| Offshore and estuarine water environment is the focus of attention of marine and coastal zone ecology,and is closely related to coastal human life,so the study of the spatial and temporal distribution characteristics of water environment elements can help to understand the state of offshore and estuarine water environment,which has important practical guidance significance.It is of great scientific value to understand how these factors affect the distribution of the water environment.Based on this,this paper analyzes the short-term effects of typhoons as a natural factor and the long-term effects of land use as anthropogenic factors,based on the characteristics of changes in the distribution of near-surface water environment elements extracted from satellite remote sensing data.The main research results are as follows.(1)This study firstly solved the problem of missing satellite data during typhoons:MODIS Aqua,MODIS Terra,and VIIRS data were used to reconstruct the daily sea surface temperature(SST)and chlorophyll concentration(CHL)data of the Yellow Sea and East China Sea and the northern part of the South China Sea by DINCAE algorithm,and the results were compared with the original data and the in site data,and the results were compared with the original data and the buoy data during typhoon transit,which have low errors(error percentage of SST were6.1%and 4.95%,and 16.02%and 13.29%for CHL,respectively).The reconstruction results can accurately reproduce the changes of sea surface elements before and after the typhoon transit.Moreover,the accuracy and reconstruction efficiency were improved compared with other reconstruction algorithms.(2)This paper also revealed the characteristics of each water environment element in China’s offshore waters:the SST of both sea areas showed an increasing trend during 2012-2019,with an increase of 0.8℃and 0.1℃,respectively;the near-shore area warming amplitude and rate were higher than that of the outer sea,with the near-shore area of the Yellow Sea and the East China sea reaching a warming rate of 0.13℃/yr,while the outer sea only had a warming rate of 0.07℃/yr.On the contrary,CHL in both waters showed a significant decline trend,with the near-shore declining more than the outer sea.The near-shore CHL in both waters declined by 0.1 mg/m~3 and 0.04 mg/m~3,respectively;for total suspended sediments(TSM),the near-shore was much higher than the outer sea,with TSM faults at 10m and 30m depths in both waters.The seasonal variation in the nearshore area is significant,while the it was stable in the offshore area.(3)The short-term effects of typhoons on the near-surface marine environment were analyzed using Japan Meterological Agency(JMA)typhoon data and reconstructed remote sensing data.After the typhoon,the spatial variation of SST and CHL decreased with increasing distance to the typhoon center,and the spatial distribution of TSS concentration increased and then decreased with increasing distance to the typhoon center.The recovery time of each water environment element after the typhoon varied,with the SST dropping to a minimum after 1-2days of transit and recovering in the following 10-15 days.CHL increased to the maximum after 3-4 days of transit and recovered in about 10 days.TSM rises to the maximum after 1-5days of transit.In addition,the effect of typhoon wind speed and movement speed on SST and CHL was significant for the Yellow Sea and the East China sea.For the northern part of the South China Sea,the decrease of SST was influenced by the typhoon wind speed and the depth of the ocean mixed layer before the transit,and the CHL was more influenced by the typhoon wind speed.When the wind speed of the typhoon is greater,the upwelling caused by it is stronger,and the upwelling brings more cold water with higher nutrient content from the bottom layer to the surface layer,which eventually leads to a higher degree of sea surface temperature reduction and higher chlorophyll concentration.At the same time,when the typhoon wind speed is higher,its wind stress is stronger,and the surface seawater flow speed increases,causing vertical flow shear,resulting in vertical mixing,and finally reducing the overall temperature of the mixed layer.When the typhoon moves at a slower speed,the mixing effect caused by it is stronger,and more cold water in the lower ocean turns up,bringing enough nutrients,which in turn leads to a greater increase in the concentration of chlorophyll.Due to the higher typhoon wind speed and shallower water depth in the East China Sea,the basic content of chlorophyll is generally higher.Therefore,on average,the sea surface cooling and chlorophyll concentration increase caused by typhoons in this area are higher than those in the northern South China Sea.(4)Further analysis of the characteristics of long-term changes in estuarine water quality and the influencing factors.The TSM concentration in the PRE showed a significant decreasing trend over the past 30 years,and the wavelet analysis results showed 4a,8a and 12a variation periods.The concentration of TSM in the PRE showed a significant negative correlation with the impervious area of the Pearl River Delta basin and the watershed area of the Xijiang River basin.The abundance of precipitation in the Pearl River Delta basin corresponds to the concentration of TSM in the PRE,while the sediment transport in the Pearl River directly determines the magnitude of TSM concentration.With the expansion of impervious area in the Pearl River Delta basin and the construction of artificial shorelines such as estuarine dams,it leads to the reduction of sediment carried by precipitation flushing,thus eventually leading to the reduction of TSM concentration in the PRE on a long-term scale.With the increase in the number of reservoir dams in the Xijiang River basin,the water area expands,and the construction of reservoir dams intercepts river sediment,thus leading to a decrease in river sediment transport,which eventually affects the concentration of the TSM in the estuary.And the concentration of TSM in the PRE will increase after the construction of the reservoir dam begins,while the TSM will show a significant decrease when the reservoir is completed and begins to store water. |
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