Photodegradation has long been considered as an important sink of dissolved organic matter (DOM) in aquatic ecosystems. In some cases, both chromophoric and fluorescent DOM (CDOM and FDOM) can be photoproduced from certain substances or in natural seawater. However, less is known about the mechanisms and the impacts of both of DOM photodegradation and photoproduction on carbon cycling in estuaries and coastal seas. In the present study, water samples collected from the freshwater endmember, mixing zone and mouth of the Pearl River estuary (PRE) were irradiated for 4 days under full spectrum (FS), UVA plus visible (UVAVis) and visible light (Vis), respectively. The results suggested that dissolved organic carbon (DOC), CDOM, humic-like (C_H) and protein-like (C_P) FDOM were significantly degraded under FS and UVAVis, with the order of the relative photoreactivities being C_H > C_P > CDOM > DOC. The relative contribution of UVB to the photodegradation of DOC and CDOM increased from freshwater to coastal water sample, while it decreased for C_H and C_P. This demonstrated the photochemical dynamics of DOC, CDOM and FDOM are different. Moreover, it favors DOC remineralization from estuaries to coastal waters with the turbidity decreasing and UVB penetration depth increasing. Under Vis, no obvious changes were found for DOC, CDOM and C_H, but significant amounts of C_P was produced. Hence, in coastal seas, it reveals that labile substances for heterobacteria would be photoproduced during daytime in the lower euphotic zone, where only visible light existing. Our results provided novel insight into the photochemical dynamics of DOM and potential carbon cycling pathways in estuarine and coastal waters.
 

Dissolved organic matter,Photochemistry,Pearl River estuary