Spatio-temporal Evolution Characteristics And Driving Mechanism Of Coastal Wetlands In The Yellow River Delta Over The Past 40 Years

The Yellow River Delta was located in the ecotone among marine,terrestrial and river ecosystems,which was rich in resources and played an important role in maintaining biodiversity,providing habitats and optimizing the environment.However,due to the special geographical location of the Yellow River Delta,its evolution was easily affected by hydrological conditions,species invasion and human activities,which leads to serious loss and degradation of coastal wetlands.Therefore,studying and discussing the evolution process and driving mechanism of coastal wetlands in the Yellow River Delta is conducive to the restoration of coastal wetlands and the healthy and sustainable development of ecosystems,and is of great significance for the rational development and effective management of coastal wetland resources.The spatial-temporal evolution characteristics of coastal wetlands were examined based on the long-term remote sensing images over the past 40 years of the Yellow River Delta.The spatial-temporal evolution driving mechanism were examined based on the data of runoff and sediment from the Yellow River(1976-2018)and long-term remote sensing(2008-2020),and explored the effect of the runoff and sediment,human activities and Spartina alterniflora invasion on the evolution of coastal wetlands.These data were analyzed by 3S technology,using methods for object-oriented,supervised classification methods,land use transfer matrices,landscape pattern statistical analysis methods and regression analysis,and the software of Envi 5.3 and Arc GIS 10.3.The results show that:(1)Over the past 40 years,the coastal wetlands of the Yellow River Delta have evolved violently.the Yellow River Delta area fluctuated between 2.33×10⁵ and 2.60×10⁵ ha during the period of 1976-2018.The evolution of the old estuary was mainly affected by erosion and area showed a downward trend,while the current estuary was mainly regulated by sedimentation during 1976-1995 and erosion during 1995-2018,and area showed a increased trend during 1976-1995 and fluctuated during 1996-2018.During the past four decades,natural wetlands,such as tidal flats and marshes,decreased dramatically by 55.0%,and these wetlands were mainly transformed into aquacultural ponds,salt pans,oil wells and dry land.The area of human-made wetlands and non-wetlands increased.The coastal wetland landscape pattern of the Yellow River Delta was disturbed by human activities.the landscape tends to be fragmented and the types of landscapes develop toward diversity and uniformity,and the heterogeneity of landscapes decreased.(2)The evolution of natural wetlands in different regions in the YRD was influenced to different degrees by natural factors and human activities.In the estuarine zone,the reduction in natural wetlands was mainly driven by natural factors such as coastal erosion,runoff and sediment load.In the old estuary,coastal erosion explained 91.2%of the changes in natural wetlands.The evolution of natural wetlands in the current estuary was mainly controlled by the runoff flux of the Yellow River and human activities,which accounted for62.7%and 19.0%of the variation in the current estuary,respectively.In the central zone,human activities,such as reclamation and urban construction,have converted 82.7%of natural wetlands into human-made wetlands and non-wetlands.(3)From 2008 to 2020,Spartina alterniflora grew rapidly and distributed in a wide range,with an area growth rate of 4.47 km² a^-1,making it a dominant species in the Yellow River Estuary wetland.Under the influence of the expansion of Spartina alterniflora,both Phragmites australis and Suaeda salsa showed a downward trend,and their area reduction rates were 0.72 ha a^-1 and 0.39 ha a^-1,respectively.During 2008 to 2012,the landscape pattern of the Yellow River Estuary was showed a fragmentation trend.After 2012,the trend was slowed down.With the invasion of Spartina alterniflora,the patches of number and density of Phragmites australis and Suaeda salsa were decreased.The index of NP of Phragmites australis was significantly related to the index of NP and SHDI of the Yellow River Estuary,indicating that the Landscape pattern of the Yellow River Estuary showed a fragmentation trend,and the landscape type is developed from a diverse to a single uneven trend.In the typical area of the Yellow River Delta,with the expansion of Spartina alterniflora from 2010 to 2020,the length and density of the tidal creeks in the typical area generally showed a downward trend.The length and density of the tidal trenches decreased by 12.84 km and 0.42 km/km²,respectively.