DENSITIES AND DISTRIBUTION OF ZOOBENTHOS IN THE LOWER REACHES(NANJING-JIANGYIN SECTION) OF CHANGJIANG(THE YANGTZE) RIVER

During October and November, 1979, a survey of zoobenthos in the lower reaches (Nanjing-Jiangyin section) of Changjiang was made. Samples were collected with a 0.1 m2 bottom sampler from 53 sampling points distributed in main stream, tributaries and outlets of the latter.Altogether 42 species belonging to 30 genera and 18 families were found in that rea ches. Among them, Aulodrilus prothecatus, Limnodrilus hoffmeisteri, Nephthys poly branchia, Procladius sp. and Coriicula fluminea are predominant.The mean density of zoobenthos at that section was 328 ind./m2 (86.59% belonging to Oligochaeta) and the biomass was 37,216 g/m2 (98.43% belonging to Molluca).Distribution of zoobenthos was found to be variable with different biotopes. Popula tion densities revealed a gradual diminution according to the following order: tributa ries>main stream>outlet of tributaries. In terms of biomass, such order was found to be tributaries>outlet of tributaries>main stream. The greater abundance of zoobenthos in the tributaries and shore area of main stream is probably due to the shallow (less than 6m deep) and slowly flowing water and muddy or mud-sandy bottom.A comparison of the results of zoobenthos from lower Changjiang with that from other rivers is presented. Other environmental characteristics of lower reaches of Chang jiang and their influences to the composition, distribution and abundance of zoobenthos are also mentioned.