Inter-annual and spatial variations of air-sea CO fluxes and acidification mechanism in the coastal waters of Qingdao, China during 2011-2019.

The interplay of global climate change and anthropogenic activities has significantly affected the carbon cycle in coastal ocean environments. Consequently, further investigation into the carbonate system, carbon source and sink processes, and acidification mechanisms is essential. This study examined the surface carbonate system offshore of Qingdao, utilizing data from nine spring cruises in 2011-2019. The inter-annual variations in sea surface temperature revealed a gradual upward trend, whereas chlorophyll a (Chl-a) and apparent oxygen utilization (AOU) exhibited alternating trends of increase and decrease. Over the period from 2011 to 2019, pH levels ranged from 8.00 to 8.19, partial pressure of CO (pCO) values varied between 187.8 and 364.1 μatm, and calcium carbonate saturation (Ω) ranged from 3.22 to 4.79. From 2011 to 2017, both pH and Ω showed a decreasing trend, attributed to the increasing influence of respiration. In contrast, an upward trend was observed for both parameters during 2018 and 2019, indicating a decline in respiration's contribution. Biological processes, especially the increase in respiration caused by the nutrients and organic matter released through human activities, were pivotal in determining spatial variations in pCO and Ω offshore, while the impact of mixing processes and temperature was comparatively negligible. Furthermore, the Ulva prolifera bloom event in July 2015 significantly affected the coastal waters of Qingdao's carbonate system when compared to the no-bloom period in March 2015. Overall, the study area functioned as a sink for atmospheric CO in the spring. The fluctuations in air-sea CO fluxes corresponded with changes in ΔpCO and wind speeds, with surface seawater air-sea CO fluxes ranging from -17.7 to 12.2 mmol m d between 2011 and 2019, yielding an average flux of -9.4 ± 5.3 mmol m d.