Abstract: Changes in the hydrological conditions cause the fluctuation of an aquatic ecosystem, and the operation of a large-scale water transfer project often changes the community structure of aquatic organisms. This study examines the community structure of macrobenthos in Nansi Lake and Dongping Lake before and after the implementation of a water transfer project. The characteristics of water age change caused by the project are simulated and analyzed, and the influence of such large-scale projects on the community of macrobenthos in the lakes is discussed from the perspective of water exchange capacity. The results show that on the annual scale, after the project’s operation for a certain period of time, a change in the macrobenthic community in Dongping Lake occurred from domination by small-size aquatic insects to domination by large-size molluscs, and the dominance of molluscs in Nansi Lake was more prominent. On the seasonal scale, the proportion of molluscs usually was decreased in the water transfer period (non-rainy season), and increased in the non-water transfer period (rainy season). The implementation and operation of the water transfer project has changed the characteristics of the hydrodynamic field in the lakes, and enhanced the exchange capacity of the water bodies on the annual scale. Superposition of the water transfer inflow and the river runoff into the lake causes a longer time of flows with high velocity in the lake, which was not conducive to the stable growth of small-size species and thus led to a gradual increase in the proportion of large-size molluscs on the annual scale. In addition, the project’s implementation mainly enhanced the water exchange capacity in the non-rainy season, but the capacity in the non-rainy season was still lower, even with an increased water volume being transferred, than that in the rainy season, which also caused a decrease in the proportion of molluscs in the rainy season and an increase in the non-rainy season before and after the implementation of the water transfer project.

Key words: hydrodynamic force, macrobenthos, South-to-North Water Transfer Project, Nansi Lake, Dongping Lake, water age