Ice Cover Characteristics And Its Impact On Nutrient Transport Process In Ulansuhai Lake In Inner Mongolia

Seasonal ice cover is one of important phenological characteristics in lakes of North China.Compared to the open season,the physical and chemical characteristics of the lake will be changed by ice-cover in winter.While,the lake ecological conditions were also changed by ice formation.When liquid water is being frozen,the nutrients in the freezing ice could be eliminated from it and enhance the unfrozen water and sediment.The ice medium promotes remarkably the increase of nutrient concentrations in the water and sediment in winter that could be an important factor impacting on the water environmental processes of the shallow lakes.The nutrients in the water body reabsorbed from sediment could exacerbate the spring bloom.Unfortunately,the research on ice-cover lake and its water environment is still in primary stage.At present,about the effect of ice-covered conditions on eutrophication of water environmental in shallow lakes is not clear.With the intensification of global warming,especially in polar region and northern high latitudes area,the researchers took more and more attention on issues of winter ice growth and melting,ice-water energy exchange,ice lake environment and polar ecosystems.In this thesis,based on the field investigation and the simulation experiment,we studied on the ice core structure,the ice growth process and the transportation of nutrient and the pollutants in typical seasonal ice-cover lake?Ulansuhai Lake?in North China.Using HIGHTSI model and degree-day empirical model,we estimated ice thickness in multi-years and pollutants load increased by frozen phenomenon in ice season.We hope the research result can supply theory provides for water environmental protection and the eutrophication control.The main results are as follows:?1?Based on the field investigation and sampling analysis,we found that there is an obvious seasonal variation of nutrient concentration in Ulansuhai Lake water.In winter,the concentration of nitrogen?N,mean value of TN is 9.07 mg/L?,phosphorus?P,mean value for TP is 0.61 mg/L?and chemical oxygen demand(COD_Cr,mean value for COD is131.3 mg/L)in water were higher than in other season.There was a positive correlation between ice thickness and distribution of N,P and the COD_Cr concentration.Meanwhile,the ice cover could block the energy and material exchanges between the atmospheric and the lake,it also weaken the light and slowdown the photosynthetic efficiency in water layer.The dissolved oxygen decreased with water depth.And it showed hypoxic condition in most part of the lake under the ice.?2?According to the investigation and the simulation experiment results,Total P and TN concentrations in the ice exhibited a C-shape or I-shape.And compared with water quality,the ice melting water are quite purity?mean value for TN is 1.006 mg/L;mean value for TP is 0.163 mg/L;mean value for COD is 32.2 mg/L?.Most of the dissolved nutrients are rejected by the growing ice and remain in the liquid water.The primary concentration solution and the temperature condition were the main factors for nutrients content in ice layer.The ice-water distribution coefficient was decreasing with solution concentration increasing and the temperature decreasing.?3?As a result of the considerable nutrient accumulation during winter,nutrient concentrations in sediment are high.The nutrient distribution in surface sediment also had an obvious seasonal variation.Sediment is the"Sink"of nutrient in ice season.And it will be the nutrient"source"under the conditions of temperature increasing and external disturbance by wind in ice melting season.These nutrients,once desorbed from sediment to the water body,could exacerbate spring blooms.?4?In order to improve analyses efficiency for ice structure,using high resolution digital camera to collecting ice slices image records and the image under polarizing microscope.Based on MATLAB image analysis tools,we extracted ice crystals information and gas bubbles information by cell recognition technology.After many times fail explorations and attempts,we successfully obtained the method for batch extraction the information of ice crystal and gas bubbles from image records.?5?According the analysis result of ice cover structure and contaminant concentration distribution in the Lake Ulansuhai:The ice cover mainly belonged to thermal growth and it was consisted of columnar-crystal ice.The ice crystal c-axis showed a random distribution,so it was an isotropic material.Along with the ice growth direction,the upper layer had fine crystal size,large number of gas bubbles and crystals and higher pollutants content;the middle layer was mainly consisted of columnar-crystal ice and low content of gas bubbles and pollutants;the bottom layer had higher pollutants content and bigger size of crystal and gas bubbles.?6?According to the analysis results of ice crystal structure and nutrients concentration in ice melting water,we believe that the contaminant migration characteristics were controlled by ice structure.And solution content in ice has a negative correlation with ice density and gas bubbles content.?7?Taking the year 2012 and 2013 as typical years,we simulated the annual heat budget and its heat component flux in Ulansuhai Lake.We found that there was a significant difference between the heat flux in ice season and in open season.The solar radiation is very high in Ulansuhai Lake,and the mean net solar radiation flux in 2012and 2013 were 210W/m² and 179W/m²,respectively.The increasing rate of solar radiation in spring will influence the ice melting speed.The variation range of langwave radiation flux in 2012 and 2013 were-68⁸1 W/m² and-71¹39 W/m²,respectively.About 280 days in each year were in the state of absorbing heat.The mean net heat flux in ice season of 2012 and 2013 respectively were-48.6W/m² and-43.5W/m²;and respectively the values in open season were 318.9 W/m² and 269.8 W/m².?8?Finally,using HIGHTSI model and degree-day empirical model,we estimated the ice growth process and ice thickness in multi-years.Compared with the measured data,the result of degree-day empirical model was close to the maxim thickness in the lake,and the result of HIGHTSI model was close to the mean ice thickness value and the modeling error was in the acceptable range.Both of them could give a good simulation for ice freezing duration,but only HIGHTSI model could give the more accurate result when ice melting.The result from degree-day empirical model for ice melting had much difference from the investigation value.?9?Combined the result of ice-water partition coefficients under different conditions and ice thickness estimation model,we built a estimation model for water environment nutrient load under non dynamic condition.It could be used to estimate the nutrient concentration under the ice during ice season,and also could supply theory support for water treatment in ice season.In summary,the ice season condition has important influence on ecological process in shallow lakes.To know the effects mechanism of ice cover growth and melting on water environment is significant to s significant to water pollution control and treatment in cold and arid regions.