| The Taiwan Strait,located at the junction of the East China Sea and the South China Sea,is a key point for water mass exchange between these two marginal seas.The plume and upwelling in summer and the Zhe-Min coastal current in spring bring a lot of terrestrial and Marine materials.The interaction between the plume and upwelling,as well as the interaction between these water masses and the warm current across the Taiwan Strait,has an important influence on the hydrological,chemical and biological production processes in the Taiwan Strait.However,the influence of plume,upwelling and inter-annual variation of Zhe-Min coastal current on carbon biogeochemistry in the Taiwan Strait remains to be further studied.In this thesis,three traditional sections A,B and C in the Taiwan Strait,which are located in the southern sea area of the Taiwan Strait,were taken as the main research area.Absorption spectra and three-dimension fluorescence spectra were used as the technical means to characterize the changes in DOM content and properties.This thesis focuses on the influence of the distribution changes of the Pearl River plumes and the upwinging along the Dongshan coastal upwelling in the southern waters of the Taiwan Strait on DOM dynamics during 20102020,and the response of the DOM across the Taiwan Strait to the change of Zhe-Min coastal current and the frontal process during the spring of 2013-2019.The main findings are as follows:(1)The CDOM absorption coefficients aCDOM(254)and aCDOM(325)in the southern area of the Taiwan Strait in summer varied from 0.57-2.93 and 0.01-0.93 m-1,respectively,and the spectral slope S275-295 varied from 0.019-0.068 nm-1.The variation ranges of peak Ac,peak C,peak M and peak T are 0.003-0.053,0-0.024,0.00100.028 and 0.001-0.054 RU,respectively.In the summer of 2010-2020,CDOM and FDOM components in the Southern Taiwan Strait have obvious interannual changes,and the FDOM components changes more greatly.Influenced by plume distribution,CDOM parameters in 2019 and 2020 in the 3 m and 10 m layer of the far-shore area have higher concentration than that in the nearshore area.In other years,CDOM parameters basically show the characteristics of nearshore high and far-shore low.In addition to the high value in the plume area,FDOM components generally show the characteristics of high near shore and low far shore.(2)Both CDOM and FDOM components in the Pearl River plume have obvious inter-annual variation,and the variation degree of FDOM components is greater.The results of the three-endmember mixing model show that the CDOM and FDOM components in the Pearl River plume region are mainly removed,and the removal degree of FDOM component is greater.The removal amount of aCDOM(254)was higher in 2019,2016 and 2012,while the removal degree of aCDOM(325)was significantly higher in 2016,2019 and 2020,when the the Pearl River plume was more widely distributed,and the hyperspectral slope S275-295 in 2016-2020 indicated that photodegradation was the main factor.The total amount of CDOM and FDOM removal in the Pearl River plume mainly depends on the volume of the Pearl River plume.The consumption of dissolved inorganic nitrogen(DIN)in the Pearl River plume suggests a biological contribution of DOM spectral components.Therefore,the net removal of DOM spectral components in the plume suggests that the effects of microbial and photochemical degradation during plume transport exceed the contribution of biological production.(3)The characteristics of high CDOM generated by the influence of the Dongshan coastal upwelling were more pronounced in 2014 and 2016,while the influence of the Taiwan Bank upwelling was more pronounced in 2012 and 2014 with higher aCDOM(254)and aCDOM(325).The results of three-endmember mixing model show that,the phenomenon of nutrient removal and the addition of aCDOM(254),aCDOM(325)and peak C were common in the coastal upwelling area,indicating that phytoplankton production activities contributed significantly in the upwelling area.The relatively active proteinlike peak T were largely consumed by microbial degradation,but the higher biological productivity in 2019 supported the "net" addition of active peak T.The addition and removal amounts of CDOM and FDOM components in the upwelling have significant interannual changes,and the volume and biological activities of the upwelling have influences on the variation of the total DOM in the upwelling.(4)The range of CDOM absorption coefficient aCDOM(254)and aCDOM(325)in spring in the Taiwan Strait were 0.9-3.8 and 0.09-1.07 m-1,respectively,and the range of spectral slope S275-295 is 0.018-0.053 nm-1.The variation ranges of peak Ac,peak C,peak M and peak T were 0.002-0.076,0-0.05,0.0010-0.083 and 0-0.119 Ru,respectively.On the whole,CDOM and FDOM in the central and northern area of the Taiwan Strait are mainly controlled by the physical mixing between Zhe-Min coastal current and the warm water in the Taiwan Strait.However,in 2019,the addition behavior of the northern protein-like components was caused by bacterial production,while in 2013,the C peak and T peak were significantly removed in the central sea area,and photodegradation and microbial degradation may be the main influencing factors.In the southern area of the Taiwan Strait,the influence of the Kuroshio input is more significant.As the Kuroshio input is strengthened in 2019,the abundance of humic-like components is lower than that in 2013,but the protein-like components in 2013 are more removed by microbial degradation.(5)The results of frontal observation voyage of cross coastal current and strait warm water in the spring of 2019 show that the nearshore side of the survey section was occupied by the Zhe-Min coastal current with low temperature,salinity,and high CDOM,FDOM,while the outer side was dominated by the strait warm water with high temperature,salt,and low CDOM,FDOM.The abundance of humic-like components decreased with the migration of Zhe-Min coastal current,and its mixing with salinity showed a conservative behavior,and the influence of frontal process was not obvious.The addition of aCDOM(254),aCDOM(325)and peak T in the water near the front and in the bottom below the front in the Zhe-Min coastal current area may be the result of the accumulation of dead phytoplankton and the release of cells after fragmentation.This indicates that different DOM components have different response characteristics to frontal processes,and its influence on the organic carbon cycle in the Taiwan Strait is worthy of further investigation. |
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