A Preliminary Study On Distribution And Seasonal Variations Of Chromophoric Dissolved Organic Matter(CDOM) In The Bohai Sea And The North Yellow Sea

Chromophoric dissolved organic matter(CDOM)is the main component of dissolved organic matter.It has significant roles on the biogeochemical processes of ocean carbon cycles and marine environmental protection.In this paper,CDOM were studied in the surface,middle and bottom layers of seawater samples in different months of the Bohai Sea and the North Yellow Sea by UV-visible absorption spectroscopy and fluorescence excitation-emission matrix(EEM)combined with parallel factor analysis(EEMs-PARAFAC).Distribution characteristics,influencing factors,and seasonal variations of CDOM in different seasons were analyzed.The main results are summarized as follows:1.In April 2016,the CDOM absorption coefficient a355 varied from 0.537 to 1.238 m-1 with an average value of 0.773 m-1.The variation range of a355 was 0.300 to 2.350 m-1 with an average of 0.855 m-1 in August 2016.In December 2016,a355 varies from 0.688 to 1.814m-1,with an average value of 1.052 m-1.The variation range of a355 was 0.111 to 3.079 m-1 with an average of 0.606 m-1 in February 2017.2.One humic-like component and one protein-like component were identified by EEMs-PARAFAC from the Bohai Sea and the North Yellow Sea water bodies in August 2016.Three humic-like components and two protein-like components were identified in December 2016.Two humic-like components and one protein-like component were identified in February 2017.The humic-like components are all derived from terrestrial sources,and the protein-like components include various sources such as terrestrial sources,phytoplankton production,and microbial decomposition.Strong correlations were found between the humic-like component,indicating that they have similar sources and the removal pathway.The total fluorescence intensity(TFI)is positively correlated with the absorption coefficient a355.No significant correlation was found between other fluorescence parameters and UV-Vis spectral parameters and DOC concentrations.3.In different seasons,the horizontal distributions of CDOM are similar,showing the characteristics of high levels in the near shore and low levels in the offshore region,which are mainly affected by the land input.According to the vertical distribution,the distribution patterns of CDOM in April and December are relatively uniform.In summer,due to the strong photodegradation,CDOM level is the lowest in surface seawater.In February,controlled by primary productivity,the lowest level of CDOM was found in bottom seawater.4.Land-based sources mainly include riverine discharge and runoff,and they have strong influence and the most extensive scope of impact in the summer season.Primary productivity will produce CDOM,which has little effect on the horizontal distribution of CDOM.In the windy spring and winter season,the strong wave made the bottom and surface water well mixed,and thus resulted in the uniform vertical distribution of CDOM.Furthermore,the bottom sediments were resuspended and then released CDOM to the overlayered seawater.The lowest DOC and CDOM concentrations were observed in the position where the double-center cold water mass areas are in the summer.5.From April 2016 to February 2017,the content of CDOM increased firstly and then decreased,and it concentration was the highest in December.Seasonal variations are mainly controlled by land input and on-site production.According to the analysis of spectral slope S275-295,the average molecular weight of CDOM in winter was the smallest in February,which was mainly related to the drastic reduction of land-based input.The strong photodegradation in summer resulted in a small average molecular weight of CDOM.6.CDOM and DOC did not show remarkable correlation in this research.This is mainly due to the fact that both CDOM and DOC are a mixture of diversified sources and complex components,and the proportion of CDOM in DOC varies greatly,and therefore they do not show a correlation.