The food web dynamics in a coastal ecosystem of Korea were predicted with Ecosim, a trophic flow model, under various scenarios of primary productivity due to ocean warming and ocean acidification. Changes in primary productivity were obtained from an earth system model 2.1 under A1B scenario of IPCC CO2 emission and replaced for forcing functions on the phytoplankton group during the period between 2020 and 2100. Impacts of ocean acidification on species were represented in the model for gastropoda, bivalvia, echinodermata, crustacean and cephalopoda groups with effect sizes of conservative, medium and large. The model results show that the total biomass of invertebrate and fish groups decreases 5%, 11~28% and 14~27%, respectively, depending on primary productivity, ocean acidification and combined effects. In particular, the blenny group shows zero biomass at 2080. The zooplankton group shows a sudden increase at the same time, and finally reaches twice the baseline at 2100. On the other hand, the ecosystem attributes of the mean trophic level of the ecosystem, Shannon's H and Kempton's Q indexes show a similar reduction pattern to biomass change, indicating that total biomass, biodiversity and evenness shrink dynamically by impacts of climate change. It is expected from the model results that, after obtaining more information on climate change impacts on the species level, this study will be helpful for further investigation of the food web dynamics in the open seas around Korea.