Effects Of The Invasion And Ecological Control Of Spartina Alterniflora On The Foodweb Of Mangrove Wetlands

Mangrove ecosystems can support high biodiversity and complicated trophic structure due to its high primary production and habitat heterogeneity, but previous studies have shown that the invasion Spartina alterniflora into mangrove wetlands could significantly change the food sources of benthic macro-invertebrates. However, there is still no study on such effect of S. alterniflora invasion on zooplankton and nekton communities. In this dissertation research, I investigated possible effects of S. alterniflora invasion on the organic carbon sources of aquatic animals in the adjacent water of mangrove including zooplankton and nekton. I also monitored changes in the structure of macrofaunal community and food sources of main macro-invertebrates species in the invased S. alterniflora stands following replacement by native mangrove species Kandelia obovata. The main results of these studies could be summarized as follows:(1) The chlorophyll a (Chi a) concentration of water in Zhangjiangkou Estuary (N23°55.719’, E117°26.436’) demonstrated large spatial-temporal variations. The highest Chi a concentrations were found in summer or spring, while the lowest values occurred in winter. A negative relationship existed between Chi a concentration and salinity (P<0.05). In winter, the compositions of particulate organic matter (POM) remained relatively stable, and the average contribution from mangrove carbon to POM decreased from30.6%to24.1%the upstream to offshore areas, while the contribution from marine phytoplankton increased from23%to28%. Benthic microalgae (BMA) averagely contributed33%-35%to the POM and the average contribution from invasive S. alterniflora was less than15%. In summer, the compositions of POM showed much larger variations. The average contribution of mangrove carbon ranged from62.1%in the upstream area to33.5%in the offshore area. Contrary to the mangrove carbon, the average contributions from marine phytoplankton (10.9%-20.9%), BMA (21.4%-31.4%) and invasive S. alterniflora (5.7%-10.7%) increased from the upstream to offshore areas. In general, the autochthonous materials including mangrove and BMA were the major carbon sources for the POM, while the invasive S. alterniflora showed much less contribution to the POM even when considering the variation range of different sources.(2) Total density of zooplankton showed significant seasonal variations (P<0.05). The highest density was found in summer while the lowest in winter. The stable isotopic analyses of zooplankton and autotroph species indicated that, in winter, mangrove forests contributed less than21%to the organic carbon of zooplankton, while the total contribution from POM and BMA ranged from15%to35%and from the S. alterniflora varied between15%-35%. The carbon sources of zooplankton demonstrated large spatial variations in summer. The average contribution from terrestrial mangrove was as high as65.6%in the upstream area, but it decreased greatly from the upstream to offshore areas, where the average contribution from mangrove was only14.5%. The average contribution from POM and BMA varied from14%to28%, while the average contribution from S. alterniflora ranged broadly from4%to45%. Major carbon sources to the zooplankton were autochthonous carbon from the mangrove forests and POM/BMA. While considering the variation range of different sources, S. alterniflora could also be the major contributor to organic carbon of zooplankton.(3) I analyzed the stable carbon and nitrogen isotope ratios of six common fish species, including detritus feeding Osteomugil ophuyseni, omnivorous Coilia grayii, Parachaeturichthys polynern and Sparus latus and carnivorous Lateolabrax japonicas and Ophichthus apicalis, in the tidal creek of Zhangjiang Estuaryto assess the relative nutritional contributions from invasive S. altemiflora and native autotrophs. The results indicated that S. alterniflora was a major contributor to all fish species. For S. latus, the minimum proportion of carbon sources from S. altemiflora was28%. For the other species, S. alterniflora contributed at least50%of organic carbon to them. Among the above6fish species, only S. Latus showed significantly negative correlation between carbon isotope ratio and its body length, demonstrating the transference of main food sources and trophic levels during growth. (4) The benthic macrofaunal community in S. alterniflora and1year restored K. obovata forest were investigated in Meizhou Bay (N25°14.552’, E118°53.314’), Minjiang Estuary (N26°01.934’, E119°37.707’) and Dazhou island. Results indicated that the preliminary physical treatment and plantation activities significantly changed the benthic community. The density and biomass of large-sized epifauna including crustacean and gastropod were impacted by the preliminary treatment, while no such effect were found on the infaunal Polychaeta, demonstrating that the small-sized organisms could be adapted to the change of habitat rapidly. The community structure and biodiversity of benthic macrofauna in S. alterniflora,1year restored K. obovata,10years K. obovata forest and40years mature K. obovata forest were compared in Dazhou island. Although no significant differences were found for the biodiversity indexes among4habitats, the community structure might be impacted by the restoration. Total density showed no significant differences among different habitats (P>0.05), but the10years K. Obovata forest had highest density of Polychaeta and lowest density of mollusk.10years K. obovata also had lower total biomass than S. alterniflora (P<0.05), but no significant differences were found between the other habitats.(5) Based on the stable isotope analyses, the variations of food sources of main gastropod and crab species in S. alterniflora and K. obovata with different ages were investigated in Meizhou Bay, Minjiang Estuary and Dazhou Island. The preliminary engineering activities did not change the food sources of macro-invertebrates, while the major carbon source for gastropod and crab species came from the remains of S. alterniflora. The relative carbon contribution from S. alternifolra to macro-invertebrates decreased with the age of K. obovata forest. However, obvious discrepancy still existed between the10year old K. obovata forest between40year old mature K. obovata forest, suggesting that it take a long period for the restoration of food source of macro-invertebrates in mangrove forests following the invasion of S. alterniflora. Furthermore, macrofaunal species with different feeding types showed different change in their food sources in response to the restoration of mangrove. The food sources of herbivorous gastropods and crab species already changed significantly in3year old K. obovata forest and their carbon contribution from S. alternifora decreased to less than50%in6year K. obovata forest. However, the detritus-feeding invertebrates did not change their food sources and derived most100%organic carbon from S. alterniflora in3year old K. obovata forest. The carbon sources from S. alterniflora to detritivorous H. latimera decreased to less than70%in6years K. obovata forest, indicating that these kind of species need at least5-6years to begin obvious change in their food sources.In general, the invasive S. alterniflroa also significantly changed the food sources of zooplankton and nekton. However, the restoration using native mangrove species to control the S. alterniflroa could restore the benthic macrofaunal community and food sources of macro-invertebrates. Although the restoration takes a long time, it feasible and beneficial to use this method to controle the spread the S. alterniflora and reverse the ecological effect.