| Coastal wetland ecosystems,one of the most important ecosystems in the world,play an important role in regulating climate,sequestering blue carbon,and maintaining sustainable development of coastal zones.Wetland landscapes are notoriously difficult to map with satellite data,particularly in highly complex,dynamic coastal regions.The Liao River Estuary(LRE)wetland in Liaoning Province,China,has attracted major attention due to its status as Asia’s largest coastal wetland,with extensive Phragmites australis(reeds),Suaeda heteroptera(seepweed,red beach),and other natural resources that have been continuously encroached upon by anthropogenic land use activities.Using the Continuous Change Detection and Classification(CCDC)algorithm and all available Landsat images,we mapped the spatial-temporal changes of LRE coastal wetlands(e.g.,seepweed,reed,tidal flats,and shallow marine water)annually from 1986 to 2018,and analyzed the changes and driving forces.Main results are as following:The total area of coastal wetlands in the LRE shrank 14.8%during the study period.The tidal flats were the most seriously affected type,the area decreased by 324.4km2,with 45.7%of its total area lost;Followed by shallow marine water,loss area of 114.38 km2(6.98%);the area of seepweed declined 10.1 km2(22.1%)from 2010 to 2018 while the area of reed had a slight increase.The decrease of coastal wetland area is mainly due to land cover change caused by human activities.The decrease of tidal flat area mainly contributed by the increase of building land,aquaculture ponds and paddy field.The reduction in shallow marine water is also due to the expansion of built-land area and the reclamation of aquaculture ponds.The area of seepweed and reed field fluctuated,which was mainly affected by built-up,aquaculture ponds and tidal flats.One of the main characteristics of wetland change was the concurrent disappearance and emergence of wetlands in different parts of the LRE,creating drastically different mixtures of wetland quality(e.g.wetland age composition)in addition to areal change.Newly emerged wetland area during the study period reached 21.7 km2 for seepweed and 95.2 km2 for reeds,accounting for 61.3%and 19.6%of their areas in 2018,respectively.The reduction and replacement/translocation of coastal wetlands were mainly caused by human activities related to urbanization,tourism,land reclamation,and expansion of aquaculture ponds.Our efforts in mapping annual changes of wetlands provide direct,specific,and spatially explicit information on rates,patterns,and causes of coastal wetland change,both in coverage and quality,so as to contribute to the effective plans and policies for coastal management,preservation,and restoration of coastal ecosystem services. |
