| Iron(Fe)is the fourth most abundant element in the Earth's crust,ranking after oxygen,silicon,and aluminum.Working as an essential micronutrient for all known living organisms,iron is widely existed in diverse biological enzyme systems,participating in metabolic processes such as photosynthesis,respiration,and nitrogen fixation.Iron is established as a key elemental resource in shaping the magnitude and dynamics of ocean primary production,and is recognized to exert influence on the global climate change.Although the marginal sea only accounts for a small proportion of the global ocean,it is of great significance to the global ocean primary production and carbon cycle,undoubtedly,iron plays an important role therein.The near-shore area is both densely populated and ecologically active,subjected to the impacts from both natural processes and human activities,thus the biogeochemical processes of iron will be affected by multiple factors under such an environment,but the impact intensity varies in different regions.Therefore,research on the biogeochemical processes of iron in diverse water bodies under different interactions between natural processes and human activities has important implications for enhancing the understanding of the iron cycle and carbon cycle in the near-shore ecosystems.In this thesis,the human high density aquacultural activities influenced Sanggou Bay(a bay of the Yellow Sea),the natural and human influenced Changjiang Estuary and East China Sea were selected as the research objects.Several field investigations were conducted and a batch of high quality dissolved iron(dFe)data were obtained.Based on these data,the spatial and temporal distribution pattern as well as the migration and transformation of dFe concentrations in the study areas were investigated,and the biogeochemical characteristics of iron in the typical water systems of Chinese coastal areas were sketchily outlined.Additionally,the published dFe concentration data of the worldwide aquaculture areas,river estuary areas,and continental shelf/marginal seas in literatures were compiled and analyzed,so as to heighten the understanding of the biogeochemical cycle of dFe in coastal waters from a global perspective.The main findings of this thesis are listed as follows:The range(average)of the dFe concentration in the spring,summer,autumn,and winter in the surface water of Sanggou Bay were 2.89-25.3(12.0±6.29)nM,2.55-14.0(5.00±2.92)nM,1.12-2.98(1.83±0.42)nM,and 1.42-9.33(3.36±2.06)nM,respectively,with significant difference(p<0.05)between seasons.The dFe budget calculation results showed that the input flux of dFe contributed from atmospheric deposition,bottom sediment,groundwater input,and riverine input to the Sanggou Bay were(8.23±5.28)×10~3 kg/yr,788 kg/yr,554±336 kg/yr,and 213±168 kg/yr,respectively,each accounting for 84.1%,8.06%,5.66%,and 2.18%of the total input,respectively.The harvest of the cultured species,exchange with the Yellow Sea,burial of the fixed carbon by the phytoplankton primary production removed/outputted7.00×10~3 kg/yr,5.48×10~3 kg/yr,and 53.1 kg/yr of dissolved iron,respectively,each accounting for 55.9%,43.7%,and 0.424%of the total output flux,respectively.The difference between dFe source and sink indicated that Sanggou Bay worked as a net source for dissolved iron.The high dFe concentration observed in the spring was probably contributed from the atmospheric dry deposition.There was no significant difference(p>0.05)in dFe concentration between different culture areas,but the utilization of dFe by the cultured species and the subsequent harvest was the largest iron sinks in the Sanggou Bay,resulting in the decrease in dFe concentration from spring to summer.The utilization of Fe by the phytoplankton in the summer and the higher exchange rate between the Sanggou Bay and the Yellow Sea in the autumn further decreased dFe concentration in the Sanggou Bay.The mathematical relationship between dFe and dissolved inorganic phosphorus(DIP)concentrations suggested that dFe concentrations in the Sanggou Bay were sufficient to support the biological utilization of DIP in spring and summer,but dFe might be a potential limiting factor for primary production in autumn.The correlation analysis between dFe and Chal-a concentrations also revealed that dFe might stimulate but limit the growth of phytoplankton in summer and autumn,respectively.The range and average of dFe concentration in the surface water of Xuliujing(XLJ),Changjiang were 15.0-159 nM and 45.5±29.2 nM,respectively,which were low compared on a global scale.On the whole,dFe delivered by the Changjiang was significantly removed from the water during the estuarine mixing process.Significant remobilization and removal of Fe occurred in the inner(salinity?1)and outer(salinity>1)region of the mouth bar area.The averaged dFe concentration in the inner salinity?1 water of the mouth bar area was quadrupled of that at XLJ,then dFe