| Connecting between land and ocean,estuaries receive both terrestrial and marine substances,which are subject to a series of biogeochemical processes.Generally,carbonates here shows geochemically dynamic due to continuous terrestrial inputs,and further organic degradation,while trace metals varied in concontraions along with particle dissolution/precipitation or adsorption/desorption.Jiulong River,is a medium-sized subtrophical river among the world's rivers,and severely suffers from anthropogenic activities locally.In 2017-2018,eight cruises were conducted by Fujian Provincial Key Laboratory of Sea-Land Interface Eco-Environment.A systematic set of samples were taken from the whole Jiulong River Estuary and Xiamen Bay,and later on analyzed for dissolved inorganic carbon(DIC),total alkalinity(TAlk),pH,salinity,dissolved oxygen(DO)and trace metals.The results are as follows(1)The carbonate system shows differently in space and seasons in Jiulong River Estuary and Xiamen Bay.pHT and aragonite saturation(?arag)are increasing gradually with increasing salility.The pH and ? in Jiulong River estuary are lower than those in Xiamen Bay.The pHT and ?arag range:6.89?8.06 and 0.46?1.28(salinity>15)in the estuary;pHT and ?arag range:7.66?8.37 and 0.73?3.36 in the bay,with little seasonal difference.The partial pressure of carbon dioxide(pCO2)shows higher at the upper estuary(as high as 6500 ?atm),while lower in the bay(140 to 1000 ?atm).Besides,elevated levels of pCO2 were observed locally,e.g.,at the upper estuary,Xiamen West bay,and Tong'an Bay,which is consistent with low DO thereTAlk ranges:603?1987 ?mol kg-1 in the estuary and 1600?2304 ?mol kg-1 in the bay.DIC ranges 750?1914 ?mol kg-1 in the estuary and 1565?2193 ?mol kg-1 in the bay.The riverine average fluxes of DIC and TAlk from 2017 to 2018 were 7.6×109 mol yr-1 and 6.3×109 mol yr-1,respectively,and the average flux of inorganic carbon was approximately 9.12×104t C yr-1(2)Dissolved trace metals decreased gradually with increasing salinity.The high correlations of each metal with salinity show that these metals' concentrations were mainly controlled by seawater dilution during estuarine mixing of fresh water and seawater,and partially by particle adsorption/desorption.Besides,laboratory experiments further show that H+concentration(acidification)is critical in controlling the dynamics of dissolved metals.In summary,the study shows that dissolved metals interact with particles geochemically as the following mechanism:in the upper estuary(salinity:0?10,H+>9×10-8;pH<7.05):trace metals could be released during dissolution of particles like manganese oxides;particles in the middle estuary(salinity:10?20,H+:2.9?9×10-8;pH=7.05?7.55),dissolved metals remain relatively constant although calcium carbonates are subject to dissolvate;in the lower estuary and Xiamen Bay(salinity:20?32,H+<2.9×10-8;pH>7.55),dissolved metals are partially released due to particle desorption besides seawater dilution under the condition of higher H+. |
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