Distribution and variation of macrobenthos from the Changjiang Estuary and its adjacent waters

The present distribution of macrobenthos from the Changjiang Estuary and its adjacent waters were studied based on the samples collected from 86 stations in four investigations during 2005~2006.A total of 330 macrobenthic species were identified from the samples,in which,122 species are members of Mollusca,83 of Polychaeta,67 of Crustacea,23 of Echinodermata,28 of Pisces and 7 of other groups.The total average abundance of macrobenthos in the research region was(146.4±22.3)ind/m2,the total average biomass was(12.8±2.3)g/m2.The average values of Shannon-Wiener's,Margalef's and Pielou's indices were 1.72±0.16,1.37±0.19 and 0.64±0.04 respectively.According to the distributions of the macrobenthos,the research region can be divided into three subareas from west to east,or from nearshore to offshore.The most westward area,nearshore and the estuarial area,was named Estuary-Hangzhou Bay Subarea,the species composition of the macrobenhos in this subarea was very simple,the indices of biodiversity were very low,implying that the macrobenthic community structure was susceptible.The area east to the Estuary-Hangzhou Bay Subarea,outside of the estuarial area to the Zhoushan Archipelago,covering most of the central part of the research region,was named Outside Estuary-Zhoushan Subarea,the species composition of the macrobenthos in this subarea was more complicated than that in the Estuary-Hangzhou Bay Subarea,the indices of biodiversity were higher,too,indicating that the macrobenthic community structure in this subarea was relatively steady.The most outside of the research region,east to the Outside Estuary-Zhoushan Subarea,was named Offshore Subarea,the species composition of the macrobenthos in this subarea was the most complicated in the research region,the indices of biodiversity were the highest,indicating that the macrobenthic community structure was the most steady.The historical variation of the distribution of the macrobenthos from the Changjiang Estuary and its adjacent waters was analyzed based on some data during the recent 50 years.The total average biomass of the macrobenthos in the diluted areas showed little variation with its value being near 20 g/m2.However,the dominant groups of the macrobenthic community in this area changed distinctly between 50 years ago and the present.The enchinodermata commonly with big body in size and long life cycles dominated significantly fifty years ago,now the polychaeta that usually having small body in size and short life cycles became the most important groups instead.