Surface oceans absorb a part of anthropogenic CO2, and it induces changes in carbonate equilibrium of seawater. The seawater is gradually acidified, and saturation status of calcium carbonate is being lowered. These changes in carbonate chemistry would be serious threats for marine organisms (e.g., calcifying organisms). Ecosystems around submarine CO2 vents; CO2 seep, would be natural analogues of ocean acidification (OA) in coastal area, and ecosystem-level studies have been carried out so far. Drastic changes in species composition of primary producers have been reported in several CO2 seep. Common features are decrease in calcifying organisms such as coral and calcifying algae, and increase in mat-forming turf algae and seagrass. Considering such shift of benthic flora, change in primary production in coastal ecosystem is expected. However, there are limited number of direct measurements for photosynthesis, and their reports had focused on only some species living both in/out of CO2 seep. Therefore, the knowledges are limited to the physiological response of the primary producers to OA. In order to quantify the impact on the primary production at ecosystem level, consideration on the shift of flora and estimation per unit community area will be required.