Abstract: Chromophoric dissolved organic matter (CDOM) is an important dissolved organic carbon reservoir in marine systems. Photodegradation of CDOM can accelerate recycling of nutrients and refractory organic matters. To examine the spatial distribution and sources of CDOM in seagrass bed in Xincun Bay, Hainan Island, the absorption coefficient \[a(355)\] and exponential slope value (S) were investigated in this region. The processes of the photo degradation of CDOM were also studied in the laboratory experiment. The results showed that the value of a(355) was highest in seagrass bed in the southern bay, followed by the small river mouth and cagefarming area, and then the center of the bay. The fluorescence intensities at 280 nm excitation \[Fn(280)\] and 350 nm excitation \[Fn(355)\] in proteinlike and humiclike substances were higher in the seagrass bed and river mouth than in the cagefarming area. These results indicated that the molecular weight of CDOM in the seagrass bed and the river mouth area were higher than that of CDOM released by phytoplankton degradation in the cagefarming area. In laboratory, after exposure to UV-B for five days, the proteinlike substances of CDOM from the seagrass bed, river mouth, and cagefarming area decreased by 44.67%, 31.75% and 37.09%, respectively, indicating that proteinlike substances in the seagrass bed absorbed UV-B more efficiently. The humiclike substances decreased by 43.64%, 36.32%, and 50.18%, respectively, which suggested the smaller molecular weight of CDOM from the cagefarming area could be degraded more easily under UV-B. Therefore, our study revealed that seagrass release, riverine inputs, and degradation from phytoplankton in cagefarming areas were the main sources of CDOM in Xincun Bay.

Key words: nitrogen and phosphorus addition, terrestrial ecosystem., nitrogen deposition, soil enzyme activity