Distribution and spectral characteristics of chromophoric dissolved organic matter in a coastal bay in northern China

The absorption spectra of chromophoric dissolved organic matter (CDOM), along with general physical, chemical and biological variables, were determined in the Bohai Bay, China, in the springs of 2011 and 2012. The absorption coefficient of CDOM at 350 nm (a₃₅₀) in surface water ranged from 1.00 to 1.83 m^-1 (mean: 1.35 m^-1) in May 2011 and from 0.78 to 1.92 m^-1 (mean: 1.19 m^-1) in April 2012. Little surface-bottom difference was observed due to strong vertical mixing. The a₃₅₀ wasweakly anti-correlated to salinity but positively correlated to chlorophyll a (Chl-a) concentration. A shoulder over 260-290 nm, suggestive of biogenic molecules, superimposed the overall pattern of exponentially decreasing CDOM absorption with wavelength. The wavelength distribution of the absorption spectral slope manifested a pronounced peak at ca. 300 nm characteristic of algal-derived CDOM. All a₂₅₀/a₃₆₅ ratios exceeded 6, corresponding to CDOM molecular weights (M_w) of less than 1 kDa. Spectroscopically, CDOM in the Bohai Bay differed substantively from that in the Haihe River, the bay's dominant source of land runoff; photobleaching of the riverine CDOM enlarged the difference. Results point to marine biological production being the principal source of CDOM in the Bohai Bay during the sampling seasons. Relatively low runoff, fast dilution, and selective photodegradation are postulated to be among the overarching elements responsible for the lack of terrigenous CDOM signature in the bay water.