Mixing behavior and photobleaching of chromophoric dissolved organic matter in the Changjiang River estuary and the adjacent East China Sea

Abstract

The distribution and chemical properties of chromophoric dissolved organic matter (CDOM) in the Changjiang River estuary and the adjacent ECS were determined in February and July 2014. CDOM concentration (aCDOM(355)) decreased along the salinity gradient, indicating that terrestrial input significantly influenced CDOM in the study area. The runoff from the Changjiang River with smaller spectral slope ratios (SR) dominated SR variability in our study area in July, and the photodegradation of allochthonous CDOM increased SR in the open sea in February. CDOM excitation/emission matrix spectra subjected to parallel factor analysis identified four components: terrestrial humic-like component (C1), autochthonous protein-like components (C2 and C4), and terrestrial tryptophan-like component (C3). C1 and C3 showed a conservative mixing behavior, whereas C2 and C4 were appeared to be influenced by chlorophyll-a (Chl-a). Our sunlight exposure experiment showed that aCDOM(355) (42% at station C1 and 16% at station A6-8) decreased and SR (27% at station C1 and 9.2% at station A6-8) increased. These findings suggested that CDOM molecules were photochemically degraded from high-molecular weight dissolved organic matter (DOM) into low-molecular weight organic substances. The fluorescence intensities of terrestrially derived C1 and C3 decreased with prolonged illumination in samples from both sites. However, autochthonous C2 and C4 moderately increased after they were exposed to sunlight. Results of this study demonstrated that terrestrially derived DOM was more susceptible to irradiation than autochthonous DOM.