Ocean Acidification And Its Driving Mechanisms In The Chukchi Sea Of The Arctic Ocean

The decrease in pH and carbonate saturation caused by increasing atmospheric anthropogenic CO2 since the pre-industrial period has become a major environmental problem,namely ocean acidification(OA),that seriously threatens ecosystem health and the sustainable development of human society.The Chukchi Sea is the largest CO2 sink in the Arctic Ocean,its air-sea CO2 flux accounts for 58%of the entire Arctic Ocean.High carbon sink and productivity in spring and summer is accompanied by the decomposition of a large number of organic compounds in autumn and winter which releases a large amount of CO2 into the water column.Along with the dilution effect of meltwater,both of which contribute to the significant ocean acidification in this region These controlling factors and the accumulation of anthropogenic CO2 make it more complex to understand the status of OA in the Chukchi Sea.Chukchi Sea is one of the richest fisheries in the world.Intensified acidification in the Chukchi Sea may lead to the cessation of calcareous plankton growth,which would further cause irreversible damage to local fisheries and ecosystems.The purpose of this study is to evaluate the characteristics and controlling mechanisms of the intra-seasonal and interannual variability of acidification in the Chukchi Sea,and therefore shed a light into the understanding of OA and its trend in the Chukchi Sea.This paper collects the field data of 33H120020718,33H120040718,CHINARE2008,CHINARE2010,CHINARE2014,33HQ20150809,CHINARE2016 and CHINARE2018 in the Chukchi Sea.Based on the field data of CHINARE2008,2010,2014 and 2016,we report the distributions of carbonate,nutrients and apparent oxygen utilization(AOU)from mid-summer to late-summer.Results show that total alkalinity(TA),dissolved inorganic carbon(DIC),AOU and nitrate are low in the surface water and high in the bottom water.In contrast,flarag and pH are high in surface water and low in bottom water.87%of the surface water are characterized by ?arag>1.5,96%of the surface water are pH>8.1,62%of the bottom water are Qarag<1.5,and 71%of the bottom water are pH<8.1.A more striking finding is that from mid-summer to late summer,TA exhibits a decreasing trend in the whole Chukchi Sea.In the southern Chukchi Sea,DIC and nitrate exhibit an increasing trend while Qarag,pH and net community production exhibit a decreasing trend;in the northern Chukchi Sea,DIC and nitrate exhibit a decreasing trend while ?arag,pH and net community production exhibit an increasing trend.In order to better understand the controlling mechanisms of acidification during warm season(i.e.,when ice melt takes place)in the Chukchi Sea,we first developed a semi-quantitative analysis of the relationship between the field data of carbonate system with dilution,photosynthesis/respiration,and precipitation/dissolution of calcium carbonate.The results show that community respiration/photosynthesis is the main cause of acidification during the warm season.we then applied a sensitivity analysis of temperature and salinity,and estimation of sea-air CO2 exchange flux to quantitatively evaluate the contribution of various factors to ocean acidification.The results show that photosynthesis/respiration contributes 63 ± 45%to the change of ?arag and 80±57%to that of pH during the warm season.The processes of sea-air CO2 exchange,changes in temperature and salinity account for within 18%,16%and 8%,respectively.In summary,we proposes for the first time that controlled by sea ice changes,completely different carbon cycles and acidification processes have occurred in the northern and southern Chukchi Sea:1)In the Southern Chukchi Sea,with the longer ice-free period,nitrogen limitation has caused the transition of the Southern Chukchi Sea from net autotrophic state to net heterotrophic state,which may be the main reason causing ?arag and pH to decrease.In the Northern Chukchi Sea,due to the low ?arag and pH in the Pacific Winter Water,and as the Northern Chukchi Sea transform from the "partial sea ice-covered state" to "ice-free state",the light limitation on phytoplankton growth is gradually mitigated.The above factors may be the main reasons causing ?arag and pH to increase.Based on field data of eight cruises from 2002 to 201 8,we find that the interannual decline rates of ?arag of Alaska Coastal Water,Seasonal Melt Water,Chukchi Summer Water and Pacific Winter Water are-0.0670 ± 0.0186 yr-1,-0.0640 ± 0.0175 yr-1,-0.0601 ± 0.0108 yr-1 and 0.0373 ± 0.0093 yr-1,respectively;and the interannual decline rate of pH are-0.0153 ± 0.0049 yr-1,-0.0158 ± 0.0047 yr-1,-0.0185 ± 0.0044 yr-1 and-0.0170 ± 0.0047 yr-1,respectively.Combining the field data with the theoretical relationship of respiration/photosynthesis,we suggest that the accumulation of CO2 in the water caused by the enhancement of net community respiration is the main factor leading to the interannual acidification of the Chukchi Sea.And the process of oxygen consumption and acidification in the Pacific Winter Water is dominated by the complete and incomplete mineralization of organic matter.In addition,the characteristics of low pCO2 of seawater covered by sea ice may be the reason for the deviation of the field data from the theoretical relationship of respiration/photosynthesis.In the context of global climate change,as the increasing melting of sea ice,the dissolved oxygen and ?arag of the Pacific winter water may be further reduced,which will have a potential impact on the local ecosystem.