Influence of Artificial Cofferdam and Spartina alterniflora Expansion on Evolution of Suaeda salsa Marsh in Yancheng Coastal Wetland of East China

Suaeda salsa is an important local species in the intertidal beach of the Western Pacific coast. However, under the artificial cofferdam and Spartina alterniflora expansion, Suaeda marsh has degraded seriously. Therefore, using Yancheng Nature Reserve as a case study area, taking ETM+ images in 2000, 2006 and 2011 as the basic data sources, we revealed the evolution characteristics of Salsa marsh which was impacted. The research results are as follows. From 2000 to 2011, Salsa marsh area in the artificial area tempestuously degraded, decreasing by 87.158%, more than 22% than those in the natural area. The landscape was fragmentized. Landscape polymerization degree index dropped from 95.780 to 65.455, more than 16% than those in the natural area. The mean patch area fell down to 21.429 ha from 389.333 ha, more than 11% that in the natural area. Compared to the steady change in natural conditions, the area was reduced by 118.167 ha/a from 2000 to 2006, while during 2006-2011, it was only 51.500 ha/a in artificial area. As for spatial change of landscape, in artificial area, the Salsa marsh centroid moved forward to the southeast with 666.350 m, but that in natural area moved forward to the north with 1042.710 m from 2000 to 2006. From 2006 to 2011, the centroid moved forward to east and north respectively. Artificial cofferdam transformed the area into freshwater ecosystem, and meanwhile the freshwater was beneficial to Reed marsh. During 2000 to 2006, in the artificial area, 539 ha Salsa marsh controlled by cofferdam transferred into reed marsh and aquaculture ponds, of which the transformation rate was nearly 4% higher than that in natural area. From 2006 to 2011, 178 ha Salsa marsh was transferred into reed marsh, the transformation rate was 20% higher than that in natural area. With rapid spreading and strong competition of Spartina species, the coastal wetland has formed the pattern of "Salsa–Spartina marshes". From 2000 to 2006, in artificial area, 15.24% of Salsa marsh was transferred into Spartina marsh, of which the transformation rate was 13% higher than that in natural area. And from 2006 to 2011, 30.07% Salsa marsh was replaced by the Spartina marsh in artificial area, the rate was almost 10% higher than that in the natural area.