concentration decreased significantly in the outer salinity>1 water.The results of laboratory simulation experiments showed that desorption from particles and salt induced flocculation were responsible for the remobilization and removal of Fe,respectively.The budget calculation results of dFe in the mouth bar area indicated that Changjiang runoff(at station XLJ),bottom sediment,Huangpujiang,Shanghai sewage treatment plants along Changjinag bank contributed(2.32±1.17)×10~6 kg/yr,(658±773)×10~3 kg/yr,(73.2±24.1)×10~3 kg/yr,55.0×10~3 kg/yr,and 48.5×10~3 kg/yr of dFe to the mouth bar area,respectively,each accounting for 73.5%,20.9%,2.32%,1.75%,and 1.54%of the total input flux.The difference of dFe source and sink indicated that the mouth bar area of the Changjiang Estuary was a net sink for dFe as a whole,but its inner salinity?1 region and outer salinity>1 region worked as a net source and a net sink for dFe,respectively.The concentration of dFe in the East China Sea ranged from 0.22 to 26.7 nM with an average of 4.48±4.96 nM.dFe concentration was high in the coastal region affected by the input from the Changjiang Diluted Water and anoxia process,with an average of about 9-10 nM,whereas dFe in the offshore area was generally lower than 2 nM.The range and average of dFe concentrations in the surface and bottom waters were 0.47-21.8 nM and 5.24±4.76 nM,0.49-26.71 nM and 5.79±6.11 nM,respectively.On the whole,the water mass mixing process controlled the distribution of dFe in the shelf water of the East China Sea,especially for the upper and middle waters.Bottom sediment,anoxia,and coastal upwelling exerted significant effect on dFe concentrations in the water column.The Fe:C of the particles in the water column of different area in the East China Sea was quite different,indicating that there might existed a great difference in Fe uptake by the phytoplankton inhabited in different water environments.The dFe budget calculation in the 0-200 m isobath in the East China Sea revealed that,the effective input flux of dFe from precipitation,Taiwan Strait Warm Water,bottom sediment,Kuroshio intrusion,dry deposition,riverine diluted water(after removal in the estuarine zone),submarine groundwater discharge to the East China Sea were(7.36±4.59)×10~6 kg/yr,(7.07±2.91)×10~6 kg/yr,6.63×10~6 kg/yr,(5.19±2.42)×10~6 kg/yr,(4.42±5.82)×10~6 kg/yr,(0.935±0.364)×10~6 kg/yr,and(0.025±0.017)×10~6 kg/yr,respectively,account for 23.3%,22.4%,21.0%,16.4%,14.0%,2.96%,and 0.081%of the total input flux,respectively.The output flux of dFe through the shelf break,Tsushima Strait,carbon burial of the primary production,and exchange with the Yellow Sea were(17.4±10.6)×10~6 kg/yr,(12.2±10.4)×10~6 kg/yr,1.93×10~3 kg/yr,and 78.8×10~3 kg/yr,respectively,account for 55.0%,38.6%,6.10%,and 0.249%of the total output flux,respectively.The compilation of the worldwide dFe concentration data reported in previous literatures showed that dFe concentrations in the culture area were mainly affected by the geology and soil type,while the influence of aquaculture activity on dFe concentration in the water has not been reported before as our case study in the Sanggou Bay,which also highlighted the specificity of our Sanggou Bay study.The concentration,yield,and output flux of dFe at the freshwater endmember of the compiled rivers differed by orders of magnitude.The rivers with high dFe yield and flux mainly distributed in the tropical and boreal regions with organic-rich environments.The“black water”river with limited basin area and discharge could produce comparable or higher dFe flux or dFe yield to large river systems.In most estuaries,dFe was rapidly removed from the water column within a salinity range of 0to 10-15,and mixed conservatively at higher salinity range.However,in a few case studies,near conservative mixing and addition processes were also observed in the estuarine area.According to the dFe budget calculation of the 0-200 m isobath continental shelf,bottom might be the most important source of dFe for the continental shelf water,followed by fluvial effective input and atmospheric deposition.The study of dFe concentrations in the near-shore areas of China in this thesis showed that human activities played a dominant role in the biogeochemical cycle of iron in the small Sanggou Bay,which was under high aquaculture pressure.Whereas,in the Changjiang Estuary and the East China Sea,natural processes including the estuarine modification,water mass mixing,bio-utilization,and bottom sediment,and the increasing anoxia phenomenon under pressures from human activities had significant influence on the dFe variation in the water body.In the context of human development and global climate change,more attention should be paied to the potential impact of human activities and their coupling effect with natural processes on the iron cycle in the oceans,especially in the near-shore areas. |
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