MONTHLY CHANGE OF COMMUNITY STRUCTURE OF ZOOBENTHOS IN XIANGXI BAY AFTER IMPOUNDMENT OF THREE GORGES RESERVOIR

The community structure of zoobenthos was investigated in Xiangxi Bay,Three Gorges Reservoir,from August 2003 toAugust 2004.Benthic invertebrate samples were sampled monthly except December 2003 with a modified Peterson grab (grab surface area 0.0625m2). The samples were passed through a sieve of mesh size 200μm and material retained was preserved in 10%formaldehyde for sorting of the invertebrates from the sediment.All sorted invertebrates were identified as far as possible.Benthicabundance and biomass were calculated per unit area of sediment. A total of 26 taxa were identified,among which,2 Lamellibranchia(belonging to two families,717% of the total taxa number),13 Oligochaeta (two families,50%,) 10 Chironomidae(38.5%) and 1 other animal (3.8%). The average density and biomass were 276 ind./m2 and 0.301g/m2, respectively.Oligochaetes and chironomids constituted the major groups,and Limnodrilus hoffmeisteri, Procladius sp., Branchiura sowerbyi,Tubificidae sp1. and Polypedilum scalaenum group sp.were the common taxa in this reservoir system.Among them,L. hoffmeisteriand Procladius sp.were the dominant species,and contributed to 37.0% and 28.3% of the total density,respectively. Whilecompared with Procladius sp., L. hoffmeisteri were influenced by impoundment more evidently. The standing crop of zoobenthosin Xiangxi Bay was relative low,compared with other similar reservoirs.While the taxa was similar.Our studies demonstrated thatthe standing crop was increased gradually along with the time. The total density and biomass were increased 7.8 and 13.3 timesas those of initial impoundment,separately. Pollutedtolerance taxa overwhelmed the system indicating that the water quality wasdegrading.Because oligochaetes may function as an indicator of high sediment loading and the relative abundance of oligochaetesand chironomids in Xiangxi Bay varied obviously among months,the sediment loading in Xiangxi Bay might be not serious.