Distribution,Migration And Transformation Of Dissolved Selenium In The Tropical East Indian Ocean And The South China Sea

Trace elements play an important role in the dynamics of marine ecosystems and carbon cycles.Most research on trace elements focus mainly on the marine science and global environmental changes.Selenium(Se),a typical trace element,is an essential micronutrient for lives,and its concentration may affect the efficiency of biological pump in the oceans.Se deficiency affects the growth of phytoplankton,which may have lead to several extinctions triggered during evolution.Therefore,the distribution and chemical composition of dissolved Se in the oceans and biogeochemical transformations of Se are of great significance to the functioning of marine ecosystems and the protection of environmental changes.In this research,the tropical East Indian Ocean(EIO)and South China Sea(SCS)are selected as the study areas.Based on the concentrations and chemical composition of Se,the temporal and spatial variation,along with the biogeochemical transformation within the marginal sea from the shelf to the open ocean were statistically analyzed.In addition,this thesis summarizes and analyzes the data of dissolved Se in the global oceans,attempting to describe the biogeochemical behavior of Se in the ocean in relation to the global ocean cycle.During the surveys which were conducted during March and May 2018,there were evident spatial differences in the concentrations of dissolved Se and its valences in surface waters collected from the Pearl River Estuary(PRE)to the Malacca Straits through the SCS.The concentration of total dissolved Se(TDSe)in the surface water varied from 0.33 nmol/L to 6.01 nmol/L(average value:1.09 nmol/L,n=67).Selenite[Se(IV),0.14?3.44 nmol/L,average value:1.33 nmol/L,n=11]was the dominant form in the PRE.Dissolved organic selenide(DOSe,0.05?0.79 nmol/L,average value:0.45nmol/L,n=25)was the dominant form in the Malacca Straits.These differences in the dominant valence states are related to the terrestrial inputs and anthropogenic activities influencing the two places.The PRE is affected by the strong industrial activities and Se(IV)results from the injection of surface loadings.In the Malacca Straits,due to the influence of the surrounding peat-draining rivers and agricultural activities,DOSe is the main form.In the surface water of the SCS,DOSe is the most dominant from.According to the surface circulation patterns of the SCS in different sampling periods,the SCS can be divided into the northern and southern parts.The Se(IV)concentrations were low in both the regions,which is related to the preferential utilization of Se(IV)by plankton.The lower DOSe concentrations found in the southern SCS may be related to a higher level of primary production and utilization.During the May 2018 sampling campaign,the concentrationof DOSe in the southern SCS,was significantly higher than the March observations,which may be related to the terrestrial inputs and the reversal of surface current after the transition of the monsoon.The dominant Se valences in the surface waters of the PRE and the Malacca Straits are mainly affected by different terrestrial inputs,while the Se valence in the surface waters of the southern and northern SCS are mainly affected by biological activities.During the surveys conducted in 2017 and 2018,the TDSe concentration in surface water of the tropical EIO ranges from 0.36 to 1.33 nmol/L(average value:0.88 nmol/L,n=113).DOSe was the dominant form,accounting for 52%of TDSe,while Se(VI)accounts for 41.5%of TDSe.The Se(IV)was the lowest(6.5%)as it was the strongly bioavailabe for phytoplankton.The spatial variation in the chemical composition of Se was significant.The Se(VI)concentration in the oligotrophic equatorial Indian Ocean was slightly higher than that of the west coast of Sumatra and the Bay of Bengal,while the DOSe concentration in the Bay of Bengal were higher than that of the oligotrophic equatorial Indian Ocean.According to the model calculation of Se(IV)_P^*,in 2018,the Se(IV)_P^*of the oligotrophic equatorial Indian Ocean was negative,indicating that surface values of Se(IV)was not enough to support the biological utilization of dissolved inorganic phosphate(DIP),and that there may experience a Se(IV)limitation of some degree.Using the global Se data of the surface waters of oceans from published literature,it can be ascertained that DOSe was the dominant form in the seas of the low-latitudes,while Se(VI)was the main present in the seas of high-latitudes in both the northern and southern hemispheres.These differences may be related to the assimilation and utilization of Se by plankton and the variations in dominant species found in different latitudes.During the surveys conducted during 2017 and 2018,the concentrations of TDSe,DOSe,Se(IV)and Se(VI)in the water column of the tropical EIO ranged from 0.36 to2.57 nmol/L(average value:1.35 nmol/L,n=395),0.01 to 1.68 nmol/L(average value:0.43 nmol/L,n=395),0.01 to 0.93 nmol/L(average value:0.32 nmol/L,n=395)and0.14 to 1.21 nmol/L(average value:0.60 nmol/L,n=395),respectively.The measured Se(IV)and Se(VI)showed a net consumption in the photic zone,and then gradually increased along with the depth,which was similar to the vertical distribution of nutrients.While DOSe is mainly found in the upper waters.Se(IV)and Se(VI)measured in water(<1200 m depth)are highly correlated with nitrate(NO₃^-),DIP and dissolved silicate(DSi),indicating the both Se(IV)and Se(VI)uptake by plankton is likely coupled with the cycling of NO₃^-,DIP and DSi.However,the ratios of Se(IV)and Se(VI)to NO₃^-,DIP,DSi in deep water(>1200 m depth)are basically constant,indicating the regeneration of Se(IV),Se(VI)and NO₃^-,DIP,DSi are according to a certain proportion.In the oxygen minimum zone(OMZ),the AOU concentrationsare significantly higher than those in the oxygen-rich water.According to the model estimation,the actual concentrations of Se(IV)and Se(VI)are 16?91%and 6?60%less than the theoretical concentrations in the OMZ,respectively.Therefore,microbial reduction of Se(IV)and Se(VI)to particulate Se⁰ might occur in the OMZ.In the deep water,the circumpolar deep water(CDW)enters the Indian Ocean,thereby the Se(IV)content gradually decreases(?0.8 nmol/L to?0.6 nmol/L)and Se(VI)gradually increases(?0.75 nmol/L to?0.95 nmol/L),and the decrease is basically equal to the increase,indicating that Se(IV)is gradually oxidized to Se(VI)during the deep-water movement.Global data also show that Se(VI)is the main form in deep-water.Along the ocean conveyor belt,the deep water receives a lot of biogenic particles from the upper layer,which subsequently degraded and oxidized to Se(IV)and Se(VI)through a step by step process during long-distance transportation.The concentrations of Se(IV)and Se(VI)in deep water gradually increase by 1.5 times and 3.2 times in the Atlantic Ocean from the north to south,1.6 times and 3.1 times in the Indian Ocean from the south to north.On the whole,the concentrations of Se(IV)and Se(VI)increased by 3.5 times from North Atlantic to Northeast Pacific.The studies conducted in the SCS and the EIO showed that the concentration and chemical format of Se in the surface water of PRE and Malacca Straits were impacted by terrestrial sources.However,the valence distribution of Se in the surface waters of open oceans is mainly controlled by biological activities.The concentration and chemical format of Se in the middle and deep waters of the tropical EIO are affected by the degradation of biogenic particles,dissolved oxygen content and circulation currents of the deep water.These results enhance our understanding of Se biogeochemistry and cycling in oceans.