Seaweed bed has an important role for the lower trophic ecosystem in coastal seas because of its high primary production. In this study, the primary production of the seaweed bed and the pelagic phytoplankton was compared in each sub area of the Seto Inland Sea. The ratio of the primary production of seaweed bed to pelagic phytoplankton was high at Aki-Nada (10.8 %) and Bisan-Seto (3.8 %), while the ratio was low at Hiroshima-Bay (0.6 %), Osaka-Bay (0.3 %) and Harima-Nada (0.2 %). Seasonally, the ratio was high during winter. The uptake rate by primary production, release rate by decomposition and stock rate as a leaves of nutrients by the seaweed bed were estimated in the southern area of Hiroshima-Bay. The quantitative contribution of seagrass bed to the nutrient budget in this area was estimated. It was estimated that 14 and 40 % of load from the river were accumulated as the less degradable organic matter for nitrogen and phosphorus, respectively.