| Water body eutrophication is an important problem with inland and nearshore waters in China and in the world as a whole.As a typical outsize river city,Shanghai faces the pressure brought to urban ecological security and sustainable social economic development from pollution and eutrophication of water body due to influences of river basin,ocean and local human activities.By virtue of assessment of water body eutrophication in Shanghai based on temporal remote sensing,research of the law of spatial-temporal differentiation and evolution characteristics of urban water body eutrophication and the identification of influence characteristics of climate change and human activities on urban water body eutrophication,this paper is of great significance for water ecological restoration,water environmental governance and control in river cities and for safeguarding ecological,environment-friendly and high-quality development of outsize cities.In this research,targeting at the five major urban water bodies of rivers,lakes,natural ponds,canal waters and aquacutural ponds in Shanghai and with the actual spectrum and water quality data measured in the field,a remote-sensing inversion model was constructed for the two key water quality parameters of concentration and turbidity of Chlorophyll-a(Chl-a)concerning eutrophication based on Sentinel-2,Landsat-8 and Landsat-5 remote-sensing images.Then,based on the temporal remote sensing date,the spatial-temporal changes and spatial differentiation of water body eutrophication over the near forty years in Shanghai was analyzed and the influencing elements of water body eutrophication such as temperature,rainfall,total planting area,livestock waste and chemical oxygen demand(COD)of waste water discharge were primarily evaluated.The main conclusions are as follows:(1)Construction of optical remote-sensing inversion models of concentration and turbidity of Chlorophyll-a with the data of Sentinel and Landsat series satellites.The model results show that the combination of the three types of satellite image data well satisfied the demand on evaluating long-lived water body eutrophication in large urban area in Shanghai.Linear function and quadratic function models of red and near-infrared bands were adopted for concentration of Chlorophyll-a in water body.With Sentinel-2 model,R~2 was 0.96 and RMSE was 4.33μg/L;with Landsat-8 model,R~2 was 0.95,RMSE was 4.63μg/L.The power function model of near-infrared models was adopted for turbidity remote-sensing inversion.With Sentinel-2,R~2 was 0.87,RMSE was 12.04μg/L;with Landsat-8 model,R~2was 0.83,RMSE was 13.01 NTU;with Landsat-5 model,R~2 was 0.87,RMSE was12.14 NTU.(2)The annual average concentration and turbidity of Chl-a in water bodies in Shanghai demonstrated great difference among different types of water bodies.In terms of Chl-a concentration from high to low,the ranking of different water bodies is as follows:farm/planting ponds>permanent freshwater lakes≈natural ponds>permanent rivers≈canal/water diversion rivers.In terms of turbidity from high to low,the ranking is:farm/planting ponds>permanent rivers≈canal/water diversion rivers>permanent freshwater lakse>natural ponds.In terms of eutrophication from high to low,the ranking is:farm/planting ponds(reach heavy level in high vale area)>permanent rivers≈canal/water diversion rivers(mild level)>permanent freshwater lakes≈natural ponds(mild to medium level).(3)The water body eutrophication demonstrated strong seasonal and interannual changes.In terms of seasonal changes,most water bodies generally show a pattern of high in summer and low in winter.In terms of inter-annual changes,the eutrophication performance of Shanghai’s urban water bodies first increased and then declined.After the Shanghai Environmental Protection Regulations were promulgated in 1995,the eutrophication level of water bodies showed an overall downward trend.In the season,except aquacutural ponds,Chl-a concentration of rest water bodies was generally less than 5μg/L in winter;about 5μg/L in spring and fall and generally greater than 10μg/L in summer;the turbidity was20-35 NTU in winter,20-45 NTU in spring and fall and generally 40-65 NTU in summer.Overall,eutrophication of all water bodies was at medium level in winter and at mild level in summer.Around 1984-1995,the value of Chl-a concentration and turbidity increased by 1.0 times;while water body entrophication aggravated from mild and medium level to medium and heavy level in summer.During 1996-2020,the Chl-a concentration and turbidity of most water bodies were relatively stable and gradually declined to the level in 1984.(4)Temperature and chemical oxygen demand of waste water discharge were the main natural and human factors of water body eutrophication in Shanghai.At the scale of a year,temperature and rainfall changes had influences on water body eutrophication in non-aquacutural ponds.Where,the influence of temperature was the greatest,with a correlation coefficient of 0.90(p<0.05).At the scale of multiple years,human activities such as total planting area,chemical oxygen demand(COD)of waste water discharge and total excrement of livestock had certain influences on water body eutrophication in non-conservation waters,lakes and reservoirs,where the chemical oxygen demand(COD)of waste water discharge was the greatest,with a correlation coefficient of 0.70(p<0.05).In case of special events such as outbreak of COVID-19,the turbidity of 85%water bodies was declined on year-on-year basis due to decreased industrial production and shipping within a short period. |
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