Lake Shihwa, which was artificially formed by the construction of dike on the intertidal flat in 1994, represent perennial eutrophication and hypoxic condition in the water columns after construction. The dense phytoplankton blooms of average value of 168 mgchl-a l-1 have occurred thrthrough out the year in Lake Shihwa. This eutrophication is resulted from the large input of nutrients from the industrial complex and populated cities in the hinter lands. Them ajor organisms of algal blooms were dinoflagellate, Prorocentrum minimum, inspring and summer and diatoms in autumn and winter. The autumn and winter diatom blooms were limited by the depletion of silicate in the lake. The primary productivities in the lake Shihwa ranged from 2650 mgCm² day to 9500 mgCm² day with an average of 3970 mgCm² day. These high primary productions were limited to the shallow euphotic zone due to the inhibition of light by high algal biomass on the surface layer. Lack of photosynthesis and the decomposition of falling organic matter under the middle of water column accelerated the depletion of dissolved oxygen in the bottom layer. The lake of mixing with in the water column was also one factor of an oxic condition in the deepest portion of the lake.