Impacts Of Spartina Alterniflora Invasion On The Benthic Communities Of Saltmarshes In Chongming Dongtan, Shanghai

Biological invasion, as one of important component of global change, is threatening natural ecosystems and becoming one of the hot spots in ecological fields. Salt marshes in Chongming Dongtan, Shanghai possess the unique natural resources and face with a big challenge from the exotic plant invasion. Benthic community in the wetlands connects with primary production and consumption and plays a major role on nutrient cycling in salt marsh ecosystems. During the 1990s, an exotic Spartian alterniflora has spread rapidly. It is important to evaluate the impacts of S. alterniflora invasion on benthic communities of salt marshes and valuable to trace the effects of the physical measures to control the spreading of S. alterniflora on macrobenthic communities in Chongming Dongtan, Shanghai.This study investigated the distribution and community constructure of benthos in the salt marshes in Chongming Dongtan during the 2007 and 2008. The Normalized biomass size spectra (NBSS) were constructed to explore the S. alterniflora invasion on benthic communities. Moreover, trace the effects of managed waterlogging on macrobenthic communities. The main results of this study were summarized as follows:(1) Investigated the distribution of benthos and constructed the NBSS to explore the influence of plant type and evelation on the benthic communities. The results showed that the distribution and species composition of macrobenthos varied with the elevation and plant community charecteristics. The benthic diversity showed a decreasing trend along the elevation gradient. The slope of NBSS for the sampling sites decreased with the decreasing elevation. Compared with the native saltmarshes and bare mud habitat, Spartina alterniflora saltmarshes exhibitated the steeper slopes indicating the more samller individuals in the saltmarshes.(2) Explored the NBSS approach to evaluate the possible impacts of Spartina alterniflora invasion on the benthic communities along gradients of intertidal zones and invasion history of Spartina alterniflora at the nature reserve. The results showed that the characters of macrobenthic communities and the variation in macrobenthic communities described by the first two CCA axes revealed clearly the gradients of elevation and invasion history of Spartina alterniflora. The differences in the macrobenthic assemblages between the Spartina alterniflara marshes and the native Phragmites australis marshes decreased with increasing of invasion history of Spartina alterniflara. The macrobenthic biomass showed a decreasing trend while the meiobenthic biomass showed a reverse trend along the elevation gradient. The macrobenthic biomass at Spartina alterniflora marshes with longer invasion history was higher than that at recently invaded Spartina alterniflora marshes, while the meiobenthic biomass were lower. The slopes of NBSS for the sampling sites showed a trend of steeper slopes with decreasing of elevation and at the recently invaded Spartina alterniflora marshes than that at marshes with longer invasion history, while the differences between the native Phragmites australis marshes and the Spartina alterniflora marshes with long invasion history tended to be diminished. The NBSS approach could be effectively used to detect possible impacts of Spartina alterniflara invasion on the benthic assemblages. This study indicated also a potential for this approach to provide valuable insights into ecosystem ecology of invasive species, which could be very important for wetland biodiversity conservation and resource management in the Yangtze River Estuary and other such impacted areas.(3) Physical measures to control the spreading of an invasive plant Spartina alterniflora are important and necessary for biodiversity conservation at the Chongming Dongtan Nature Reserve. A field survey was carried out to trace the impact of managed waterlogging on the macrobenthic communities at a demonstration site. The results showed that managed waterlogging treatments had caused significant decreases in the density, biomass and diversity of macrobenthic community. Moreover, the community structure of macrobenthos was different drastically with the control site. After 12 month waterlogging and breaking the dike, the density, biomass and diversity of macrobenthos could recover gradually as compared with the control site. The result of multivariate analysis DCA ordination revealed that differences in community structure between the recovery site and the control site reduced gradually. The results from this study indicated that the control measure of managed waterlogging could have a negative impact on the macrobenthic community under saltmarsh vegetation, while such negative effects could be recovered by natural processes after ending the waterlogging treatment.