Air-sea CO₂ Fluxes And Ocean Acidification In Prydz Bay,Antarctica

The uptake of anthropogenic CO₂ by the ocean decreases seawater p H and carbonate mineral saturation state,a process known as Ocean Acidification?OA?.The carbonate system in the polar ocean changed greatly under the global change.Prydz Bay is a typical area of OA research in the Sourthern Ocean,and combination of sea ice process,biological process,hydrological dynamics makes it complicate to study the capacity of CO₂ uptake and influence of OA in Prydz Bay.Therefore,we examined the dynamics of the carbonate system and air-sea CO₂ exchanges in 31 th Chinese National Antarctic Research Expedition?CHINARE?during February 2015.Based on temperature,salinity,depth,measured total alkalinity?TA?,total dissolved inorganic carbon?DIC?and calculated ?arag,we evaluated the status of OA and the impact on marine calcareous organisms.In Prydz Bay and its adjacent waters,a significant CO₂ undersaturation state existed with an average seawater pCO₂ value?265±68 ?atm?which was lower than the atmospheic pCO₂ value?382±3 ?atm?.The average air-sea CO₂ flux was-58±34 mmol C m-2 d-1 in Prydz Bay which was the strong sink of atmospheric CO₂.Minimum pCO₂?140 ?atm?was observed in the inside region with an average value 212±42 ?atm and highest value?380 ?atm?in the outside region with an average value 324±38 ?atm due to the different environmental factors in Prydz Bay.In the outside of the bay,the productivity of phytoplankton was low due to the limitation of iron and light conditions,and the CO₂-riched circumpolar deep water?CDW?upwelling is the controlling factor of enhanced surface pCO₂ and air-sea CO₂ flux variation.In the inside of the bay,the photosynthesis of phytoplankton consumed a large amount of CO₂,which greatly reduced the surface water pCO₂,so the biological activity of upper water column became the main controlling factor of the pCO₂ and air-sea CO₂ flux.With the variation of the circulation and water masses,the exchange of air-sea CO₂ has influence on the spatial distribution of carbonate system in Prydz Bay and its adjacent waters.Overall,the parameters of the carbonate system were consistent with the distributions of water masses.Surface water temperature,salinity and TA showed a minimum value in the Four Ladies Bank due to meltwater dilution,.However,the variations of DIC were: inside region < outside region and it was controlled by the phytoplankton photosynthesis rather than the effect of meltwater.Therefore,the ?arag and pH,which are determined by the temperature,salinity,TA and DIC,showed opposite distribution trends in the surface water: the average values of ?arag and pH were 1.40±0.28 and 8.06±0.09 in the inside of the bay,respectively,which were higher than the average values of ?arag?1.12±0.23?and pH?7.96±0.08?in the outside of bay.Under the surface seawater,Carbonate system south of Antarctic Slope Front?ASF?were controlled by the low temperature and high salinity shelf water.TA and DIC maintained the low concentrations due to the strong biological activity,while the ?arag showed high value and insignificant vertical variations.Carbonate system north of ASF were controlled by the high temperature and salinity CDW and Antarctic Bottow Water.The upwelling of CDW carried high CO₂ improved the concentritions of DIC,thus,the ?arag showed the minimum values?0.93±0.16?.The surface distributions of ?arag were affected by the sea ice melting,biological activity,water masses mixing and air-sea CO₂ exchange.The meltwater percentage which dilution effect is obvious can reach 8% in upper water column.Meanwhile,air-sea CO₂ exchange reduced the ?arag ⁰.32 in the surface water.However,the photosynthesis of phytoplankton in summer resulted in a seasonal increase of ⁰.59 in ?arag which effectively dampened the negative influences of meltwater and air-sea CO₂ processes and slowed down the ocean acidification processes in the upper water column of Prydz Bay.In the deep water of Prydz Bay,the accumulation of anthropogenic carbon caused the ?arag to decrease by ⁰.1,indicating that the strong carbon sink make the absorbed anthropogenic carbon intrude into the deep ocean through the high-density shelf water and accelerating the OA process of the deep seawater.