Optical properties as tracers of riverine dissolved organic matter biodegradation in a headwater tributary of the Yangtze

The optical properties can characterize the chemical composition, bioavailability and origins of dissolved organic matter (DOM), and thus indicate its biodegradability in natural waters. However, there remains a large gap in how DOM optical properties (UV-visible and fluorescence spectra) trace and predict the dissolved organic carbon biodegradation (%BDOC) in river systems. Over a 56-day laboratory incubation experiment, we examined the variations of %BDOC and DOM optical properties in a headwater tributary of the upper Yangtze River. We found that DOC biodegradability was significantly higher in the tributaries (20 degrees C: 38.9 +/- 19.8%; 30 degrees C: 58.6 +/- 19.5%) than that in the main stem (20 degrees C: 26.1 +/- 18.2%: 30 degrees C: 41.8 +/- 16.3%) (p < 0.001). The %BDOC, exhibiting coincident peaks in the agricultural regions and troughs in the ecologically fragile regions, increased by a factor of two-fold with a temperature gradient of 10 degrees C. The optical properties SUVA(254) and S275-295 decreased, while the S350-400, FI and BIX increased after the incubations, implying the synchronous biodegradation and production of young DOMs via microbial respiration. The %BDOC:DOC tightly linked to a(254), S275-295, S350-400 and BIX, illustrating that optical properties could predict the riverine %BDOC. The findings demonstrated that optical properties could be regarded as the useful tracers of DOM biodegradation in river systems.