Spatial and seasonal variability of organic carbon transport in the Yellow River, China

In this study, we examined the spatial and seasonal variability in the concentrations of dissolved and particulate organic carbon (DOC and POC) of the Yellow River. Weekly samples of water and suspended solids were collected along the main stem channel between July 2011 and July 2012 for the upstream Toudaoguai and Tongguan stations, and between August 2008 and July 2012 for the downstream Lijin station near the river mouth. The DOC export at the upstream two stations was primarily controlled by hydrological events such as melting of ice and snow with high DOC concentrations occurring in spring. In contrast, it was more affected by human activities, mainly reservoir regulation, at the lowermost Lijin station. Lower DOC concentration in the wet season indicates that most of the leachable DOC in surface soils may have largely been flushed away by spring floods. In addition, it is also likely due to dilution effect of the rapidly increased water discharge. As a result of low organic carbon content in the parent soils, the Yellow River sediments were characterized by low POC content (POC%). The averaged POC% at Toudaoguai, Tongguan, and Lijin was 0.48%, 0.47%, and 0.37%, respectively, which is significantly lower than the global mean of around 0.95%. The POC% decreased exponentially with total suspended solids (TSS) concentration. This is likely due to the dilution of riverine POC, because high TSS generally means a higher proportion of coarse sediments that have more mineral matter. During the study period, the total DOC and POC fluxes into the ocean were estimated at 0.06 × 1012 g/yr and 0.41 × 1012 g/yr, respectively. Combining our previous estimate of dissolved inorganic carbon (DIC) export shows that the Yellow River transports a carbon flux of 1.52 × 1012 g/yr into the Bohai Sea, accounting for about 0.19% of the global total riverine carbon flux (DOC + POC + DIC). The extremely low DOC/POC ratio represents the lowest level among major world rivers, which is consistent with its intense soil erosion and highlights the effect of soil erosion on organic carbon export.