Carbonate chemistry variability in the southern Yellow Sea and East China Sea during spring of 2017 and summer of 2018

Highlights: • Variability in DIC and TA was caused by physical and biological processes. • pCO₂ change was primarily dominated by temperature and primary production. • The study region went from a CO₂ sink in spring to a weak source in summer. • Carbonate parameters were controlled by different mechanisms in each subregion. Marginal seas are highly productive and disproportionately large contributors to global air-sea CO₂ fluxes. Due to complex physical and biogeochemical conditions, the southern Yellow-East China Sea is an ideal site for studying carbonate chemistry variability. The carbonate system was investigated in the area in spring of 2017 and summer of 2018. Dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations were higher in the SYS than the ECS due to material from carbonate weathering and erosion carried by the Yellow River. High pH and low DIC and TA were observed in the Zhe-Min Coastal Current in spring due to high primary productivity caused by Changjiang River input and the Taiwan Warm Current. Temperature and biological activity were the primary drivers controlling the partial pressure of CO₂ (pCO₂) in the SYS, pCO₂ was controlled by primary productivity related to nutrients carried by the Changjiang River and physical mixing in the Changjiang River plume and inner/middle shelves of the ECS, whereas temperature was the dominant factor determining pCO₂ distributions in the ECS outer shelf waters influenced by the Kuroshio Current. Overall, the entire study area shifted from a CO₂ sink (−4.18 ± 5.60 mmol m⁻² d⁻¹) to a weak source (1.02 ± 4.87 mmol m⁻² d⁻¹) from spring to summer. Specifically, the SYS and ECS offshore waters changed from CO₂ sinks in spring to sources in summer, while the Changjiang River plume was always a CO₂ sink.