Abstract: Biological integrity is the ability of an aquatic ecosystem to support and maintain a balanced, integrated, adaptive assemblage of organisms having a species composition, diversity, and functional organization comparable to that of natural habitats within a region. Vegetation with the advantages including ease of identification and immobility is the key feature of wetland ecosystem and can be used to assess the health status of the wetland ecosystem. Lake Liangzihu, as the Wetland Nature Reserve of Hubei Province, plays an important role in biodiversity protection by offering habitat for waterbirds, macrophytes and fish. To evaluate the health status of the riparian wetland of Lake Liangzihu, a vegetation index of biotic integrity (VIBI) was developed from data collected by vegetation survey. To establish the VIBI calculation, 21 candidate vegetation metrics based on species richness and composition, vegetation abundance, ecological affinity, and species tolerance were tested for their ability to discriminate between impaired and least-impaired habitat conditions. Seven discriminatory metrics, Number of emergent species, Number of perennial species, Percent exotic species, Shannon-weaver diversity index, Floristic Quality Assessment Index(FQAI), Percent tolerant plant species and Percent intolerant plant species, were finally selected with a range test, box-and-whisker analysis and Pearson correlation analysis. We then assigned scores for each metric based on the statistical analysis by using three scoring method, andthe scoring criteria were based on the distribution of the metrics' values across all sites. The total VIBI score was obtained by summing the scores from the 7 metrics, resulting in a maximum score of 35. The criteria of health ranking were finally determined based on the 25% percentile of VIBI value in reference sites. Four quality classes (Good, Fair, Poor, and Very poor) of the health status were defined. The results of vegetation survey showed that there were 182 plant species belonging to 52 families and 128 genera in Lake Liangzihu, including 147 hygrophytes, 20 emergent macrophytes, 6 floating-leaved plants, 9 submerged macrophytes, respectively. The dominant plant species were Conyza Canadensis, Cynodon dactylon, Carex argyi, Echinochloa crusgalli, Juncus effuses, Zizania caduciflora, Typha orientalis and Trapa bispinosa. The dominant association were Trapa bispinosa Ass., Vallisneria natans-Trapa bispinosa-Nymphoides indica Ass., Phragmites australis-Zizania caduciflora Ass. and Vallisneria natans-Najas minor-Hydrilla verticillata Ass. The results indicated that in the 22 sampling sites we have investigated, only 3 and 6 were in good and fair, respectively, totally covering a percentage of 41%, while 59% of which were not satisfactory. In general, the health status of eastern Liangzi Lake, Niushan Lake, Shanpo Lake and northern part of Qianjiang Lake were good, Zhangqiao Lake were in poor, and the six worse sites were along the coast of Ninggang Lake and southern Qianjiang Lake. The main factors affecting the biological integrity of vegetation were sediments and human activities such as water level regulation, enclosure aquaculture and grazing.

Key words: vegetation IBI, Lake Liangzihu, health assessment, wetland