Environmental Changes And Urbanization Progress Evidenced By Remote Sensing And Sediment Chracteristics Of Futian Mangroves Wetlands In Shenzhen Bay

Mangroves are particularly sensitive to environmental change due to their location at the interface of terrestrial and marine ecosystems.Futian Mangrove wetland is situated in the center of Shenzhen City,and it faithfully record the urbanization process of the city during past decades.In this study,four sediment cores?Cores HS,QQ,BGR within the wetland and TT from nearby tidal flat?were obtained from Futian mangrove wetland,and the sediment proxies such as TN,TP,TOC and?¹³C were analyzed,and the land use changes in Futian District of Shenzhen City were quantitatively extracted through remote sensing technology.All these data were used to reconstruct the history of environmental changes in Futian mangrove ecosystem and their ecological response to the rapid urbanization and industrialization in Shenzhen City over half a century.The main results are as follows.1.Using the CIC?constant initial concentration?model in ²¹⁰Pb dating,the average sedimentation rate of Core HS was drawn at 0.88 cm/a,and accordingly the sediment bottom was dated as 1959AD.Through the parallel correlation of the vertical profiles of magnetic susceptibility,the average sedimentation rates of Cores BGR,QQ and TT were obtained as 0.94 cm/a,0.92 cm/a and 0.81cm/a,respectively,and their bottom ages were also determined at 1961AD,1960AD and 1954AD,respectively.2.The changes in land use and vegetation coverage in Futian District of Shenzhen City were explored by remote-sensing analysis of the Landsat satellite serial images from 1987 to 2014.The areas of mangrove forest and construction land showed an increasing trend,increased by 1.15 times and 81.19%,respectively,from 1987 to 2014.Vegetation coverage change.The NDVI?Normalized Difference Vegetation Index?reveals a reduction in vegetation coverage from 1987 to 1994,which indicates a landscape transition from rural to urban in Futian District of Shenzhen City.3.The sediment grain size components?percentages of clay,silt and sand?and characteristics?Mz,Md,?i,Kg,Skf?of Cores BGR,QQ and TT showed that the clay and silt dominate the BGR and QQ cores,and the gentle variation pattern in these two cores is possibly a result of mangrove functions to reduce waves and promote siltation within the system that facilitate deposition of fine particles.On the contrary,Core TT is absent from vegetation coverage and seems more vulnerable to waves and storms,resulting in a higer percentage of sands in the sediments.4.Spatio-temporal analysises were conducted on sedimentary TN and TP in Futian mangroves.Along the time axis,the TN showed similar increase trends in Cores BGR,QQ and TT,while a gruadual decrease trend was observed in TP,which are in agreement with most previous studies.In the spatial distribution,the TN and TP contents in TT core are typically lower than those in the other two cores?BGR,QQ?,which can be attributed to more coaser particles in Core TT strongly influenced by waves due to an absence of protection from mangrove.This indication is further evidenced by significant negative correlations between nutrient contents and particle size in the core samples.5.The sources of sediment organic matter in Futian mangroves were determined by TOC,C/N ratio and?¹³C.The Bayesian model was used to estimate the contribution of organic matter sources in the columnar sediments of Futian mangroves.It was found that the land source input increased gradually,the input of sea source organic matter gradually decreased,and the input of mangroves dominated.Especially in the 1980s,the rapid development of urbanization in Shenzhen led to the importation of a large amount of terrestrial organic matter into Shenzhen Bay.After the 1980s,terrestrial organic matter increased rapidly.