| Chromophoric dissolved organic matter (CDOM) is the main component of marine dissolved organic matter. It has great significance on reaching the ocean carbon cycle and marine environmental protection. In recent years,CDOM has been taken seriously by the oceanographer. The composition, distribution characteristics and sources of chromophoric dissolved organic matter (CDOM) in the Bohai Sea and the Yellow Sea in summer and autumn were evaluated by fluorescence excitation-emission matrix (EEM) combined with parallel factor analysis (EEMs-PARAFAC). The main results obtained are summarized as follows:1. Three humic-like components(C1 (Ex/Em/nm, (275) 365/420),C2 (325/395) and C3 ((260)395/515)) and one protein-like component (C4(280/345)) were identified by EEMs-PARAFAC from the Bohai Sea and the Yellow Sea water bodies and pore waters in summer and autumn.2. The horizontal distribution patterns of thefour components were almost similar in both summer and autumn, showing decreasing with increasing distance from shore, especially in the southwest of the Bohai Sea near the Yellow River Estuary, the maximum of the fluorescence intensity is appeared. Secondly.thehigh value areas of the fluorescence intensity are observed in the following areas:the haizhou bay, southeast of Shandong peninsula, Yancheng of Jiangsu province. These high value areas are in conformity with the low salinity and high concentration of the chlorophyll-a areas, this may attribute to the terrigenous input, human activities and so on. A large number of nutrients were carried into these areas leading to the massive growth of the phytoplankton, but relative to the terrestrial input, the effect of the phytoplankton is small. In the regionsthat the effect of terrigenous input serious, fluorescence intensity value is higher in summer than that in autumn. The high value area in autumn is southward and narrower than summer, this is mainly attributed to the prevailing northerly winds and the stronger coastal current in autumn.According to the vertical distribution, the high values were appeared in the surface and middle layers in the nearshore area, corresponding to the low salinity and high chlorophyll-a concentration, indicating that terrigenous input and human activities were the main source of CDOM in these areas. Besides the inshore areas, C2 and C4 are also showed high values on the offshore area, which are corresponding to the low salinity and high chlorophyll-a concentration. It indicates that C2 and C4 are affected by both terrigenous input and the primary production of phytoplankton. In the bottom layers in the nearshore area, high values also appeared that attribute to sediments resuspension, resulting the CDOM was released from the pore water into bottom water. For shallow water area in autumn, vertical distribution is homogeneous; stratification phenomenon is serious in summer, these may attribute to the high water stabilityin summer.The correlationship between salinity and the relative fluorescence intensity (Pn) is introducedto characterise the differences among all the components.Pl, P2 stayed constant or reduced with the increase of salinity, showed that in addition to terrigenous input, C1 and C2 also affected by other processes such as sediment resuspension, human activities etc. P3 stayed constant with the increase of salinity all the time, indicating that C3 was mainly controlled by terrestrial input. P4 increased with the increase of salinity, inferringthat the effect of biological activity on C4 was significant.3. By discussing the humification Index (HIX), the Biological Index (BIX) and their relationship with CDOM components to study the humification degree and the stability of the four fluorescent components in the Bohai Sea and the Yellow Sea. Further to understand the source of CDOM, the molecular characteristics and so on. The distribution of HIX is similar with CDOM fluorescence intensity that showing decreasing with increasing distances from shore, and has positive relationship with fluorescence intensity. The correlationship between HIX and C3 is the most significant and then C1 (C2), C4 is the worst, indicating that C3 has the strongest stability, highest molecular weight and the longest time stay in the environment, and C4 is just the opposite. The BIX showed opposite trends with HIX and verify the same results.4. A strong correlation occurred between C1 and C2, C3 indicating that they have similar sources and the removel pathway, a weak correlation appeared between C1 (C2, C3)andC4, reflecting their similarity is weak. But the correlationship between C2 and C4 is better than that C1 (C3) and C4, indicating that C2 and C4 also have similar sources and the removel pathway to some extent.5. Fluorescence intensity of pore water CDOM is far higher than that of seawater, and the fluorescence intensity of C4 is the highest, at the same time, compared the two seasons CDOM fluorescence intensity can found that the values in autumn is higher than summer, especially the protein-like component C4. These can be attributed to the biodegradation, in autumn, microbial decomposition of organism debris, producing a large number of amino acids to show protein fluorescence.In autumn, the high value areas is southward and narrow relative to summer, this is mainly explained by the northerly winds prevail in autumn and the coastal current is stronger in autumn than summer, the coastal current limits high concentration CDOM in inshore and hard to extent outward.6. According to the correlation analysis between salinity and the absorption coefficient a355, showing that no matter from the horizontal or the vertical data, the salinity has a negative relationship with the absorption coefficient 3355, and the absorption coefficient has a significant positive relationship with the total fluorescence intensity. In the areas where the influence of theterrigenous input is stronger, the S (350-400) valueswere higher, and the SR values were lower. From the inshore to the offshore, from surface to the bottom, the SR values increases gradually,explained by the terrigenous impact force is abate, leading to the high molecular weight CDOM decreased and small molecular weight CDOMincreased, causing the SR values increasing. SR values were higher in autumn than that in summer, because,in summer photodegradation is strong, parts of the larger molecular weight CDOM was decomposed into small molecular weight organic compounds. And in summer, the appropriate temperature and the richnutrient were very favorable for the growth of phytoplankton, makes the new generation of CDOM increasing, causing the SR value increased in summer. |
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