|Other Abstract||The Yellow River Delta (YRD), the youngest, extensive and well-preserved wetland, is a typical newborn coastal wetland ecosystem in the warm-temperate zone of eastern China. The YRD can serve as a migration station in the East Asia-Australia bird migration route, and a wintering habitat and breeding farm for migration birds due to its rich natural resources. Therefore, the YRD has unique environmental and biological features, showing its essential international function in the wetland system. Meanwhile, the YRD possesses the characteristics of primitiveness, fragility and rarity, making it relatively fragile and vulnerable to interference from human activities and exotic species invasion. Spartina alterniflora was introduced into the YRD in 1990 to protect mudflats and promote land siltation, which received certain ecological benefits during the following period. However, it spread out rapidly in recent years, posing severe threats to local biodiversity and the stability of the wetland ecosystem. As the critical component of the coastal wetland, macrobenthos can be a proxy to indicate the ecological process of the coastal wetland, and assess the ecological quality of the coastal wetland by the changes of community structure. We collected biotic and abiotic samples across seasons in different invasion years of S. alterniflora in the YRD, including sediment pore-water, sediments, macrobenthos and their potential food sources, to analyze the changes in sedimentary physicochemical properties, the structure of the macrobenthic community, and the benthic food web under the stress of S. alterniflora invasion. Present work aimed to provide a scientific basis for protecting and managing the Yellow River Delta Nature Reserve and controlling S. alterniflora scientifically. Three main results are as follows:
(1) Invasion of S. alterniflora significantly changed the physicochemical properties of sediment in different seasons and invasion years. The content of ammonium in the non-invasion area was twice higher than that in the invasion area. However, the content of organic matter in surface sediment was higher in the invasion area because S. alterniflora had abundant aboveground and underground biomass. With the increase of the invasive years, the content of nitrate was getting higher, but the organic matter content was declining because the density of S. alterniflora decreased, contributing to endogenous organic matter input reduced. The content of organic matter in surface sediment was higher in spring, and the dissolved inorganic nitrogen and phosphate in pore-water were higher in autumn than in spring and summer, but the contents of silicate in different seasons were of no differences.
(2) Invasion of S. alterniflora changed the benthic habitat then affected the macrobenthic community structure. Compared to the non-invasive area, the species number decreased because S. alterniflora well-developed roots limited the growth of organisms with stronger active ability, such as crabs. The species numbers were 13 and 6 in the non-invasion and the 12-year invasion area. The dominant species changed to pollution-resistant species feeding on sediment and debris within the invasive area. The abundance was the highest in the non-invasion area, which was 417 ind./m2. The highest biomass value (57.04 g/m2) of macrobenthos happened in the 2-year invasion area. With the increase of the invasive years, the macrobenthic community was dominated by those species with small body sizes. The macrobenthic community was moderately or even seriously suffered from disturbances after S. alterniflora invasion.
(3) The invasion of S. alterniflora changed the food sources of macrobenthos. S. alterniflora and its debris became essential food sources for some benthic animals because the C/N and tannins content of S. alterniflora were lower. Besides, S. alterniflora invasion also increased the complexity of the benthic food web within the early invasion area, but this complexity declined with the increase of the invasive year.
In conclusion, S. alterniflora invasion changed the physicochemical properties of the sediment to a certain extent, the macrobenthic community structure, and the structure of benthic food web. Different invasive years of S. alterniflora caused different impacts on the local wetland system. With the increase of the invasive year, an uncomplicated community dominated by S. alterniflora gradually emerged in the YRD. The organic carbon food sources for macrobenthos experienced a shift from multiple sources to a single source, which resulted in an irreversible impact on the stability of the ecosystem.|