Abstract: This study was based on the data of four surveys in the northern Beibu Gulf (107°51′04?~109°56′07?E; 19°44′12?~ 21°27′46?N) in 2006 and 2007, using Ecopath with Ecosim model to study the biological community structure and select the keystone species in the northern Beibu Gulf. The results showed that the ecosystem trophic levels ranged from 1.00 to 4.03, and the highest trophic level was Elasmobranchs. There were three food chains with the origin of detritus, phytoplankton and bacteria. The mixed trophic impacts among the functional groups ranged from -1.09 to 0.54, and 61.60% species had competitive relationship to each other. This result showed that there were complex competitions among the functional groups in the northern Beibu Gulf. In the end, with the keystone index of the Ecopath model, Flaccisagitta enflata was not only the keystone specie of zooplankton but also the keystone specie among all the functional groups; Parargyrops edita had the highest keystone index among fish functional groups, and the fourth highest keystone index among all the functional groups; phytoplankton had the third highest keystone index in the ecosystem. This research may provide certain reference for the trophodynamics and fishery activities in the northern Beibu Gulf.

Key words: the northern Beibu Gulf, Ecopath, food web structure, mix trophic impacts, keystone species