A Comparative Study Of Seasonal Acidification In Southern Nearshore And Central Offshore Waters Of The North Yellow Sea

At present the atmospheric carbon dioxide rise has induced ocean acidification,threatening marine biodiversity and ecosystem health.The ocean acidification has become a major environmental problem that seriously affects the sustainable development of human society.The North Yellow Sea?NYS?is a productive shallow-water marginal sea of the western North Pacific,around which fast-developing and highly populated marine aquacultural activities are involved.Previous studies have reported that the NYS cold water mass accumulates CO₂ produced by community respiration in summer and autumn,resulting in critically low aragonite saturation index(?_arag)values of less than 1.5 in a large area of sea water below the surface,which may severely affect those acid-sensitive bivalves and the aquacultural activities.This study is conducted to further evaluate the temporal and spatial variability of sea water acidification and its regulating mechanism in the NYS,and to predict local responses to the ocean acidification in the next 30 years.In 2017,the author investigated monthly variations in dissolved oxygen?DO?and carbonate system in southern neashore waters of the NYS from spring?May?to autumn?September?,and collected DO and carbonate system data in central waters of the NYS in October.By combining two other central NYS surveys in autumn?September?and winter?December?in 2017,the author also has an opportunity to analyze monthly variations in carbonate systems from September to October in this offshore water area.By comparing and analyzing the seasonal acidification conditions of the two water areas,the regional differences of seasonal acidification in the NYS bottom waters and its regulating mechanisms were studied.The results showed that bottom-water seasonal acidification was mainly driven by community respiration in the two regions.In southern neashore waters,rainfall dilution and calcification also affected ?_arag in summer.Autumnal net CaCO₃ dissolution occurred in central offshore waters.To help industrial people to easily deduce seawater ?_arag in the NYS,the author constructed a mechanistic model which connected ?_arag with seawater temperature,salinity,dissolved oxygen and wintertime atmospheric CO₂ concentration,considering acidification effects of atmospheric CO₂ rise,respiration and dilution.For central offshore waters,?_arag? 0.00885×[?61.745×Salinity + 320?-?DIC^Winter + 106/138×AOU?]+ 0.46;and for southern neashore waters,?_arag= 0.00961 ×[73.288×Salinity-?DICWinter+ 106/138×AOU?]+ 0.52,where DICWinter is the air equilibrated DIC in bottom water at the beginning of every year,and AOU is the apparent oxygen utilization.The reconstructed ?_arag values below the seasonal thermocline were in good agreement with the standard Qarag values calculated from dissolved inorganic carbon and total alkalinity,with a standard deviation of 0.15?n=237 for central offshore waters and n=99 for southern neashore waters?.This approach may have applicability in other coastal systems with similar physical and biogeochemical conditions.Incorporating historical data,this paper quantifies the driving factors of rapid acidification in the NYS from spring to autumn,including the effects of community respiration,calcification and calcium carbonate dissolution on water acidification.Results show that the contributions of the NYS cold water mass community respiration?A??Re-p?,calcification???Calc?and temperature changes????T?to the monthly ??_arag from spring to summer were 84±9%,30±16%,and-13±10%,while the ??Resp,???T and contributions of CaCO₃ dissolution???Diss?to the bimonthly ??_arag from late summer to autumn were 103%,1%and-5%.Meanwhile,the contributions of the NYS cold water mass community respiration?ApHResp?,temperature changes???H?T?and calcification??pHCaIc?to the monthly ApH from spring to summer were 50±23%,39±23%,and 13±4%,while the ApHResp,ApHAT and contributions of CaCO₃ dissolution??pHDiss?to the bimonthly ApH from late summer to autumn were 110%,-5%,and-5%.The author also developed a positive correlation between net community calcification rate and ?_arag in the NYS cold water mass.And the results suggested that the NYS cold water mass net calcification rate declined to nearly zero when ?arag reached a critical level of 1.5-1.6.Projection results showed that very low ?_arag values of<1.5 may exist all year round in the NYS subsurface waters by the 2050s,causing great environmental stress to its benthic faunal community.