Shifting of dissolved organic matter components and sources in precipitation into an intensified anthropogenic influenced embayment: Interpretation from spectral characteristics and dual stable isotopes

Abstract

Dissolved organic matter (DOM) is a ubiquitous group of organic compounds in rainwater that contributes to DOM pools and plays a vital role in the marine biogeochemical cycle. Here, we presented the rainwater DOM components and sources attribution (in δ13C-DOMSPE and δ15N-DOMSPE perspective) to an intensified anthropogenic influenced embayment, and discussed the biogeological effects of atmospheric DOM deposition. The results showed an apparent seasonal variation in the DOM concentration dominated by precipitation. Moreover, the excitation-emission matrix spectroscopy coupled a parallel factor analysis (PARAFAC) results revealed that five fluorescent components (C1-C5) of the rainwater DOM. C1 and C5 are protein-like DOM components and derived from biogenic emissions. The fluorescence intensity of C1 and C5 were significantly higher in the wet season, whereas C2 and C3, which are humic-like substances with higher molecular weights and aromatization, were mainly derived from fossil fuel combustion. The δ13C-DOMSPE and δ15N-DOMSPE results indicated that fossil fuel contributed approximately 80% of the DOM in the precipitation in the dry seasons, whereas the marine emission contribution of DOM was higher in the wet seasons due to the influence of air masses originating from the South China Sea. However, the maritime emissions only contributed 4% to the rainwater DOM. The deposition fluxes of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) were 799 mmol·C·m˗2·yr˗1 and 46 mmol·N·m˗2·yr˗1, respectively, and they played a substantial role for the DOM input flux of Daya Bay (28% of DOC and 12% of DON, respectively). Overall, our results suggest that atmospheric wet deposition represents a potentially critical source of organic carbon and nitrogen for coastal waters, particularly in intensified anthropogenically influenced embayments characterized by substantial monsoon shifts between seasons.