Distribution And Optical Characteristics Of Dissolved Organic Matter In Summer In The Yellow And Bohai Seas

Dissolved organic matter(DOM)is the largest organic carbon reservoir in the ocean and plays an important role in the marine carbon cycle,which is of great significance to the global carbon cycle and climate change.Based on the investigation of the DOM in August-September 2017,the spatial distribution characteristics of dissolved organic carbon(DOC),chromophoric dissolved organic matter(CDOM)and fluorescent dissolved organic matter(FDOM)were studied in the Yellow Sea and Bohai Sea.The main results obtained are summarized as follows:1.From the distribution characteristics of DOC and CDOM,both of them are near-shore high and low-shore,with the highest concentration in the Bohai Sea,the second in the North Yellow Sea and the lowest in the South Yellow Sea.The high-value areas aPPear in the Yellow River estuary and Liaodong Bay mouth.The high-value area of the Yellow River estuary coincides with the low salinity area,indicating that the river input has a significant impact on the area.The difference between DOC and CDOM in the bottom water of Bohai Sea was very small,which is significantly lower than that in the South and North Yellow Sea.This may be related to the shallower and more vertical mixing effect of the Bohai Sea.In addition,in the Yellow Sea and Bohai Sea,the CDOM absorption coefficient a33o has a good correlation with DOC,which indicates that the CDOM in the bottom water is more constant in the DOC than in the surface water.2.Four fluorescent components were identified by the EEMs-PARAFAC model:two humic-like components C1(Ex/Em:250(310)/390 nm),C3(Ex/Em:260(360)/450 nm),two protein-like components:C2(Ex/Em:280/336 nm),C4(Ex/Em:275/308 nm).Among them,the humic-like components(Cl vs C3)and the protein-like components(C2 vs C4)have good correlations,especially the humic-like components C1 and C3,which indicate that each type of fluorescent substances has similar sources and biogeochemical behaviors.3.The distribution characteristics of humic-like component(C1+C3)and protein-like component(C2+C4)are consistent with CDOM absorption coefficient a330,and have a good correlation with absorption coefficient a330,especially humic-like component.The surface water in the northeastern area of the Yangtze River estuary have a relatively high-value area of CDOM absorption coefficient a330 and protein-like component,which may be due to factors such as river input and local microbial activity.4.Compared with the surface water of the South Yellow Sea and North Yellow Sea,the humic-like component increased in the same amount(0.03 R.U.)in the bottom water,while the protein-like component was reduced 0.01 RU and 0.03 RU.Compared with the surface water,the humic-like component of the bottom water increased by 7%and 8%respectively,while the protein-like component was the opposite.The humic-like component and protein-like component had little difference between the surface water and the bottom water in the Bohai Sea.In addition,the contribution of protein-like component to the FDOM fluorescence intensity(C2+C4(%))in the surface water and bottom water of the Yellow Sea and Bohai Sea was higher than that of the humic-like component,and it was the highest in the South Yellow Sea,followed by the North Yellow Sea and the lowest in the Bohai Sea.5.The molecular weight of CDOM and the aromaticity of DOM in the Yellow Sea and Bohai Sea are both high near-shore and low-shore,with low surface water and high bottom water.The molecular weight of CDOM and the aromaticity of DOM in the surface water,and the aromaticity of DOM in the bottom water were the highest in the Bohai Sea,followed by the North Yellow Sea and the lowest in the South Yellow Sea.The molecular weight of CDOM was similar in the bottom water of the Yellow Sea and Bohai Sea.Due to CDOM photobleaching,not only decreased the absorption coefficient of CDOM a330,but also decreased the molecular weight of CDOM and the aromaticity of DOM in the central water of the surface water in the South Yellow Sea.In addition,the river input of the Yangtze River contributed to the higher molecular weight of CDOM and the aromaticity of DOM in the northeastern area of the Yangtze River estuary,which confirmed that the river input had a significant impact on the DOM in the water.6.The aromaticity of DOM in the Yellow Sea and Bohai Sea was closely related to the humic-like component(C1+C3),which was higher than that of the protein-like component(C2+C4),indicating that the proportion of aromatic substances contained in humic-like component(C1+C3)was higher than protein-like component(C2+C4).