Contrasting seasonal variability of net community production between the Southern Yellow Sea and the Northern East China Sea during spring and summer.

The Yellow Sea (YS) and the East China Sea (ECS), which comprise continental shelves with depths of 200 m or less, are recognized as some of the most productive coastal areas globally. Although this high productivity can contribute to carbon sequestration, the spatiotemporal variability of the biological pump remains unclear. To investigate this variability, net community production (NCP) in August 2020 was estimated based on high-resolution O/Ar measurements. The study area was divided into Yellow Sea Water (YSW) in the southern YS, Changjiang Diluted Water (CDW) with salinity < 30, and Cheju Warm Current Water (CWCW) in the northern ECS. In YSW and CDW, the seasonal variation in NCP during the warm period was evaluated in conjunction with the results from the previous spring, revealing contrasting trends between YSW and CDW. In YSW, the summer NCP of 25 ± 6 mmol O m d was lower than the spring NCP of 49 ± 19 mmol O m d, primarily due to nutrient depletion in the mixed layer caused by stronger stratification. Conversely, in CDW, the summer NCP of 67 ± 28 mmol O m d exceeded the spring NCP of 26 ± 13 mmol O m d, mainly due to nutrient inputs from Changjiang River discharge. Meanwhile in CWCW, which is not influenced by river, the summer NCP was 32 ± 21 mmol O m d, showing little seasonal variation between spring and summer. To assess the efficiency of the biological pump, we measured the net community production (NCP) and gross primary production (GPP) using O in vitro method for the first time in this region. The (NCP/GPP) ratios ranged from 0.20 to 0.27 in YSW and 0.21 to 0.26 in CWCW, indicating similar efficiencies. These efficiencies were approximately twice those of the open ocean. The (NCP/GPP) was 0.35 in CDW, likely due to increased productivity driven by the Changjiang River during summer. This efficiency not only approaches the global upper limit but is also comparable to regions affected by episodic events such as upwelling and spring blooms, highlighting the potential of CDW as a significant carbon sink. This study provides valuable insights into the role of the continental shelves as a carbon sink and helps reduce uncertainties in shelf carbon uptake. Furthermore, to enhance the spatial applicability of high-resolution observations, underway NCP data were integrated with satellite-derived oceanographic variables to develop a Random Forest model. This approach underscores the potential of utilizing observational datasets to estimate large-scale NCP, reinforcing the critical role of continental shelves in global carbon cycling.