Impacts of Physical Mixing Combined With Biological Activity on Spatiotemporal Evolution of CO2 Uptake Within the Plume Discharged From the Changjiang River in the Summer of 2016

Abstract

Understanding carbon dynamics in the river-dominated marginal seas remains challenging due to limited temporal and spatial in situ measurements and complexities of coupled physical-biogeochemical systems. In this study, a coupled physical-biogeochemistry regional ocean model was used to investigate the underlying drivers of the spatiotemporal evolution of oceanic CO₂ uptake within the Changjiang River Plume (CRP) during the summer of 2016. Three comparative experiments were conducted to quantify the contribution of physical and biological processes and air–sea gas exchange within the CRP. In the estuary, strong turbulent mixing due to tides largely inhibited biological production despite being supplied with a high concentration of riverine nutrients, making this area a vigorous source of atmospheric CO₂. On the other hand, phytoplankton growth was rapidly promoted by weakened turbulent mixing on the nearshore slope, leading to the strongest drawdown in the partial pressure of CO₂ (pCO₂) due to biological processes. In the offshore region, the air–sea CO₂ exchange contributed to increasing pCO₂ (approximately 10%), which finally altered the offshore water to a weak CO₂ source into the atmosphere. An additional experiment without tides further demonstrated that strong fluctuations in turbulent mixing in the nearshore slope modulate surface flows with a spring-neap cycle of stratification and destratification, resulting in the formation of an elongated chlorophyll front and its periodically undulating behavior of extending seaward or retreating shoreward. Our findings highlight the deep impacts of tidal modulation combined with biological activities on spatiotemporally evolving biogeochemical responses in river-dominated marginal seas.