai,which are the important components in the material cycle in the lake.Finding out the distribution characteristics and migration process of the main pollutants in lake during the ice-sealing period and clarifying the mechanism of lake pollution in the cold regions are of great significance for the protection and restoration of lake water ecological environment.In order to reveal the distribution and migration characteristics of nitrogen,phosphorus,and salinity at the water-sediment interface in the Lake Ulansuhai during the ice-sealing period,after the ice thickness reached its maximum in the early February 2018,samples of subglacial water and sediments were collected at seven sampling sites in the Lake Ulansuhai.Ammonia nitrogen(NH4-N),dissolved phosphorus(DTP,DIP)and total solubility in the water body and sediment in the late freezing period were obtained.Based on the concentration of dissolved solids(TDS),the diffusion fluxes of NH4+-N,DTP,DIP,Na+,and Cl-at the water and sediment interface were estimated.The results showed that:(1)The concentration range of NH4+-N in the water was 0.55-1.60 mg/L,with an average of 1.05 mg/L.The concentration of NH4--N in the sediment(0-5cm)was 6.64-18.63 mg/L with an average of 11.92 mg/L,which was 10 times more than that in the water.The diffusion flux of NH4+-N from the sediment to the water was estimated to be 1.282-4.269 mg/(m2.d).(2)The concentration range of DTP in the water was 0.036-0.204 mg/L,with an average of 0.075 mg/L.The concentration of DTP in the sediment was 0.164-0.334 mg/L with an average of 0.254 mg/L,which was 3.4 times higher than that in the water.The diffusion flux of DTP from the sediment to the water was estimated to be 0.033-0.147 mg/(m2.d).(3)The concentration range of DIP in the water was 0.032-0.197 mg/L,with an average of 0.070 mg/L.The concentration of DIP in the sediment was 0.142-0.329 mg/L with an average of 0.231 mg/L,which was 3.3 times higher than that in the water.The diffusion flux of DIP from the sediment to the water was estimated to be 0.028-0.146 mg/(m2.d).(4)The mean distribution coefficients(K)of TDS,Na-and Cl-between the ice and water were 0.02,0.03 and 0.01,respectively.The average diffusion fluxes of Na+and Cl-from the water to the sediment were estimated to be-229mg/(m2.d)and-676 mg/(m2.d),respectively.In conclusion,N,P and TDS in the Lake Ulansuhai were at a high level,and the diffusion flux of DTP is slightly higher than that of DIP.During the freezing process of lake water,with the gradual precipitation of ice crystals,salinity gradually migrated and concentrated to the water body.As a result of this,the concentration of the pollutants increased in the water,and then migrated to the sediment through the driving force of concentration gradient.As the sediment accepted a large amount of salinity as an endogenous source of pollution,it could become the main source of lake water pollution during the ice-freezing period,the ice-melt period,and the period after the ice-melt.