Study Of Large Benthic Community Ecology Of The Yangtze River Estuary And Its Adjacent Waters

A quantitative analysis on the distribution pattern and diversity of macrobenthos communities in the Yangtze Estuary (YE) and adjacent waters (YEAW) was performed based on survey data from2006to2011, in terms of spatio-temporal properties, sediment types, and dissolved oxygen (DO). Several multivariate methods were applied to explore the macrobenthos distribution and community structure in relation to the environmental factors, such as the analysis of variance, Bray-Curtis similarity-based cluster, multidimensional scaling (MDS), analysis of similarity (ANOSIM), and canonical correspondence analysis (CCA). Besides, all the methods of curve for abundance and biomass comparison (ABC), macrobenthos pollution index (MPI) and diversity index were used to evaluate the environmental quality of the YEAW on the basis of the present macrobenthos data.The main results described as follows.Regarding the temporal influence on macrobenthos, no significant seasonal and annual variations in the species composition in the YEAW were observed. The high-value region of species number located around the front in the YE. However, the dominants varied with temporal change, and Maldane sp. were found to be the dominant species at the initial stage of community development. These Polychaeta group then gradually died out at the later stage and were finally replaced by Apionsoma trichocephala. No apparent difference was observed in the biomass, whereas the total abundance and secondary productivity show remarkable temporal differences. Community structure analysis reveals that macrobenthos in each season could be divided into estuarine and inshore assemblages. The estuarine community in summer and winter can be further subdivided into riverine and estuarine groups. The results indicated no evident interannual and seasonal variations in macrobenthos in the YE in recent years.Regarding the spatial influence on macrobenthos, the Yangtze estuary is divided into estuarine, inshore, and offshore subregion. We found that these three subregions have significant different species number, biomass, and density and the former presenting the lowest values for these community parameters. According to SIMPER analysis results, the dominant species of macrobenthos varied geographically. The community structure in the estuarine region was relatively simple with obvious dominant species, whereas the inshore region showed complicated community structure without absolute dominantsand Capitella capitata dominated in all the seasons. In the offshore region, the dominants were evidently differ in each season. The results indicate thai human activities have a huge impact on the estuanme area, some effects on the offshore area, and minimal influence on the far shore area. Regarding the influence of the sediment types on macrobenthos, a total of293species are identified in four different types of sediment. Polychaeta and mollusca were the main groups in each type of sediment. Two-way analysis of variance indicates that polychaeta, mollusca, crustacea, echinodermata significantly differ from all other groups in terms of the substrate type of sediment. However, the species of other groups except for polychaeta did not show apparent seasonal differences. SIMPER results reveal that different types of sediment are dominated bydifferent species. The dominant species in silt-substrate-typed sediment were small-sized polychaeta and crustacea, whereas the dominants in silt-mud substrate-type sediment were large-sized polychaeta, echinodermata, crustacea, and so on. Two-way analysis of variance also indicates that the biomass, density, and diversity index evidently vary with different substrate types of sediment. The biomass from low to high is silt, silt sand, silt mud, and sandy silt, in turn, whereas the abundance is silt, silt sand, sandy silt, and silt mud, in turn. Community ANOSIM indicates no evident differencein different seasons, except in autumn. CCA shows that themacrobenthos in different sediment types are affected by different environmental factors. The main factors influencing the macrobenthos in silt substrate include nutrients, salinity, water depth, and DO (negative correlation); in silt sand are DO and pH; in sandy silt is temperature; and in sandy mud are phosphate, silicate, DO, and pH. Results demonstrate that variations in food availability in different sediment types, habitat heterogeneity, and deposition rate significantly effect the macrobenthos distribution.Regarding the influence of DO on macrobenthos, the species number, abundance, and biomass of polychaeta and mollusca in hypoxic zones are much higher than in non-hypoxic zones. A higher percentage of polychaeta and mollusca exists in hypoxic zones, whereas higher crustacea exists in non-hypoxic zones. SIMPER analysis results show that the dominant species are different between these two zones. Especially in summer, the dominant species in hypoxic zones were small-sized polychaeta (e.g., C. capitata and Heterospio sinica). However, dominant species in non-hypoxic include not only polychaeta but also the Ogyrides striaticauda. Both the analysis of clusterand MDS indicate that communities in these two zones have no significant difference, which is also confirmed through the ANOSIM. CCA results indicate that the polychaeta in hypoxic zones is principally affected by salinity, whereas crustacea is depth, temperature, total organic carbon, and so on. In non-hypoxic, the polychaeta is mainly influenced by salinity and depth, whereas crustacea is suspended matter and inorganic nitrogen. Annual variation analysis shows no, and so on. Variations in the dominant species are observed, although there was no evident annual variation in biomass and abundance. These findings indicate that a short-term, moderately low-oxygen supply does not necessarily lead to decreased total abundance of macrobenthos but can result in the community reconstruction and evident variation in assemblage composition.Three methods (diversity index, MPI, and ABC curve) were used to study the effects of pollution on macrobenthos. Diversity index evaluation shows that the stations presented heavy pollution in each season is above one third, whereas the slight pollution and cleanness is below20stations. MPI evaluation indicates that the heavy-pollution stations in each season is>80%, whereas the influence of slight-pollution and cleanness is below10stations. ABC curve reveals that the community in the offshore area is disturbed only in spring and winter. These results indicate that the three methods used to evaluate environmental quality have both merits and demerits. However, the MPI method may overestimate the pollution degree in the YE with low diversity. Compared with other domestic estuaries and harbors investigated at the same period, environmental quality of the YE is only better than that of Hangzhou Bay and Jiaojiang Estuary. Therefore, the environmental protection in the YE allows of no delay.