| Microcystin produced by microcystis in water poses a serious threat to public health.The toxins produced by microcystis mainly include isomers such as MC-LR,MC-RR and MC-YR,and MC-LR has the strongest physiological toxicity.Microcystins(MCs)not only have a strong tumor-promoting effect,but also can be toxic to the kidneys and intestines,thus posing a serious threat to the health of humans and livestock.Therefore,it is of great significance to monitor the information of total microcystin in fresh water,especially to realize continuous macro monitoring of microcystin concentration by using remote sensing technology,which plays an important role in protecting people's life safety and eutrophication water treatment.Based on Taihu lake as the study area,this paper based on field measured data and in situ observation in 2017 Sentinel-3A OLCI and MERIS image,constructed for Taihu total of algal toxin concentration(TMC)remote sensing estimation model,including empirical model and analysis model,and evaluate the accuracy of each model validation,finally choose three-band model is applied to the image,get the Taihu lake in 2017 spring,summer,autumn and winter seasons as well as the 2003-2011 TMC space-time distribution,and analyze the driving forces of the time and space change of TMC.The main conclusions are as follows:(1)Optical characteristics and sensitive bands of Taihu lake TMCIn the wavelength range of 500-750 nm,the remote sensing reflectance of different TMCS showed obvious differences.In general,with the increase of TMC,the depth of reflection valley around 675 nm is also increasing.TMC has a good correlation with remote sensing reflectivity,with the largest negative correlation near 520 nm,the correlation coefficient reaching 0.75,and the largest positive correlation near 709 nm,the correlation coefficient reaching 0.8.(2)Construction of Taihu lake TMC inversion modelBased on spectrum analysis of the measured data,has been clear about the sensitive to changes in TMC,the characteristics of band based on features band and band combination build the three-band model of TMC estimation in Taihu lake,partial least-squares regression model,and half an analysis model based on radiative transfer theory,and use the Taihu unified independent validation data set for evaluation of the accuracy of each model are verified.The results show that RMSE value of three-band model is the lowest and that of semi-analysis model is the highest.From the perspective of MRE,the same three-band model has the lowest MRE value,and the partial least squares regression model has the highest MRE value.The three-band model has a good inversion effect.(3)The spatial and temporal change characteristics of TMC in the four seasons of Taihu lake in 2017Based on the OLCI image,the spatial and temporal change characteristics of TMC in the four seasons of Taihu lake in 2017 were obtained.From the perspective of space,the distribution characteristics of TMC in Taihu lake were roughly in line with the trend that the north was higher than the south,and the middle area was lower than meiliang bay and zhushan bay,and the TMC values in coastal waters were higher than the concentration in the lake.From the perspective of seasons,the TMC of Taihu lake water body is higher in summer and autumn,and reaches the highest in summer,while the TMC value is relatively low in spring and winter,and the lowest in winter.(4)Interannual trend,seasonal trend and irregular trend of TMC in Taihu lake from 2003 to 2011Based on the MERIS images from 2003 to 2011,the temporal and spatial distribution characteristics of Taihu lake were analyzed,and time series decomposition was introduced to analyze its interannual trend,seasonal trend and irregular change.The interannual trend change showed that TMC increased slightly from 2004 to 2006,and increased significantly from 2006 to 2008,followed by a rapid decline,and showed a slow decline from 2009 to 2011.The annual mean value of TMC in the open area was lower than that in the other two areas.The annual mean value of TMC increased continuously from 2003 to 2006,reached the highest value in 2006,and decreased continuously after 2006.The annual mean value of total microcystin in meiliang bay was between that of zhushan bay and the open area,with the highest value of TMC in 2008 and the lowest value in 2011.The annual mean value of TMC in zhushan bay was the highest in the three regions,with the highest value in 2007 and the lowest value in 2011.In spring,the TMC in the west of Taihu lake is the highest,and gradually decreases from west to east,and the TMC in the middle of the lake is the lowest.In summer,the west of Taihu lake still has the highest TMC,mainly because the high temperature of Taihu lake is conducive to the growth of toxic algae.The distribution of TMC in autumn is similar to that in summer.In winter,the TMC in Taihu lake reaches its minimum value,and the growth of toxic algae is greatly restricted mainly at low temperature.The irregular component has no obvious change rule,and it keeps a stable trend from 2003 to 2007,but in 2008,it shows obvious spatial variability and the short-term maximum value of the irregular component.After 2008,it declines rapidly,and then it shows a stable trend again from 2009 to 2011.In the north meiliang bay and the northwest zhushan bay,relatively high TMC variability of irregular components,which has a great impact on the total variance.(5)Analysis of driving factors of interannual trend,seasonal trend and irregular trend change of TMC in Taihu lakeFor the interannual trend change,total nitrogen(TN)in the open area from 2003 to 2005 was consistent with the change of TMC,and the response of TMC to the change of TN lagged for one year after that,and in 2010,the change of TMC and TN showed an opposite trend,indicating that the concentration of TN and total phosphorus(TP)were not the main influencing factors of the change of TMC in 2010.In meiliang bay,the change trend of TP was consistent with that of TMC.The high content of TN was not the limiting factor of TMC change,while the low content of TP had a greater impact on phytoplankton growth and became the limiting factor of TMC change.In zhushan bay,the ratio of TN to TP is more than 20,obviously higher than the other two regions,indicating the existence of nitrogen limitation conditions.However,the ratio between TN and TP has a low correlation with TMC in the three regions.As for the change of seasonal trend,due to the difference of nutrients in different seasons,meiliang bay and zhushan bay are greatly affected by runoff in spring and summer,and TP concentration has a more significant impact on TMC content at high temperature,so TMC in zhushan bay reaches its maximum value in summer.Open areas are less affected by seasonal factors due to lower nutrient content.For the irregular variation,it was mainly due to the irregular variability in 2008,mainly due to the outbreak of cyanobacteria bloom caused by the abnormal high temperature and low wind speed in 2008,which resulted in the highest concentration of total microcystins. |
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