Dissolved organic carbon content is lower in warm seasons and neutral sugar composition indicates its degradation in a large subtropical river (Nantong Section), China

Riverine organic carbon export plays a major role in the global carbon cycle. The biodegradability of riverine dissolved organic carbon (DOC) determines both greenhouse gas emissions and the amount of refractory DOC. However, seasonal changes in organic carbon and factors controlling DOC biodegradation are still not well understood in large rivers. Here, we examined the monthly changes in concentrations of particulate organic carbon (POC) and DOC in a large subtropical river (Changjiang River, Nantong Section, China) in 2017. We also measured biodegradable DOC (BDOC), specific UV absorbance (SUVA(254)), neutral sugar concentrations and their compositions. The results showed that POC had no significant relationship with water temperature, precipitation, and discharge. DOC concentrations were significantly negatively correlated with all these parameters, and water temperature explained 61% of the variance in DOC. BDOC concentrations varied between 0 and 14.01 +/- 1.22mg C/L and were closely associated with DOC (R-2=0.85, p<0.001). The negative correlation between BDOC% and SUVA(254) (R-2=0.63, p<0.01) suggested that high aromaticity was associated with low bioavailability. Water temperature was positively correlated with total neutral sugar yield (R-2=0.43, p<0.001), which was mainly composed of glucose and deoxysugars in the warm season; thus, neutral sugar composition is a useful indicator of DOC decomposition. Taken together, our study showed that POC has no clear seasonal change, while DOC exhibits a strong seasonal change, and high water temperature leads to low BDOC in a large subtropical river.