| Over the past 250 years,more and more anthropogenic CO2 have been released into the atmosphere,which is partially eased by oceanic uptake.However,this process causes "ocean acidification".Ocean acidification will have a serious impact on marine ecosystem.As a key parameter indicating the acidity of seawater,pH has received extensive attention from scholars around the world.Based on systematically analysis of the role of physical factors?especially temperature and salinity?,chemical composition and biological processes played on seawater pH,we propose a new pH parameter named pHin situ?25??to represent the acidity of seawater.This,we analyzed pH and other parameters of station ALOHA in the North Pacific Ocean from 1992 to 2016 and the adjacent waters of the Changjiang Estuary at all seasons from 2015 to 2016,and discussed pH seasonal changes,influencing factors and long-term acidification trends of the eutrophic adjacent waters of the Changjiang Estuary and the oligotrophic station ALOHA with correlation analysis,regression analysis and time series analysis.The main results are as follows:?1?A new parameter named "pHin situ?25??" was defined to indicate the acidity of seawater at in situ temperature,which can be calculated by the formula:pHin situ?25??=pHin situ-pK*w?in situ?/2 + pK*w?25??/2.The relationship between pHin situ?25??and temperature is opposite to the pHin situ,that is,the higher the temperature,the higher the pHin situ?25??,which is coupled to the fact that calcium carbonate saturation increases with temperature rising.Salinity is of vital importance for the absolute value of the current seawater pH which ranges from 7.5 to 8.3.pHin situ is the negative logarithm of the absolute concentration of hydrogen ions in seawater.The pH of water decreases significantly with increasing temperature,and water remains neutral.Therefore,pHin situ can not indicate the acidity of seawater at in situ temperature.pHin situ?25??is a parameter normalize the hydrogen ion concentration at in situ temperature to 25?,it can indicate the effect of temperature on the acidity of seawater.The temperature coefficient of pHin situ is approximately-0.0127 pH/?,the one of pHin situ?25??is about+0.0032 pH/°C.This completely changed previous understanding of the effect of temperature on seawater pH or acidity.As the temperature increases,the seawater is more alkaline,and the effect of temperature on pHin situ?25??is reduced to a quarter of the effect of temperature on pHin situ.Although salinity is a conservative parameter,it also has an important influence on seawater pH.The experimental and calculation results show that when the sea surface salinity is 0 and the composition of the carbonate system does not change,the pH will be as high as 9.4,which reveals the significant role of salinity played on seawater pH.?2?Seasonal change characteristics of surface pHin situ?25??of the high-productivity Changjiang Estuary adjacent waters and low-productivity surface layer water at station ALOHA is coincident,which are the highest in summer and lowest in winter.It is mainly related to the exchange of sea-gas CO2,the seasonal growth of phytoplankton and the effect of temperature on seawater acidity.Surface pHin situ of Changjiang Estuary adjacent waters is higher in summer and lower in winter;while it is on the contrary in the low-productivity sea area.Seasonal change of surface pHin sit,in the high productivity sea area is relative to the seasonal variation of concentration of Chi a,while surface pHin situ in the low-productivity sea area is significantly affected by temperature.Surface pHin situ?25??of the waters adjacent to the Changjiang Estuary was 7.81± 0.15 in spring,8.18 ± 0.14 in summer,7.83 ± 0.09 in autumn and 7.79±0.09 in winter,which was higher in summer and lower in winter.pHin situ?25??of the surface water of station ALOHA reached the highest value in summer and the lowest in winter.Lower CO2 solubility at higher temperature and biological drawdowm effect on pCO2 lead to lower concentration of carbon dioxide in summer.Moreover,higher temperature means higher pHin situ25?).All of these processes lead to higher pHin situ?25??in summer than that in winter whether it is high-productivity sea area or low-productivity sea area.Surface pHin situ of the waters adjacent to the Changjiang Estuary is higher in summer and lower in winter,and it is the opposite at station ALOHA.The seasonal change of pHin situ in the adjacent waters of the Changjiang Estuary is controlled by the seasonal growth of phytoplankton because of its high production.While the seasonal change of surface pHin situ at station ALOHA depends more on seasonal temperature changes,which will affect the dissociation constant of seawater carbonate and then change pH.?3?The increasing salinity of the surface water at station ALOHA accelerates the pH reducing,while the increasing sea surface temperature slightly eases the acidification at station ALOHA.The pH reduction rate of subsurface water?235-265 m?at station ALOHA was significantly higher than surface layer.It is mainly caused by the higher increase rate of CO2 and the weaker buffering capacity.From 1992 to 2016,the sea surface temperature of station ALOHA increased at a rate of 0.013?/yr,and the salinity increased at a rate of 0.010/yr.The increase in salinity contributed approximately 9.1%to the decrease of pHm situ?25??,and the contribution of temperature was-0.2%.In the past 30 years,the subsurface?235-265 m?pH?25??of station ALOHA has been reduced at a rate of-0.0027 pH/yr,which is about twice as much as the pH decrease rate of surface water.The higher acidification rate of subsurface seawater is related to its higher CO2 increase rate and weaker buffer capacity.The rate of increase of nDIC?nDIC=35DIC/Salinity?in subsurface seawater is about 1.61?mol/kg/yr,compared to 1.08 ?mol/kg/yr in surface water.The nDIC average value of subsurface seawater was 2082.8 ?mol/kg.compared to 1978.8?mol/kg in surface layer.The higher nDIC means higher CO2 and weaker buffer capacity. |
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