Temporal And Spatial Variation Of Total Phosphorus Concentration And Its Influencing Factors In Taihu Lake

Taihu Lake is the third largest freshwater lake in China,and Taihu Lake Basin is China's most important economic core area and urban dense area.Since the 1970s,with the development of agriculture and urbanization,a large amount of nutrient material has been discharged into Taihu Lake,and the eutrophication of water has become increasingly serious,which has caused Taihu Lake to become a heavily eutrophic lake.Taihu Lake is also one of the three lakes which needs to be focused on governance.Phosphorus is the main cause of eutrophication in Taihu Lake.Therefore,understanding the distribution of total phosphorus(TP)in Taihu Lake,controlling the input of TP,purifying water quality and improving water environment is an important direction for Taihu Lake.However,eutrophication is a long-term cumulative change process.Only by mastering the temporal and spatial variation of TP in Taihu Lake,we can further analyze the influencing factors of these changes.The development of satellite remote sensing technology provides a large amount of multi-source remote sensing data for the monitoring of TP concentration in the study area,which makes it possible to explore the relationship between lake eutrophication and terrigenous human activities from the perspective of long-term sequence variation of TP concentration.In this paper,the inland water bodies such as Taihu Lake were taken as the research object,the empirical estimation model of TP concentration for wide-band remote sensing and the TP concentration estimation model for hyperspectral remote sensing were constructed respectively.The temporal and spatial distribution and variation of TP concentration in Taihu Lake was explored.The influence mechanism of climate change on the spatial and temporal distribution of TP concentration in different time scales of different areas in Taihu Lake was analyzed.Combined with the estimation of TP load from the input source,the response of TP concentration in Taihu Lake to land-source input is studied.The research would have positive significance on the monitoring of TP concentration in inland eutrophic lakes represented by Taihu Lake,which can provide a reference for the water environment and water ecological restoration of eutrophic lakes in the middle and lower reaches of the Yangtze River in China.The main contents and conclusions of this article are as follows:(1)Remote sensing estimation models for TP concentration in inland water bodies suitable for different sensor band settings were constructed.For multi-spectral remote sensing,hyperspectral remote sensing and aqua sensor band setting,an empirical estimation model for TP concentration in wide-band and a TP concentration estimation model were constructed respectively.Among them,two kinds of estimation models were constructed for hyperspectral resolution,which is the TPconcentration estimation model based on optical classification and the semi-analytical model of TP concentration based on absorption coefficient.The accuracy was verified by the ground measured data,the results shown that the MAPE and RMSE based on the optical classification model were 26.68%and 0.06 mg/L respectively,and the MAPE and RMSE based on the absorption coefficient model were 16.8%and 0.04 mg/L respectively.The semi-analytical model based on the absorption coefficient had higher estimation accuracy in the low-value region of TP concentration.(2)Achieving long-term monitoring of TP concentration in Taihu Lake using multi-source satellite remote sensing imagery.Combined with GOCI,MODIS,TM and OLI sensors,TP concentration in Taihu Lake from 1984 to 2016 was estimated.Among them,the semi-analytical model based on absorption coefficient was applied to GOCI and MODIS sensors,and TP concentration monitoring data from 2002 to 2016 was obtained.The MAPE of the models applied to GOCI and MODIS data was 21.31%and20.22%respectively.The RMSE was 0.052 mg/L and 0.051 mg/L respectively;the empirical model of TP concentration in wide-band remote sensing was applied to Landsat's OLI and TM data,and the time for satellite observation of TP concentration was advanced to 1984.The MAPE of the model used in the two types of data was 24.13%and 24.35%respectively,and the RMSE was 0.054 mg/L and 0.055 mg/L respectively.(3)The tempor-spatial distribution and variation of TP concentration in Taihu Lake were discussed.From the spatial distribution of TP concentration in Taihu Lake,the concentration of TP in Zhushan Bay,Meiliang Bay and the west coast of Taihu Lake was higher,the concentration of the south coast of Taihu Lake and Gonghu Bay was second,and the concentration of TP in the center of Taihu Lake was the lowest.Judging from the intra-day variation of TP concentration in Taihu Lake,all areas of Taihu Lake basically showed a trend of decreasing from morning to afternoon.From the monthly variation of TP concentration in Taihu Lake,the TP concentration in Zhushan Bay,Meiliang Bay and the west coast of Taihu Lake was higher than the whole lake concentration during the period from March to October and was significantly higher than the rest areas of Taihu Lake.The TP concentration in the south coast and the center area of Taihu Lake was lower compared with other areas.From the interannual variation of TP concentration in Taihu Lake,the concentration of Zhushan Bay was higher than the other areas in almost all years,followed by the west bank of Taihu Lake and the Meiliang Bay,which was higher than the average concentration of the whole lake.The TP concentration in the south coast of Taihu Lake in 2006,2007,2008 and 2009 was higher than the mean value of the whole lake.The TP concentration in Gonghu Bay and Datai Lake was low in all years.(4)The influencing factors of the temporal-spatial variation of TP in Taihu Lake were analyzed.The diurnal variation of TP concentration in all areas of Taihu Lake was mainly affected by light and temperature.The monthly variation of TP concentration in Zhushan Bay,Meiliang Bay and the west coast of Taihu Lake was mainly affected by temperature,and the monthly variation of TP concentration in Gonghu Bay was mainly affected by total precipitation,while the main influencing factors of the monthly variation of TP concentration in the south coast and the center area of Taihu Lake were wind speed;the interannual variation of TP concentration in Taihu Lake was mainly affected by annual average wind speed.The interannual variation of TP concentration in Zhushan Bay,Gonghu Bay,the south coast and the center area of Taihu Lake was mainly affected by wind speed,while Meiliang Bay and the west coast of Taihu Lake was less affected by meteorological factors.(5)The relationship between the TP concentration in Taihu Lake and the input of land source into the lake was revealed.The TP concentration in the estuary of the western part of Taihu Lake was the highest in autumn,and the range of high concentration area was larger than other seasons.It was mainly distributed in the coastal and northern areas.Rain erosion would bring a lot of nutrients from the surface to the river,and finally the TP concentration in the estuary increased through river transport;the land use in the western lake basin of Taihu Lake changed greatly from 1984 to 2014,and the water and cultivated land area decreased by 23.92km~2 and 739.72km~2respectively.The wasteland and forest land increased by 412.60km~2,218.60km~2 and132.43km~2 respectively.At the same time,with the population and economic growth,the TP load of terrestrial resources showed a gradual increasing trend,and the corresponding TP concentration in Taihu Lake also increased from 1984 to 2014gradually.In 2006,the concentration reached the maximum.The Zhushan Bay,Meiliang Bay and the western coast were areas with high TP concentration.The TP concentration in Taihu Lake had a high fitting accuracy with the TP surface source load in the corresponding year,and R~2 reaches 0.87,indicating that the input of phosphorus in the surface source of the western lake basin of Taihu Lake was an important factor affecting the increase of phosphorus content in Taihu Lake.The western coast of Taihu Lake was the most affected,followed by the southern coast and the center area of Taihu Lake.