Spatiotemporal Variation Of Colored Dissolved Organic Matter And Its Control In The Bohai Sea And The North Yellow Sea

Colored dissolved organic matter(CDOM)is a fraction of DOM that have absorption in the UV-Visible wavelength range.It plays an important role in marine carbon cycle and climate change.Optical properties of CDOM are usually derived from UV-Visible absorption spectrum and three-dimensional fluorescence spectrum combined with parallel factor analysis.These methods are efficient and effective so that they have been widely used in marine CDOM research.Using these methods,we investigated the spatial distribution and seasonal variation of CDOM in the Bohai Sea and the North Yellow Sea based on three cruises in summer 2018 and in summer and winter 2019.Additionally,photodegradation process of CDOM derived from Ulva prolifera was studied by exposing it to simulated sunlightHorizontal distribution of absorption coefficient acDOM(330)in the Bohai Sea and the North Yellow Sea in summer and winter were similar,with a high level near the coast and a low level in the central area.S275-295 showed an opposite horizontal distribution to that of acDOM(330),indicating that the molecular weight of CDOM was higher near shore and lower offshore.The acDO(330)and S275-295 varied in a wider range in the surface layer than in the bottom layer in summer whereas they were well mixed vertically in winter.Due to terrestrial input,CDOM in the Bohai Sea showed high abundance,high molecular weight,and small seasonal variations.Additionally,algal blooms increased the abundance of CDOM in local region.On the contrary,the North Yellow Sea was less affected by terrestrial input so that CDOM showed obvious seasonal variation in line with seasonal variation of primary production.PARAFAC modeling decomposed the EEMs into three fluorescence components one terrestrial humic-like fluorescence component C1(370/455),one marine humic-like fluorescence component C2(325/365),and one protein-like fluorescence component C3(290/320)produced in situ.Both C1 and C2 had high levels near the coast and low levels in the central area,while C3 distributed unevenly.Vertical distributions of the three fluorescent components were complicated,while they were well-mixed vertically in winter.The intensities of two humic-like fluorescence components C1 and C2 were higher in the Bohai Sea than in the Northern Yellow Sea,while the protein-like fluorescence component was similar between these two areas.C1-C3 in the Bohai Sea showed weak seasonal variation in the Bohai Sea than in the Norther Yellow Sea.Three fluorescent components were identified in the Ulva prolifera extract,including one long-wavelength humic-like fluorescent component C1(365/455),one short-wavelength humic-like component C2(345/395)and one protein-like fluorescent component C3(285/335).These components were similar to those identified in the field investigation,indicating that CDOM from biological sources could play an important role in the distribution and seasonal variation of CDOM in the investigated area.Dissolved organic matter derived from Ulva prolifera extract had strong photodegradation activity and they were more sensitive in the UVB wavelength range.Their half-lives were less than those in the coastal water,and much less than the turnover time of terrestrial organic matter in the marine mixing layer.Photodegradation could decrease the absorption ability of CDOM and reduce its molecular weight,so that large-molecular fluorescent matters would degrade.Fresh CDOM derived from algal blooms could be easily photobleached or photodegraded in a short period.