Comparison of seawater CO2 system in summer between the East China Sea shelf and the Peter the Great Bay of the Japan (East) Sea

Continental shelves are active sites of air-sea CO₂ exchange and represent an important component of the global carbon budget. In this study, we investigated the CO₂ system and pertinent hydrographic parameters in two distinct continental shelf systems in the Northwest Pacific in summer 2014: the East China Sea shelf (ECSS) and the Peter the Great Bay (PGB) of the Japan/East Sea. The results show that the average temperature, pH, chlorophyll a and nutrients in the ECSS are higher, but salinity, dissolved inorganic carbon, and fugacity of CO₂ are lower than those in the PGB. Meanwhile, the ECSS acted as a sink of atmospheric CO₂, but the PGB was a source. We suggest that the observed divergent behaviors in terms of CO₂ absorption between the ECSS and the PGB may be associated with their difference in riverine runoff. Under the influence of the Yangtze River, the nutrient discharge into the ECSS is much higher than that into the PGB, where only a few small rivers empty into. The high nutrient discharge into the ECSS may stimulate high biological production, which may drawdown CO₂ and thereby driving the ECSS to act as a CO₂ sink despite high temperature in summer. On the contrary, the warming effect may dominate over the effect of biological production in the PGB due to the limited nutrient discharge, and thus turn the PGB to be a source of atmospheric CO₂. The results of this study imply that riverine nutrient discharge may exert a large control on net ecosystem productivity in shelf areas, which may subsequently play a critical role on determining whether a shelf system acts as a source or a sink of atmospheric CO₂.