The water quality before and after a typhoon was investigated as a part of a study on the increase of organic matter and on the fundamental measures to counter chemical oxygen demand (COD) in the eutrophic Soho coastal seawaters of Gamak Bay. The dissolved oxygen (DO) saturations were <54% when water column was stratified. The DO saturation was similar at surface and in the bottom layer after a typhoon $(78sim88%)$, and a very high DO saturation (234%) was observed in the surface water after mass phytoplankton growth. The highest values of $NH_4-N,;NO_3-N,;PO_4-P$, and $SiO_2-Si$ were 18.22, 38.90, 1.58, and $52.10{mu}M$, respectively, when freshwater entered after heavy rainfall events. In addition, high concentrations of $NH_4-N,;PO_4-P$, and $SiO_2-Si$ were detected with low DO saturations in bottom water (>5m). A maximum chlorophyll (Chl.) concentration of $311.0{mu}gl^{-1}$ was observed after typhoon, when a high-density Scrippsiella trochoidea red tide occurred with cell density of 42,000 cells $ml^{-1}$. The algal growth potential (AGP) was high after the typhoon. Nitrogen was always a limiting nutrient for phytoplankton growth. The highest COD level was $10.55mgl^{-1}$, and the main reason of the variation in COD was likely to be phytoplankton growth $(r^2=0.612,;p=0.000)$. Organic matter, which entered the water column when the typhoon stirred the sediments, seems to have little effects on COD increase.