Photochemical processing of naturally dissolved organic matter (DOM) has been mainly considered as a significant source of green house gases from estuaries. Recent studies increasingly realized that photo-transformation of DOM can contribute to the formation of refractory DOM in estuarine environments. Diverse sources of DOM and heterogenous nutrients conditions together make rivers and estuaries a complex system to be examined. This study investigates the Zhangjiang River estuary and upstream regions in Fujian Province, as well as sedimentary DOM sourced from mangroves adjacent to the watershed and DOM derived from vascular plants located upstream. Water samples containing various levels of nitrogen nutrients (including nitrate, nitrite, and ammonium) were subjected to photo-irradiation and subsampled at different time points. Bulk analysis of carbon and nitrogen, and molecular level characterization of DOM, were conducted to assess the transformation and migration of nitrogen during the photochemical reactions, and to evaluate the photochemical reactivity of different DOM components. Results indicated that the incorporation of various nitrogen-containing nutrients exerted distinct influences on the rates of photobleaching and photomineralization of dissolved organic matter (DOM). The finding of newly formed nitrogen-containing DOM during photochemical processing provides novel insights regarding the relation between terrestrial DOM and marine DOM.
 

Dissolved organic matter,Photochemistry,nutrients,Estuarine processes