Biogeochemical Processes Of Dissolved Selenium And Isotopes In The Coastal Areas

Trace elements serve important roles in the marine ecosystem,which is the main focus areas for the marine science and global environmental change research.Moreover,the non-traditional isotopes are a front field of isotope research in the world.Selenium exhibits dual beneficial and toxic effects on organisms,and it is also bioaccumalted along the food chain.The studies of the distribution of selenium in the seawater and its behavior along the food chain,are important for understanding biogeochemical cycles of selenium and its impact on the marine ecosystems.In this study,in order to explore the biogeochemical cycles of selenium in the coastal waters of China,the spatial and temporal distribution of dissolved inorganic selenium concentration and isotopic compositions were investigated in the Changjiang Estuary,East China Sea and Sanggou Bay in Yellow Sea by multiple field observations,as well as the selenium isotopes along the food chain.Stable selenium isotopes are potential geochemical tracers with great isotopes fractionations,while selenium isotopic composition in seawater has not been measured directly.An accurate method for the robust and precise determination of selenium iso-topic composition in seawater was developed in this study for the first time.The new thiol resins were used to preconcentrate and separate Se(IV)from the seawater matrix,and a hydride generator combined with a desolvation device was coupled to multiple collector inductively coupled mass spectrometry to measure Se isotopes precisely.The precisions for ?82/78Se,?82/77Se,and ?82/76Se were measured to be 0.07‰,0.10‰,and 0.16‰(2sd),respectively,which were better than previously reported values.The?82/76Se values of standard solutions agreed well with results obtained by other interna-tional research groups.No detectable Se isotopic fractionation of spiked seawater sam-ples was observed during the preconcentration and separation procedure.In the Changjiang Estuary,dissolved inorganic selenium(DISe = Se(?)+ Se(?))concentrations ranged from 0.35 nmol/L to 4.39 nmol/L in summer(July 2011)and 0.49 nmol/L to 4.75 nmol/L in winter(March 2012),with Se(VI)being the predominant species.The DISe concentrations was high inside the river mouth and decreased grad-ually outside the river mouth.Moreover,the DISe concentration distributions in the Changjiang Estuary were controlled by three water masses,each with a specific Se(IV)/Se(VI)ratio signature:the Changjiang Water input,the Taiwan Warm Current,and the Yellow Sea Coastal Current.The data showed that DISe,Se(?)and Se(?)concentrations in the estuary behaved strictly conservatively during winter but non-conservatively during summer.During the estuarine mixing process,non-conservative processes of DISe resulted from preferential adsorption of Se(IV)by suspended partic-ulate matter and assimilation by phytoplankton.In the East China Sea,the average ?82/76DISe was 0.82‰(ranges:-1.14‰ to 1.66‰)in August 2013,which was first data sets of selenium isotopic composition in seawater.DISe concentrations ranged from 0.30 nmol/L to 2.3 nmol/L in spring(May 2011)and 0.26 nmol/L to 2.2 nmol/L in summer(August 2013),with Se(VI)being the predominant species.Spring DISe concentrations was comparable with those in sum-mer,and the Se(IV)concentration was lower in summer.The distribution of DISe in the East China Sea is regulated by the Changjiang Diluted Water,the Kuroshio Water and Taiwan Strait Warm Water,and high concentration of DISe was found along the coast,and decreased across the shelf in both spring and summer.The preliminary box model indicated that the main source of DISe in East China Sea was incursion of the Kuroshio Water and Taiwan Strait Warm Water,which contributed nearly 90%of the DISe input.The ?82/76DISe decreased from 1.59‰ to 0.4‰ with depth in the Okinawa Trough.Heavier 882/76DISe values in the euphotic zone clearly suggest that phytoplank-ton preferentially accumulate the lighter Se isotopes,and the change in ?82/76DISe value over the depth range is probably mainly caused by the conversion of particulate organic selenide to dissolved organic selenide,with the lighter isotopes being released into the dissolved phase.Negative values was found in the near bottom water(-1.66‰)in Changjiang Estuary which suffered from hypoxia,and the offset were 1 to%2‰ indi-cating biotic reduction of Se(?).In the Sanggou Bay,DISe concentration ranged from 0.21 nmol/L to 1.36 nmol/L in the surface water of Sanggou Bay,with Se(?)being the predominant species,in April,July and October 2013.The DISe and Se(?)concentrations varied temporally,with lows in summer and highs in spring and autumn.Concentrations showed strong horizontal gradients from the coast to offshore areas within the Bay,as significantly influenced by the intensive and widespread seaweeds and bivalves aquaculture activity in the Bay.The main source of DISe in Sanggou Bay was water exchange with the Yellow Sea,followed by ground water,whereas the most important sink was the strong biological activities,which removed 53 ± 12%of the incoming selenium from bay waters.The total dissolved selenium(TDSe = DISe + DOSe)isotopic ratio(?82/76TDSe)ranged from 0.38‰ to 1.06‰,while the ?82/76DISe was relatively negative,with ranges of-2.77‰ to-2.05‰ in the surface water of Sanggou Bay.The Se isotopes varied from-3.26‰ to 3.46‰ in the biological samples,and the lowest ?82/76Se values was observed in fish(-1.11±2.02‰),followed by zootoplankton(0.76±0.17‰),phytoplankton(0.80±0.10‰),oyster(1.24±0.30‰),scallops(1.67± 0.23‰),Gracilaria lemanei-formis(1.88±0.21‰)and kelp(3.19±0.22‰).Without the artificial bait,the seaweeds and phytoplankton Se isotope ratios are heavier than ?82/76DISe of the water,being en-riched in the heavier isotopes.The ?82/76Se values of bivalves was also enriched in heav-ier isotopes compared to seaweeds and phytoplankton.The ?82/76Se values of fish mus-cles increased with the increase of selenium content and ?15N values,indicated be en-riched in heavier isotopes.Based on the studies of the Changjiang Estaury,East China Sea and Sanggou Bay,the dissolved selenium was influenced by physical water mass mixing,utilization by phytoplankton,adsorption by the suspended particles,the oxidative-reductive condi-tions and etc.,in the coastal areas of China,and the selenium enriched the heavier iso-topes along the food chain.This study provides a basis for selenium isotopes as effec-tive geochemical tracers.