A multispecies model for shellfish polycuhure in the Sanggou Bay in China used for large-scale long-line cultivation of the Chinese scallop Chlamysfarreri,the Pacific oyster Crassostrea gigas and the kelp Laminariajaponica is presented.The model includes key physical processes which are the transports of matter at the system boundary,and the main biological process that is the primary production and nutrients release from the bottom.By the model,the seasonal fluctuations of phytoplankton biomass and dissolved inorganic nitrogen(DIN) in 1994 are simulated.Furthermore,if the kelp culture scale is kept constant and the Chinese scallop and the Pacific oyster culture scales are adjusted,virtual shellfish farms are funded and responses of phytoplankton to the large- scale shellfish culture are simulated.According to these simulated results,the room limitation,and the hypothesis that shellfish will not grow well if the phytoplankton biomass is less than 8.2 mg/m3,the expandable multiple of scallop culture k and that of oyster culture y are determined as k=-0.276 5y+4.690 5 and 0.133 3k+0.006 6y≤0.667 5,where,k (or y) is equal to 1, the culture scale of scallop (or oyster) is 8.8×109 individuals (or 66 ha,with a density of 59 ind./m2),and the kelp culture scale is 3 300 ha with a density of 12 ind./m2.