With impoundment of Three-Gorges Reservoir (TGR) in June 2003, drastic hydrological changes ensued. After the impoundment, downstream stretches of many tributaries were inundated by the reservoir to form bays. The hydrodynamic conditions of these bays were all influenced by hydrodynamics in the mainstream of the reservoir. Therefore, these bays are all closely connected with the mainstream. On the other hand, two reservoirs, Gudongkou Reservoir 1 and Gudongkou Reservoir 2, which were located upstream of Xiangxi Bay, constituted a cascade system with the latter. Longitudinal hydrological gradients were observed among them. The present study investigated benthic macroinvertebrates both in the Hubei portion of TGR and in the cascade of reservoirs in Xiangxi River Basin to explore the ecological effects of large hydropower projects, and supply corresponding theoretic foundation for the improvement of reservoir management. The main results are as follows:
The benthic macroinvertebrate community structure in TGR and Xiangxi Bay during a three-year period after the reservoir became operational was investigated. The time when the benthic community of the mainstream was stabilizing was similar to that of Xiangxi Bay. However, the succession patterns were different between them. An apparent annual cycle in the benthic community of the mainstream was observed from the second year, and the community responded to the hydrological conditions. In Xiangxi Bay, the benthic community was stable form October 2004 and the community responded to eutrophication.
Effects of sedimentation on benthic communities in Xiangxi Bay were investigated during one year. However, no variable related with sedimentation appeared to be associated with differences in the community.
Spatial and temporal patterns of benthic macroinvertebrate communities of the three bays (Tongzhuang Bay etc.) were investigated during one year. The seasonal pattern of benthic communities was similar to that of the mainstream of the reservoir, implying the impact of the mainstream on the bays.
Relative production of macroinvertebrates in the mainstream and Xiangxi Bay during a three-year period was calculated with an empirical model. The mainstream is allochthonous dominated; whereas Xiangxi Bay is autochthonous dominated.
Macroinvertebrate ecology was investigated along a cascade of three reservoirs in Xiangxi River Basin during one year. Benthic communities differed in the three reservoirs. The study suggested that the biodiversity of the system could be improved by increasing disturbance in water body.
The results from analyses of a benthic macroinvertebrate community in the bays of TGR showed that the mainstream zone is the hinge between the mainstream and the bay. This newly distinguished mainstream zone is worthy of attention in reservoir ecosystem management.