Chromophoric dissolved organic matter in the estuaries of populated and mountainous Taiwan

• Dynamics of DOM in the mountainous rivers and their estuaries are largely unknown.
• The level and mixing behavior of DOM changed notably across different estuaries.
• The least populated mountainous rivers have low CDOM levels.
• The CDOM level correlates with population density and chlorophyll a.
• Protein-like fluorophores show notable additions and removals in the estuaries.

Absorption and fluorescence of chromophoric dissolved organic matter (CDOM) in 16 Taiwan estuaries are reported for the first time, so as to examine the influence of catchment characteristics on the CDOM level and to study the estuarine behavior of CDOM. The absorption coefficient (a350) in the freshwater end-member ranged from 0.15 to 12.5 m− 1 among estuaries. The a350 and fluorescence intensities correlated positively with population density and the chlorophyll a concentration, likely due to the additions of CDOM from anthropogenic activities and autochthonous production. The freshwater a350 also correlated negatively with precipitation, headwater altitude and freshwater discharge. The a350 generally decreased linearly with increasing salinity in most estuaries. Humic-like fluorescent components had intensities strongly correlated with a350, while protein-like components received notable additions in five estuaries but were removed in another five estuaries. The absorption spectral slope (S275–295) and slope ratio (SR; 275–295 nm slope: 350–400 nm slope) generally increased with increasing salinity, likely reflecting a decrease in the average molecular weight. The contribution of protein-like components to the total fluorescence changed greatly among different estuaries, which was in part associated with river population density and precipitation. The results showed that both the catchment characteristics and estuarine biogeochemical processes contributed to the variations of CDOM level and composition in estuarine environments.