Within the ecosystem of macroalgal bed of Gouqi Island offshore,small invertebrates such as zooplankton,amphipod,polychaetes and echinodermata were important parts of the food web,which bore the responsibility of transferring the energy of the primary producers to advanced consumers.To find out the food sources of copepoda,Euphausia pacifica,Acetes chinensis,Acetes chinensis,Halosydna brevisetosa,Mytilus coruscus,Turbo cornutus,Anthocidaris crassispina,gammaridae,caprellidae,Spongia,gammaridae larvae etc.,this research analyzed the carbon and nitrogen stable isotope composition of small invertebrates and primary producers including phytoplankton,macroalgae,microalgae etc.Meanwhile,the research estimated the food source contribution rate of primary producers to small invertebrates by using IsoSource multi-source linear mixed model.The results showed the δ¹³C value of primary producers was between -21.7 ‰ and -13.8 ‰,and the δ¹⁵N of primary producers was between 2.3 ‰ and 11.6 ‰. The stable isotope composition of primary producers had different varitions between the period of autumn and winter and that of spring and summer.Among them,the δ¹³C value of SOM,phytoplankton,POM and Sargassum horueri had no significant seasonal difference(P>0.05),while the δ¹³C value of epiphyte,Ulva pertuca and benthic microalgae had significant seasonal difference(P<0.05).Among all the primary producers,only the δ¹⁵N values of epiphyte had very significant seasonal difference,while the δ¹⁵N value of all the others had no significant seasonal difference.The δ¹³C value of small invertebrates was between -20.3 ‰ and -15.4 ‰,and the δ¹⁵N of small invertebrates was between 2.5 ‰ and 8.1 ‰.Meanwhile,among all the small invertebrates,only the δ¹³C and δ¹⁵N value of copepoda had significant seasonal difference(P<0.05),while the δ¹³C and δ¹⁵N values of all the others had no significant seasonal difference(P>0.05).Based on the multi-source linear mixed model,the research found phytoplankton was the main carbon source of Spongia and zooplankton such as Copepoda,Euphausia pacifica and Acetes chinensis.Caprellidae and Gammaridae larvae etc.mainly fed on epiphyte.As to small benthic invertebrates,such as Gammaridae,Anthocidaris crassispina and Turbo cornutus etc.,their main nutrition sources were macroalgae and benthic microalgae.