Sea Surface PCO₂ Dynamics In The Nearshore Area Of The East China Sea Based On Buoy Time Series Observations

Accounting for a small part of the ocean though,coastal oceans play an essential role in the marine carbon cycle.Located in transional zones featuring dynmical exchanges between the ocean,estuary and land where human activities are intense,coastal oceans are often subject to highest CO2 variability and much more complex controlling processes as compared to the open ocean.Therefore,long-term and high-frequency time-series observations are essential to resolve multiple scale processes in coastal oceans.We examined the CO2 dynamics in the East China Sea based on a buoy system equipped with multiple sensors deployed in the East China Sea at 122.8°E,31°N off the Changjiang Estuary.Data were collected from November 3,2015 to July 23,2017.Our atmospheric and seawater pCO2 data were sampled with an interval of 3 hours along with other parameters at a higher frequency.Seawater pCO2 at the study site was the highest in autumn,the lowest in spring.It was mostly stable in winter but had highest variability in summer.On a short-time scale,seawater pCO2 in summer was negatively correlated with chlorophyll while in the other three seasons seawater pCO2 mirrowed the semi-tidal cycle.The daily amplitude in summer is also larger than those in other three seasons.The study site was overall a weak sink of atmospheric CO2 with an average annual flux of 1.4 ± 14.7 mmol m-2 d-1 in 2016.Seasonally,it was a source in summer and autumn,but became a sink in winter and spring.Based on the high-frequency continuous data,this thesis evaluated the contribution of seawater and atmospheric pCO2,wind speed and other factors to the uncertainty of sea-air CO2 flux.The uncertainty of sea-gas CO2 flux mainly depended on sea surface pCO2,which is ± 515%.The contribution of atmospheric pCO2 and wind speed is ± 77%,± 86%,respectively.The uncertainty caused by sea surface salinity and temperature is almost negligible.The controls of seawater pCO2 differ seasonally.Biological activities resulted in reduction of seawater pCO2 in early autumn.Seawater pCO2 decreased with temperature in late autumn and early winter.In late winter and early spring when temperature was below 15.5 ?,water mixing was a dominant control showing an increase of pCO2 originated from the Changjiang Dilution Water(CDW).While in spring when temperature increased to over 15.5 ?,the role of biological activities becames signicfiant on top of water mass mixing in controlling the changes of seawater pCO2.Such a characteristic continued in summer when both biological activities and CDW significantly modulated the level and its variations in seawaterpCO2.On a short-time scale,or intra-seasonally,seawater pCO2 in summer was regulated by biological activities.In winter and spring with low chlorophyll concentrations,the diel variation of seawater pCO2 is mainly affected by tidal-driven water mixing.However,when the chlorophyll concentration peaked in spring,biological activities largely contributed to the diel variation of seawater pCO2.In addition to the tidal cycle,vertical mixing notably during typhoon had a fundamental impact on the daily variation of seawater pCO2 by bringing high CO2 water to the surface in autumn.Our quantitative analysis further shows that temperature changes during the rapid cooling periods could account for 55%of the variation in seawater pCO2 from November to January with a monthly drawdown rate of 28?70 ?atm.Water mixing dominated the seawater pCO2 variation in February,accounting for 46%.Biological activities dominated the seawater pCO2 variation in July and August,accounting for 50%and 41%respectively,which lowered seawater pCO2 by 113 to 278 ?atm per month.In other months,multiple processes affected the seawater pCO2 variation.For example,in April 2016,the effect of water mixing and biological activities could offset the increase of seawater pCO2 caused by the temperature rise,resulting in reduction of seawater pCO2.Comparing the buoy located in the outer Changjiang estuary as decribed above,our data collected from the buoy deployed in the mid-shelf of the East China Sea showed a lower pCO2 with more stable values.Here combined with salinity data in spring and summer,it showed that seawater pCO2 with salinity below 25 had large variations and were significantly affected by CDW with high pCO2 and nutrients.Seawater with salinity around 30 was affected by photosynthesis and had a lowerpCO2.At salinity>33,it showed higher seawater pCO2 in summer than in spring due to seasonal warming.We also conducted two sets of CO2 perturbation experiments using DIC-enriched seawater in an enclosed system with a capacity of 300 liters in the lab in order to examine how the variation of seawater DIC would affect the air pCO2 via sea-air CO2 exchanges.The amount of DIC reduction in seawater is similar to the CO2 increase in the air when the bulk seawater system is perturbed by adding high DIC water,suggesting that excess DIC of the seawater with contant temperature,salinity and total alkalinity can be entirely converted to atmosphere CO2 via CO2 sea-air exchange.Our lab experiments further suggest that using buffer factors alogn with the variation in seawater DIC,we are well able to predict the variation of seawater pCO2.Using a simplied numerical simulation scheme,we further showed that the estimated variation of pCO2 is slightly smaller than the actual variation measured.Only when the amount of seawater DIC variation is small such as less than 20 pmol/kg,the difference between the estimated pCO2 variation and the actual variation will be within 10 ?atm.To sum up,this study demonstrated the dynamics and controls of seawater pCO2 in the nearshore the East China Sea with large temporal variations.Such temporal variations and their controls differ significantly between the outer Changjiang estuary and the mid-shelf of the East China Sea.The continuous high frequency observation provides reliable data for quantifying the effects of controlling processes on seawater pCO2.