Seawater monitoring, geochemical and benthic foraminiferal analysis of sediments were conducted to understand the occurrence of hypoxia and effect of mussel farming on benthic ecology in spring season of Gamak Bay. Bottom dissolved oxygen (DO) of surface sediment ranged 1.42-7.05 mg/l with average 4.90 mg/l and increased from northwest to south area. Hypoxia occurred at northwest area of station 1 and 3 with 1.8 mg/l, 1.4 mg/l, respectively. Contents of total organic carbon with 1.20% of average was decreased from northwest toward south area. Ratio of total organic carbon to total sulfur with 4.82 ranged 2.40-8.26 and increased from northwest toward southern part of bay. Polluted reductive conditions were gradually diffused from northwest toward south. These diffusion patterns were observed in species diversity, abundance frequency, and benthic foraminiferal assemblages composed of E. subarcticum-T. hadai (northwest and Daegyeongdo area), E. subarcticum-A. beccarii (central area) and E. subarcticum-E. clavatum (central and south area). It was thought that these organic pollution caused by the mussel farm distributed northwest area.