Using data from four field investigations between 2003 and 2009 along the Yellow River mainstream, we examined the transport features and seasonal variations of organic carbon, with a focus on contrasting the impacts of human activities with those of natural processes. Particulate organic carbon (POC) in the Yellow River originated mainly from the Loess Plateau, and thus the POC content in suspended sediments was much lower than in the world's other large rivers. Owing to both natural and human influences, dissolved organic carbon (DOC) has only a weak correlation with discharge. DOC varied as a result of human activities such as agricultural irrigation and pollution in the whole basin except for the upstream Qinghai–Tibetan Plateau. Our study also suggested that while reservoirs are a POC sink over short periods, a long-term POC storage flux cannot be easily estimated as discharge and sediment regulations have completely changed the relationship between the fluxes of water, sediments, and rainfall. However, this carbon sink can be obtained reliably through high-frequency sampling over long time periods. In addition, the annual water and sediment regulation (WSR) scheme has imposed an extremely severe human disturbance on the transport pattern of river organic carbon. Our study demonstrated for the first time that in a WSR event of less than 20 days, large proportions of the annual DOC (35%) and POC (56%) fluxes of the Yellow River were transported to the estuarine and coastal zone, potentially influencing estuarine and coastal geochemistry and ecosystems profoundly

Based on four field investigations during 2003 and 2009 along the Yellow River mainstream, we examined the distributions, seasonal variations and transport features of organic carbon, with a focus on the impacts of human activities (reservoir construction and regulation scheme). The results showed that organic carbon transport processes in the Yellow River were different from other major rivers. Particulate organic carbon (POC) dominated in the Yellow River and it mainly originated from the Loess Plateau. POC levels in suspended sediment (POC %) ranged between 0.25% and 2.20% and more than 80% of POC concentrated in the particles with grain size smaller than 16 μm. On time scale, dissolved organic carbon (DOC) correlated negatively with discharges, indicating the influence of dilution effect. Along the river on spatial scales, DOC in the Qinghai-Tibet Plateau was closely related with temperature while DOC in the Loess Plateau showed the concentration effect, due to the abundant human input and the high ratio of evaporation to precipitation. Organic carbon in the Yellow River was very refractory and about 90% of POC and 70% of DOC cannot be degraded. Due to the high turbidity, the Yellow River suffers more impacts from the reservoirs in the transport of total suspended solids (TSS) and organic carbon. Ratios of DOC/POC ranged between 2.0 and 12 in the reservoirs and organic carbon was mainly in the dissolved from. POC deposited in the reservoirs of the Yellow River achieved 0.0033 Gt a^−1, about 8 times its annual POC flux discharged to the ocean. During the 2008 Water and Sediment Regulation (WSR) period, DOC and POC fluxes was as high as 1.1 × 10¹⁰ g and 2.2 × 10¹¹ g respectively, accounting for 35% of annual DOC flux and 56% of the annual POC flux to the ocean. Discharges and material fluxes to the ocean decline sharply due to the reservoir construction while large amounts of water and sediment are transported to the ocean in such a~short WSR period. These two human disturbances totally altered the processes of substance transport in the Yellow River, and may change the water chemical characteristics in the coastal zones