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In this study, we investigated the CO 2 system in two distinct continental shelf systems in the Northwest Pacific in the summer of 2014. The East China Sea (ECS) is a river-dominated shelf system, and the Peter the Great Bay (PGB) of the Japan/East Sea represents an ocean-dominated shelf system. The results show that the PGB acted as a source of atmospheric CO 2 , while the ECS was a sink. We suggest that the observed divergent behavior, in terms of CO 2 absorption, may result from differences in the river runoff received. Under the influence of the Changjiang River, the nutrient discharge into the ECS is much higher than that into the PGB, where only a few small rivers act as inputs. The high nutrient discharge may stimulate high biological production, which may consume CO 2 , driving the ECS to act as a CO 2 sink despite its high temperature in summer. The temperature effect may dominate the effect of biological production in the PGB because of its limited riverine nutrient inputs. We found that external CO 2 sources from the adjacent open ocean could explain the source status of the PGB. This finding agrees with the “Ocean-dominated Margin” (OceMar) hypothesis, which has only been tested using data from tropical marginal seas. The present data set from the PGB, thus, provides the first example to support that the “OceMar” hypothesis may also be applicable in high latitude shelf systems. [ABSTRACT FROM AUTHOR]