Key points are not available for this paper at this time.
We examined the sub-seasonal to interannual variability and multi-year trend of sea surface CO2 partial pressure (pCO2) and air-sea CO2 flux at a coastal site of the East China Sea (31N, 122.8E) based on high-frequency time-series data collected by a buoy since 2013. Seasonal average sea surface pCO2 was highest in autumn, but the lowest value can appear in winter or spring, depending on the biological productivity in spring. The seasonal amplitude of pCO2 was up to 123 atm. Based on property-property relationships and a simple mass budget model, we found that temperature change, biological activity, water mixing and air-sea CO2 exchange all made significant contributions to the seasonal variation of pCO2. From winter to summer, seasonal warming and atmospheric CO2 uptake elevated the pCO2, while net biological production, weakened vertical mixing and the retreat of the Yellow Sea Coastal Water (YSCW) lowered the pCO2. Conversely, from summer to winter, seasonal cooling and CO2 emission lowered the pCO2, while respiration, enhanced vertical mixing and the YSCW intrusion raised them up. Over short-term timescale, biological production and respiration frequently drew down or elevated the pCO2 by 150-400 atm within 5-10 days during warm months. When biological activity was suppressed during cold months, such short-term variations were dominated by water mixing with a smaller pCO2 amplitude of 5-60 atm within 2-6 days. This site was a sink of atmospheric CO2 in winter and spring, but a CO2 source in summer and autumn. Annually, it was a moderate CO2 source in 2014 (air-sea CO2 flux was 2.88 11.02 mmol m2 d1), a weak CO2 sink in 2016 (-0.21 12.23 mmol m2 d1), and a weak CO2 source in the combined year of the first half of 2017 and the second half of 2018 (0.40 9.11 mmol m2 d1). The relatively high CO2 source in 2014 was likely due to the weaker biological production in spring and more typhoon passage in autumn. From 2013 to 2019, the wintertime sea surface pCO2 didnt follow the increasing trend of the atmospheric pCO2, leading to an enhancing carbon sink in winter.
Luo et al. (Sat,) studied this question.