Spatial-Temporal Pattern And Driving Mechanism Of Coastal Wetlands In Shanghai Over The Past 40 Years

Coastal wetlands are located in the intersection of terrestrial and marine ecosystems,and have various ecological service functions such as regulating climate,maintaining species diversity,providing biological habitats,and protecting coastal zones.However,due to the combined effects of human activities and natural factors such as coastal enclosure,urbanization,biological invasion,reduction in sediment deposition,sea level rise,etc.,the pattern of coastal wetlands is prone to rapid evolution,which seriously affects the stability of the coastal zone.Therefore,it is of great guiding significance for the protection and sustainable development of coastal wetlands to carry out research on the pattern evolution of coastal wetlands and its influencing factors.This paper selects the coastal wetlands in Shanghai as the research object,and analyzes the spatial-temporal dynamics and evolution of the coastal wetlands in Shanghai in the past 40 years(1986-2020)by means of remote sensing interpretation,digitization of charts,and field investigation;and focused on the analysis of the impact of three main factors,including reclamation,changes of annual runoff and sediment transport,and the invasive species Spartina alterniflora.According to the needs of future protection and development,the counter measures and suggestions for the protection of coastal wetlands in Shanghai are put forward.The main results of the study are as follows:(1)From 1986 to 2020,the land cover area and types of coastal wetlands in Shanghai have changed significantly.Firstly,in the past 40 years,the area of coastal wetland in Shanghai has increased first and then decreased slowly.The area was the largest in 1990(431775.43 ha)and decreased to 355572.88 ha by 2020.Secondly,affected by coastal human activities,land cover types gradually changed from a single coastal wetland to coastal wetland,swamp wetland,constructed wetland,farmland and construction land;In the past 30 years,a total of 28548.95 ha,20963.50 ha and 8467.22 ha coastal wetlands have been transformed into swamp wetlands,constructed wetlands,farmland and construction land.The most significant period for the transformation of coastal wetlands into other land cover was from 2005 to 2010(a total of 17324.19 ha coastal wetlands had been transformed),and Changxing and Hengsha Island were the areas with the largest transformation area(13254.16 ha coastal wetlands have been transformed).(2)Affected by the reclamation,the area of coastal wetlands in Shanghai has decreased by 64539 ha from 1986 to 2020.The areas with the largest reclaimed area were Chongming Island,Nanhui Dongtan,Changxing and Hengsha Island,resulting in the cumulative reduction of 26440 ha,21890 ha and 18754 ha of coastal wetlands.Because the reclamation were mainly in the high tide flat,it had a great impact on the saltmarsh.From 1986 to 2020,the coastal reclamation had led to a cumulative reduction of 11364.1 ha in the area of saltmarsh.The reclaimed coastal wetland has lost the tidal dynamic process and has been transformed into other land cover types such as inland wetland(herbaceous marshes,ponds,breeding ponds),farmland and construction land under the influence of human activities.Affected by the reduction of sediment inflow from the river basin,the total area of coastal wetlands in Shanghai(the area of tidal flat above-6m)showed a decreasing trend from 1986 to 2020.Especially after the completion and operation of the Three Gorges Dam in 2003(the sediment discharge to the Yangtze River Estuary decreased by 62%),the annual average sedimentation rate of the total area of coastal wetlands in Shanghai decreased to 213.62 ha/a;The area of the tidal flat above 0 m beach and saltmarsh still showed an increasing trend,but the increasing rate decreased.Since the introduction of alien species S.alterniflora in Shanghai coastal wetland in the 1990 s,its community area has shown a steady increase trend.By 2020,the area has reached 15275.99 ha,accounting for 46.22% of the total area of saltmarsh in Shanghai coastal wetland,becoming the largest dominant species in Shanghai coastal wetland area.After the invasion of S.alterniflora,the spatial distribution pattern of saltmarsh also changed from the original " mudflat — Scirpus Mariqueter —Phragmites australis " to "mudflat — S.mariqueter — S.alterniflora— P.australis" or "mudflat — S.mariqueter — S.alterniflora".(3)Based on the research on the pattern evolution and influencing factors of coastal wetlands in Shanghai in recent 40 years,in order to realize the high-quality development and sustainable utilization of coastal wetlands in Shanghai,it is suggested to protect the coastal wetland resources in Shanghai through the scientific utilization and management of sediment resources,the control of alien species invasion,the scientific development of coastal biological silt-promoting projects,the control of large-scale reclamation.and strengthening of publicity and education to the public.It provides successful experience and cases for the protection and restoration of fragile ecosystems in China’s coastal zone.