Program

Coastal waters, although accounting for a small part of the ocean, play an important role in the marine carbon cycle. However, it remains a big challenge to elucidate the dynamics of sea surface pCO₂ and its controls in coastal waters. Long-term and high-frequency time-series observations are essential to resolve multiple scale processes. We examined the CO₂ dynamics in the East China Sea based on the buoy system equipped with multiple sensors, located in the East China Sea at 122.8°E, 31°N on the inner shelf off the Changjiang Estuary. Data were collected from November 3, 2015 to October 20, 2016.Ourair and sea surface pCO₂ data were sampled with an interval of 3 hours along with other parameters at a higher frequency.

Atmospheric pCO₂(average 403 ± 17 μatm) showed a negative correlation with temperature during the study period. Sea surface pCO₂ (average 394 ± 108 μatm) showed a decreasing tendency from November to February and began to be under saturated around 18 December with respect to the atmospheric CO2. Then it slightly increased on March and April and showed large fluctuations from May to October from 69.4 μatm to 806.4 μatm. The rapid cooling process during late November to early January could account for most of the variation in pCO₂. On early November and April, biology activity contributed the most drawdown of sea surface pCO₂ while during late Jan to March, mixing between the plume and ECS water played an important role on the sea surface pCO₂ variability.

         We used a diagnostic approach to semi-quantitatively assess the pCO₂ variation between seasons with different controls including temperature effect, sea-air exchange, physical mixing and biological activities. We adopted the temperature dependent coefficient proposed by Takahashi to calculate the inter-seasonal changes in pCO2 modulated by temperature.The air-sea exchange effect was estimated based on the sea-air CO₂ flux and dissolved inorganic carbon (DIC) in the mixed layer. According to the variations between salinity and DIC, we could distinguish the effect of physicalmixing and biological activities. At the inner shelf site, from winter to spring, the totalpCO₂ variation was-29 μatm, most of which was contributed by the biological activities that decreased the pCO₂by -86 μatm (net biological production). While the temperature effect and the sea-air exchange leveled up the pCO₂ by 46 μatm and 33 μatm respectively due to the fact that this area was warming by 2.6 ℃ and a sink of atmospheric CO₂ during this period. The physical mixing drew down the pCO₂ by -3 μatm. Unfortunately, the salinity data missed for summer and fall, so we couldn’t calculate the physical mixing term and biological activities term. From spring to summer, the total pCO₂ variation was -8 μatm, while the temperature effect significantly enhanced the pCO₂ with 210 μatm due to the rapid warming period (from 14.3 ℃ in spring to 25.1 ℃ in summer). The sea-air exchange increased the pCO₂ by 10 μatm. Physical mixing and strong biological activities with high Chl_a level drew down the pCO₂-228 μatm together, which could counteract the temperature and flux effect from spring to summer. From summer to fall, the total pCO₂ variation was 123 μatm. The temperature effect and the sea-air exchange decreased the pCO₂ by -37 μatm and -5 μatm respectively since that this area declined in temperature about 2.6 ℃ and changed into a source of atmospheric CO₂ during this period. From fall to winter, the total pCO₂ variation was -86 μatm. The temperature effect decreased pCO₂ by -175 μatm due to the rapid cooling (from 22.5 ℃ in fall to 11.7 ℃ in winter), and the sea-air exchange effect was relative minor about -9 μatm.