Different Responses of Dissolved Black Carbon and Dissolved Lignin to Seasonal Hydrological Changes and an Extreme Rain Event

Hydrology, especially extreme hydrological events, has been recognized as an important driver of the land-to-ocean export of terrigenous dissolved organic matter (tDOM). Nevertheless, how various types of tDOM that differ in source and reactivity respond to changes in hydrology is not known. Seasonal and event exports of dissolved organic carbon (DOC), dissolved black carbon (DBC), and dissolved lignin were studied in a small subtropical river. We found that seasonal variations in DBC concentration were significantly related to hydrology, while DOC and dissolved lignin were not. In contrast, DOC, DBC, and dissolved lignin changed similarly during an extreme rain event. The variation magnitudes of DOC, DBC, and dissolved lignin concentrations were in the lower end compared to other rivers, which may be related to the limited coverage of wetlands and riparian vegetation and poor development of organic-rich soil. Dilution effects were observed when the runoff exceeded 0.4mm/hr, and the fluxes of both DBC and dissolved lignin decreased during the runoff peak, which was caused by surface flow and potentially by removal processes during peak discharge. Our results suggest that the influence of hydrology varies with tDOM source and reactivity and that high enough runoff (e.g., 0.7mm/hr in the Jiulong River) may not enhance the export rate of tDOM. However, our study was carried out in a small watershed with limited wetlands and riparian vegetation, and more studies are needed to verify whether this trend is consistent among global rivers.