Distribution, source, and transport of particulate organic carbon in the Yellow River estuary as affected by the water-sediment regulation

The Yellow River, one of the highest sediment-laden rivers, discharges a huge amount of terrestrial particulate organic carbon (POC) to the sea. However, the reservoir regulations over recent decades have significantly affected the downstream hydrology and POC delivery. The Water-Sediment Regulation Scheme (WSRS) has become a critical time window for the impulse discharges of water, sediment, and POC to the estuary. We investigated the impacts of the WSRS on the estuarine dynamics and POC distribution based on the sampling and observations in the Yellow River estuary in July 2020 corresponding to the two stages (water-regulation stage and sediment-regulation stage) of the WSRS. The distributions of grain-size composition and POC content of surface sediments were presented according to the datasets of in-laboratory analysis. Results indicated that there was a clear turning point of variation in sediment source, grain-size composition, and the POC distribution at the two stages. At the water-regulation stage, the water discharge was high and sediments were relatively coarser and mostly deposited nearshore (＜12 km off the river mouth) with low POC content, while at the sediment-regulation stage, fine-gained sediments were delivered offshore with significant increase in POC content. POC in surface sediments at the water-regulation stage was sourced maily from the lower river erosion, while that at the sediment-regulation stage was composed of the dam-released soil carbon and C₃ vascular plant debris. The rapid changes in POC source and distribution in the Yellow River estuary were controlled by the reservoir regulation, which significantly changed the downstream hydrology and sediment transport. Therefore, human intervention can play an important role in regulating the seaward POC delivery and distribution in the coastal sea.