Biogeochemical Cycle Of Silicon And Its Stable Isotopes In Estuaries And Coastal Waters

Estuaries and coastal waters harbor high primary productivity,thus playing a key role in the oceanic carbon cycle.Diatoms account for approximately half of the primary production and up to 40%of carbon export in coastal waters,making the biogeochemical cycle of silicon(Si)crucial to the marine ecosystem and carbon cycle.In the recent decades,the Si cycle in coastal systems has been extensively perturbed by human activities.Substantial DSi decrease in some regions has led to serious awareness of a community structure shift from diatoms to harmful algae,thus revealing how and to what degree the coastal Si cycle responds to environmental changes caused by human activities is of great significance in marine ecosystem study.However,previous studies have shown large spatial and temporal variability of the Si cycle pattern in coastal waters,hampering prediction of future changes in the Si cycle under intensified anthropogenic pressure and global warming.In the marine environment,Si isotope fractionation occurs during the uptake of dissolve silica(DSi)by diatoms to form their siliceous frustule(Biogenic Silica,BSi)through preferential uptake of lighter Si isotopes over heavier ones and its subsequent dissolution releasing primarily lighter Si isotopes.Isotopic fractionation would also occur during formation of authigenic clay minerals in sediments between the precipitates and the pore waters.Therefore,stable Si isotopes(?30Si)are a powerful tool for identifying Si sources and tracking Si processes over various temporal and spatial scales.On the basis of field data of stable Si isotopes including ?30Si of DSi(?30SiDSi)and BSi(?30SiBSi)in different estuarine and coastal systems,combined with culture experiments and various model calculations,this dissertation investigates the behaviors of Si and its stable isotopes and their controlling mechanisms in coastal waters,in order to i)compare the peculiarities of different systems and reveal the feature of coastal Si cycling and ii)understand how Si cycling responds to anthropogenic pressure and global warming for better uture prediction.Firstly,estuarine systems are of key importance for the riverine input of Si to the ocean,which acts as a limiting factor of diatom productivity in coastal waters.This study presents a field data set of ?30SiDSi in surface water obtained in the estuaries of three world's largest rivers,the Amazon(ARE),Yangtze(YRE),and Pearl(PRE),which cover different climate zones.While ?30SiDSi behaves conservatively in the YRE and PRE supporting a dominant control by physical mixing,significantly increased?30SiDSi signatures resulting from utilization of DSi by diatoms are observed in the ARE and reflect Si isotope enrichment factors 30? of-1.0±0.4 ‰(Rayleigh model)or-1.6±0.4 ‰(steady state model),which modify the ?30SiDSi values of the DSi delivered to the ocean by +0.7-0.8 ‰.?30SiDSi of the river water endmember is heavier in the YRE(+1.8±0.2 ‰)than in the ARE(+1.2±0.2 ‰)and PRE(+1.4±0.2 ‰),presumably due to stronger fractionation in the Yangtze River catchment induced by a combined effect of weathering,biological utilization,and anthropogenic activities,as suggested previously.In addition,seasonal variability of Si isotope behavior in the YRE is observed,most likely resulting from changes in temperature,light level and water residence time.Based on the 30? value obtained for the ARE,we estimate that global estuaries modify the ?30SiDSi values of the DSi delivered to the ocean by+0.2-0.3 ‰.Secondly,coastal upwelling systems are thought to harbor high diatom productivity and high efficiency of the silica pump.However,the diatom frustule BSi dissolution to production ratio(D:P)in euphotic zones,essential in characterizing the silica pump efficiency has been observed to be largely variable in different coastal upwelling system,spanning a low of 0.1 to a high end of 1.In this study we investigated the distribution of ?30SiDSi and ?30SiBSi on a cruise to the coastal upwelling system off Hainan Island conducted in August 2012,to explore the silica pump efficiency in this upwelling system.A two end-member mixing model and two fractionation models were used to calculate ??30SiDSi*,which is defined as the difference between observed fractionation and theoretical fractionation when there is no dissolution(i.e.,D:P=0,net enrichment factor 30?net=-1.1‰).Given that any increase in BSi D:P would lower the net fractionation(30?net)of ?30Si during diatom growth,the more negative??30SiDSi*value indicates higher BSi D:P,i.e.lower silica pump efficiency.Silica pump was strong in the nearshore upwelling waters and weak in the subsurface water(3060m)of the downstream upwelling circulation.We hypothesize that such difference is caused by the difference in diatom growth stages at different locations,due to longer seawater residence time downstream than in the nearshore upwelling waters.To test these hypotheses,we conducted a series of culture experiments on BSi D:P variations during diatom growth,including a mesocosm study in eutrophic subtropical settings and an on-deck incubation in the SCS.?30SiDSi in all cultures gradually increased during DSi consumption period.Net fractionations in all cultures(30?net =-0.33±0.06,0.33±0.04,-0.39±0.11 and-0.77±0.34 ‰)were smaller than the theoretical fractionation(30?upt=-1.1±0.4 ‰)during diatom production,reflecting the influence of diatom dissolution and the isotopic fractionation during this process.We employed a new fractionation model with variable 30?nct assuming linearly increasing BSi D:P ratio during the DSi consumption phase in the culture experiments.In all cultures,model results fit well all observed ?30SiDSi values verifying that BSi D:P ratio increases along with the diatom growth and DSi consumption,which indicates that silica pump efficiency changes during different diatom growth stages.Lastly,Si cycling in coastal seas has been strongly influenced by human activities in the past decades,which in turn alters primary production and nutrient export efficiency to the open ocean.In this study we present a field investigation conducted in March 2016 on the distribution of stable Si isotope of dissolved silica(?30SiDSi)in a highly eutrophic coastal system,the Baltic Sea.Under the influence of both physical mixing and different stages in the spring diatom growth,?30SiDSi values in the mixed layer decreased gradually from the shallow straits in the west(?+2.2‰)to the deep central basin in the east(?+1.4 ‰).An uncommon vertical distribution pattern of DSi was observed with heavier ?30SiDSi in the deep waters(+1.57 to+1.95 ‰)than in the surface waters(+1.24 to+1.68 ‰)in the central basin.A box model was used to simulate variations in DSi concentration and ?30SiDSi values and examine the impact of different human disturbances including damming,eutrophication and stratification,on the Si dynamics.Based on this model,we show that although damming had an impact on the ?30SiDSi values,the uncommon vertical distribution pattern of ?30SiDSi was mainly attributed to the strong internal recycling of Si driven by eutrophication.Intensive diatom production exported highly fractionated Si from the surface to deep waters,and subsequently,strong benthic recycling brought heavier ?30SiDSi back into the deep water due to the fractionation during authigenic clay formation.In summary,the biogeochemical behaviors of Si and its isotopes displayed high spatial variabilities,reflecting to some extent changing efficiency of the silica pump among different regions,which is related to a variety of factors such as the nutrient supply,water body residence time,sediment-water interface process and diatom growth stage.This dissertation shows that the systematic modification of riverine Si isotopic compositions during estuarine mixing,as well as the seasonality of Si isotope dynamics in a single estuary,need to be taken into account for better constraining the role of large river estuaries in the oceanic Si cycle.Our findings also provide solid evidence on the response of Si cycling to environmental changes and important implications for future development of the Si cycle in coastal systems.However,due to the large spatial and temporal variability of silica pump efficiency revealed in this dissertation,further investigations on the Si cycle in different regions are necessary for understanding the role of Si in the biological pump and its coupling with carbon,and also crucial for accurately predicting the behavior of the Si cycle in estuaries and coastal waters under human disturbance and global warming.