Seasonal variations and exchange fluxes of particulate organic carbon in the Bohai Sea and Yellow Sea

The ocean is the largest carbon reservoir on Earth and plays an important role in mitigating global warming by regulating the amount of CO₂ in the atmosphere. The Bohai Sea and Yellow Sea are typical semi-enclosed continental shelf marginal seas in China. Therefore, an in-depth study of the characteristics of particulate organic carbon （POC） changes and transport fluxes in the Bohai Sea and Yellow Sea is of great significance to understand carbon cycling and ocean carbon flux. The seasonal variations in spatial distribution and carbon pools of POC in the Bohai Sea and Yellow Sea were studied based on the measured data of different seasons from 2010 to 2016. Results show that the concentration of POC was the highest in spring on average of 0.34 mg/L, while it was 0.30 mg/L in summer, 0.27 mg/L in autumn, and 0.17 mg/L in winter. The spatial distribution of POC in different seasons showed the same pattern. The concentration of POC was higher in the nearshore, lower in the offshore area, and the highest in the bottom layer. The areas with higher concentrations were mainly located near the Huanghe （Yellow） River Delta, the nearshore area off the east coast of the Shandong Peninsula, and the area near the northern Jiangsu Shoal in the north of the Changjiang （Yangtze） River estuary. The lower concentration areas were mainly concentrated in the deepwater area of the central part of the South Yellow Sea. The factors affecting the distribution of POC varied in season. In spring and fall, runoff input, sediment resuspension, and phytoplankton metabolism dominated the distribution of POC; in summer, the spatial distribution of POC was mainly controlled by the primary production of phytoplankton; and in winter, the main factor was the resuspension of sediments. In addition, POC carbon pools of the Bohai Sea and Yellow Sea showed significant seasonal variations, with the highest reserves in spring, being 1.32×10⁶ t and 6.71×10⁶ t in the Bohai Sea and Yellow Sea, and the lowest in winter for 7.21×10⁵ t and 3.39×10⁶ t, in the Bohai Sea and Yellow Sea, respectively. About 6.55×10⁵ t POC entered into the Yellow Sea from the Bohai Sea through the Bohai Strait every year and mainly concentrated in summer.