Interannual Variations In Carbonate System In The Pearl River Estuary

Rivers and estuaries are the primary link between lands and ocean,which are also the most dynamic areas with complex physical and biogeochemical processes.We studied the carbonate system in a subtropical large river estuary(the Pearl River estuary,PRE)under the influences of biogeochemical processes,including organic carbon oxygenation,nitrification,CaCO3 dissolution,etc.Additionally,the inter-annual variations of the carbonate system between now and decade ago is also revealed.In January of 2019,we conducted a cruise to the PRE.Carbonate system parameters,dissolved oxygen(DO),nutrients and[Ca2+]were measured.In addition,incubations on bulk oxygen consumption and nitrification rates were also conducted.Dissolved inorganic carbon(DIC)and total alkalinity(TA)were>2000 ?mol kg-1 in the upper reaches of PRE near Guangzhou,and decreased with salinity in the upper eatuary(salinity<5),while they increased with salinity in the area of salinity>5.At the seawater endmember,DIC and TA were 2011 ?mol kg-1 and 2221?mol kg-1.The low DIC and TA values at salinity?5 were attributed to the mixing of low-carbonate East River.In the Guangzhou section of the estuary,DIC and TA showed an addition of 699.3 and 546.2 ?mol kg-1 compared to the source North River water.Ammonia and nitrate concentrations were>150 and>400 ?mol kg-1 in the upper estuary.Although they generally decreased with salinity,nitrate concentration in the area of salinity<5 was scatter and relatively high,suggesting nitrification.The Guangzhou section was taken as a case to estimate the influences of the biogeochemical processes on the carbonate system parameters in the water.Organic carbon oxidation,CaCO3 dissolution,sediment release and CO2 degassing to the atmosphere contributed 132.5,159.9,491.3-912.3,and-130.5 to-536.7 ?mol kg-1 to the DIC concentration.CaCO3 dissolution and sediment release contributed 319.8 and 457.2-849.2 ?mol kg-1 to the TA.Nitrification decreases TA by 57.7 ?mol kg-1.The above biogeochemical processes contribute 653.2-668.0 and 759.5-1111.5 ?mol kg-1 to DIC and TA,which were generally consistent with the measured DIC and TA addition.These biogeochemical processes also influenced pH25 of PRE water.Organic carbon oxidation and nitrification decreased pH25 by 0.228 and 0.132,while CaCO3 dissolution,CO2 degassing to the atmosphere and sediment release increased pH25 by 0.769,0.164-0.477 and 0.003-0.019,suggesting that CaCO3 dissolution and CO2 degassing are important processes buffering acidification in the upper PRE.Compared with January 2005,both DIC and TA in the upper estuary decreased by?1000 ?mol kg-1 in 2019.The ammonia concentration decreased from>800 ?mol kg-1 to?150 ?mol kg-1.The DIC and TA addition by biogeochemical processes decreased from 1039.9 and 1146.3?mol kg-1 in January of 2005 to 699.3 and 546.2?mol kg-1 in in January of 2019.Although the bulk water oxygen consumption rates were similar in January of 2005 and 2019(42.2-55.7 and 49.5-54.4 ?mol O2 L-1 d-1),nitrification rate doubled from 4.2-5.4 ?mol L-1 d-1 in 2005 to 11.5-12.4 ?mol L-1 d-1 in 2019.Comparison of the influences of biogeochemical processes showed that the decrease in CaCO3 dissolution(778.9 to 159.9?mol kg-1)was the major driving force of the lower DIC and TA addition in 2019 compared to 2005.