Spatiotemporal Characteristics of Carbon Source–Sink Dynamics in the Coastal Waters of the South Yellow Sea, China

Continental-shelf marginal seas play a pivotal role in the global carbon cycle. The coastal South Yellow Sea (SYS), located in an intense land–ocean interaction zone, is a key region for coastal carbon cycle research. The issue addressed in this region is that strong land–sea interactions have resulted in pronounced spatiotemporal heterogeneity of marine carbon sources and sinks, leading to high uncertainty in regional carbon flux estimation and an incomplete understanding of the underlying driving mechanisms. Therefore, the objective of this study was to systematically analyze the spatial variations, interannual trends, and key influencing factors of carbon sources and sinks in the coastal SYS. Methodologically, the following approach was taken: We combined shipboard in situ monitoring with multi-source environmental datasets, and developed a LightGBM machine learning model to map surface seawater partial pressure of carbon dioxide (pCO2) and air–sea CO2 fluxes at a spatial resolution of 1 km. The results indicated that surface seawater pCO2 in the coastal SYS exhibited a clear spatial gradient of “high in the south and low in the north, and higher in nearshore waters than in offshore waters”. The average pCO2 across the study region during summer was 409.9 μatm. On an annual scale, the Nantong coastal area had the highest pCO2 level, while the Lianyungang coastal area maintained a relatively low and stable pCO2 level. The air–sea CO2 flux followed a typical seasonal pattern of “winter carbon sink, summer carbon source”: the Lianyungang sea area acted as a weak carbon sink throughout the year, whereas the Nantong sea area functioned as a carbon source in summer. Over the past 20 years, pCO2 levels in the coastal waters have shown a gradual increasing trend. Nutrients and primary productivity were the core factors regulating regional carbon fluxes, with correlation coefficients of 0.58 and 0.64 between carbon flux and nutrients and chlorophyll-a in the Nantong sea area, respectively. The evidence allows us to conclude that eutrophication induced by terrestrial nutrient inputs, combined with organic matter decomposition, is the dominant driver of carbon release in the southern coastal waters. This study provides valuable technical support for coastal pollution management, carbon sink enhancement, and ocean acidification response in the SYS and analogous marginal sea systems.