[Spatial Distribution and Degradation of CDOM in the Minjiang River in Summer]

The composition, distribution, and degradation of chromophoric dissolved organic matter (CDOM) in the lower reach and estuary of the Minjiang River were analyzed using fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC) and microbial and photochemical degradation experiments. The results show that there are three types of fluorescence components in the study area:humic-like, tyrosine-like, and tryptophan-like. The humic-like components are the main components in the river zone, while the protein-like components become dominant with increasing salinity in the estuary. The change of the CDOM abundance shows a notable spatial distribution pattern. The absorption coefficient a(280) of CDOM is lower in the river, increases after entering the urban area, shows a decreasing trend in the suburbs, and rapidly declines in the estuary. A conservative estimate of the contribution of the Fuzhou urban area to the CDOM of the Minjiang River is 8%. The a(280) in the river is susceptible to microbial and photochemical degradation and the degradation percentages are (28±8)% and (44±7)%, respectively. The bioavailability and photochemical reactivity of a(280) are much higher in the river than in the estuary. The humic-like, tyrosine-like, and tryptophan-like components show a higher photochemical reactivity in the river, with degradation percentages of (75±0.5)%, (58±21)%, and (73±3)%, respectively. The fluorescent components are not labile with respect to microbial degradation and humic-like components are accumulated after 28 days of microbial culture.