From May to November 2007, we investigated the seasonal and spatial variations of temperature, salinity, optical properties, chlorophyll-α and turbidity in Suo-Sound of the Seto Inland Sea in relation to the Manila clam (Ruditapes philippinarum) production. The bottom turbid layer was found throughout the Suo-Sound, particularly in the western shallow area (bottom depth: about 10 m) which chlorophyll a concentrations were significantly higher than offshore. Generally, the entire water column of western area (bottom depth: less than 20 m) was mostly the euphotic zone. As the fishery grounds of the Manila clam located in the mud flat and the areas less than 5 m water depth, they inhabit in potentially productive area of phytoplankton and benthic microalgae. Also, the highest chlorophyll-α concentrations were observed in October, approximately a few weeks before their autumn spawning period. These results indicate that the marina clam's production is apparently coupled with local environmental factors and seasonal events related to the phytoplankton production.