In order to investigate the changes in nutrient availability after the introduction of oyster farms in 2015, an investigation of seawater analysis was carried out from March 2011 to December 2012 for the before and from September 2015 to April 2018 for the after. Although there was a significant increase in nitrite, dissolved inorganic nitrogen was significantly decreased to half of what it had been before. The spatial discrimination of each nitrogenous nutrient was observed in CV% for ammonia, nitrite, and nitrate twice as much as before but not for DIN. No significant difference was observed in dissolved inorganic phosphate but not in CV%, which rose from 12.67% to 36.62%. Consequently, the chlorophyll-a concentration significantly decreased from 2.242 μg/L to 1.244 μg/L, negatively correlated with salinity while positively with nitrate and DIN. The grazing pressure from the introduced oyster farm has significantly decreased the phytoplankton stocks and successively increased metabolite sedimentation. The metabolite has been remineralized by oxidation-reduction reaction rather than bioturbation in hyperoxic sediment of a shallow tidal zone. As well as food availability, nutrient availability from top-down control should be considered for maximum sustainable yield when introducing oyster farms in closed waters.