Study On The Regional Differentiation And Evolution Of Coastal Wetland In Yellow River Delta

The development of Yellow River Delta coastal wetland (YRDCW) is typical, but itsecological system is fragile and the ecological environment problem is increasingly prominent.Therefore the study about the Yellow River Delta (YRD) is concerned by many scholars,especially the modern YRDCW. Nevertheless, the comprehensive study about YRD from theangle of regional differentiation has not been reported. The frequent diversions of YRD formedthe different development condition of coastal wetland. In order to better understand and masterthe environmental characteristics and evolution mechanism of YRD under the differentdevelopment condition, this article selects the ancient area, abandoned area and estuary area astypical study areas. Applying of RS, GIS spatial analysis techniques and landscape ecologytheory, comprehensively analyze the regional differentiation characteristics and evolution ofYRDCW in recent30years. The main conclusions are as follows:Landforms are the basis of wetland development and evolution. The estuary, abandonedand ancient area respectively represent three different evolution stages of YRD landforms, witha significantly difference of erosion-siltation characteristics in different stages. The estuary areais the development period of evolution and wholly in the weak silting-up state, under the impactof the rivers and ocean dynamics. From1986to2010, the net formed land area is32.24km~2andthe annual average siltation rate is1.33km~2/a. The abandoned area is the adjustment period ofevolution and strongly scoured under the function of wave. From1986to2010, the net erosionarea is63.81km~2and annual average erosion rate is2.63km~2/a. Under the joint action of waveand flow, the ancient area is developed maturely with a straight coastal line and stable landform.The west of ancient area is slightly eroded with the mean erosion rate of0.08km~2/a and the neterosion area of1.78km~2.The regional differentiation characteristics of the grain size and chemical characteristics ofsediments are obvious. Material composition of sediments in the estuary area is the finest withthe average grain size of7.59Φ and the worst sorting, and the content of heavy metals is high. Inthe abandoned area, the average grain size is6.60Φ with a bad sorting and unimodal distribution,in which the content of heavy metals is low but the oil pollution is most serious. In the ancientarea the sand content increased, the average grain size is6.16Φ with a better sorting and zoning characteristics, and the content of heavy metals is higher than the abandoned areas. The qualityevaluation results indicate that the overall quality of the surface sediments in YRDCW is ingood condition and in low-pollution state. The potential ecological risk of heavy metals is in lowlevel. The mean value of comprehensive potential risk index in the ancient area is the highest,which in the estuary area is of second, and the abandoned area minimum. Cr and Cd arerespectively the region’s major heavy metal pollution factor and potential ecological risk factor.Over the past30years, the benthic species was significantly reduced. In the autumn of2005and2007, the benthoses in YRDCW were identified of64pecies, of which the abandonedarea includes38species, the ancient area of26species and estuary of22species. This speciesare members of Annelida, Mollusca, arthropods and other animal populations. The speciescomposition and the individual distribution are uneven. The diversity index of benthos in orderis the ancient area (2.04)> the abandoned area (1.19)> the estuary area (1.05).The vegetation species diversity in the ancient area is the highest, but with the fastestdeclining rate. The main vegetation types are halophytes and mesoxerophytes in the ancient area,while the abandoned and estuary areas are mainly with hygrophytes and halophytes. Successionsof vegetation are different because of the different habitats. In ancient area there are twosuccession patterns, one of which is halophytic vegetationâ†'psammophytic vegetationâ†'weedvegetation trends with the development of cheniers, the other is halophytic vegetationâ†'swampvegetationâ†'weed vegetation trend from sea to land. At present, under the impact of humandisturbance, the main vegetation succession in the ancient area is retrograde succession. Due toerosion and destruction of coastline, the abandoned area is mainly with retrograde or secondarysuccession. The vegetation of the estuary area is interweaved by the succession of the halophyticvegetation from sea to land and hygrophilous vegetation from river channel to both sides,mainly is the anterograde succession of natural vegetation.The four periods of remote sensing image interpretation and landscape analysis from1986to2010showed that the regional differentiation of YRDCW landscape pattern and evolution issignificant, and the transformation between different types of landscape is frequent. The impactsof human activities on landscape are increasing; meanwhile, the degree of landscapefragmentation is deepened. In addition to sea, the main landscape types in the ancient area areculture ponds, beaches and salt pan in which is most obvious impacted by human activities,showing as the rapid growth of the artificial landscape and quickly faded for natural landscape,as a result, landscape diversity decreased continuously. The main landscape types in theabandoned area are the same with the estuary area, which are mainly the beach, coastal marshand farmland. The natural landscape accounted for a large proportion, with a low landscape fragmentation degree. Landscape change is that the natural landscape area is with downwardfluctuations and artificial landscape area with upward fluctuation. The landscape diversity andfragmentation degree in the estuary area is the highest. The natural landscape areas and theartificial landscape areas change slowly.The development and evolution of YRDCW is influenced by many natural and humanfactors, including the coastal erosion and siltation, water and sediments of the Yellow River,reclamation, fishing, resource development etc. The tidal flat reclamation and resourceexploitation in the ancient area most significantly impact the coastal wetlands and wetlanddegradation is most obvious; Influence factors of the abandoned area mainly are coastal erosion,construction of oil field and engineering facilities; the estuary area is mainly affected by thevariation of Yellow River water and sediments, and the land-making rate.