Gwangyang Bay, a semi-closed water channel, has habited various shellfish species and has been much influenced because of introduction from Sumjin River and sewage discharge from Gwangyang City, as well as enriched nutrients from industrial complex. We have determined the analysis of long-term monitoring data in water qualities including nutrients in Gwangyang Bay during 1987-2010, except for 1991, 1995, 1996, 1998, 2002 and 2003 of which these years do not enough water quality data to analysis in this study. In May, June, July, August, September and October were carried out the analysis of temperature, salinity, DO, transparency, SS, DIN, DIP, COD, Chl-a, and ocean climate. Temperature, salinity, DO, DIN, DIP, COD were analyzed in surface and bottom. These water parameters were showed in yearly fluctuations, in particular, salinity and DIN were a significant difference between surface and bottom based on One-way ANOVA because of intensive precipitation and irradiation in summer. However, the period of summer is the highest number of larvae in shellfish which possibly has increased their mortality because of osmotic pressure from freshwater. Higher SS and lower transparency were associated with decreasing the photosynthesis in phytoplankton, but higher Chl-a in this study was possibly contributed to more introduction of pico or nano-phytoplankton than that of commonsize phytoplankton. Consequently, it is necessary for long-term monitoring in Gwangyang Bay to produce a sustainable shellfish farming and maintain marine eco-system.