DOC fluorescence properties and degradation in the Changjiang River Network, China: implications for estimating in-stream DOC removal

Abstract

Dissolved organic carbon (DOC) in large river networks is an important carbon pool in the global biogeochemical cycle. DOC compositions vary spatially at global and regional scales, which influences DOC degradation and subsequent in-stream removal in river networks. The Changjiang River Network (CRN) is the third largest river system worldwide and plays an important role in DOC transformation and transportation. However, the relationships between DOC composition and degradation, as well as the quantification of in-stream removal, remain poorly understood. In this study, we chose the CRN to study spatial patterns of DOC fluorescence, as proxies for chemical composition, and degradation. Laboratory degradation experiments were conducted to explore DOC degradability and mechanisms of photodegradation and microbial respiration. We found that DOC fluorescence properties and degradation showed significant spatial differences in the lower reaches of the CRN. DOC with higher proportions of humic acid-like materials showed higher photodegradability. In degradation experiments, the total DOC loss was 49.1–66.0% over about the first 6 to 7 days. Microbial respiration removed 33.0–47.3% of total DOC, while photodegradation removed 9.0–35.3%. The relative contributions of photodegradation and microbial respiration to DOC degradation varied with different DOC fluorescence properties. The experimental values of DOC decay rate constants were 0.217–0.250 d−1. Our study improves understanding of DOC degradability, linking to its composition and providing a reference for estimating DOC in-stream removal in large river networks.