Implications of the different submerged extent of typical sandbars to local DOM and SDOM during the pre-flood season in the middle reaches of Yangtze River.

In large rivers, sandbar evolution is mainly driven by changes in discharge, flow velocity, and sediment concentration in the water, which can shape the implications of local dissolved organic matter (DOM) and sediment dissolved organic matter (SDOM). After the completion of the Three Gorges Dam (TGD), the water level in the middle reaches of the Yangtze River changes rapidly during the year, and the submerged extent of the Yangtze River sandbars are significantly affected by the change in water level, so that when the water level rises to the defense level or higher, the sandbars may be completely submerged. As a result, different submerged extent such as periodically and long-term have occurred on the typical sandbars. However, the extent to which the submersion of typical sandbars affects the source, composition and content of regional DOM/SDOM in large rivers remains unclear. This study was conducted in the middle reaches of the Yangtze River, the characteristics, sources, and influencing factors of DOM were studied using methods such as three-dimensional fluorescence spectrum (EEMS) and parallel factor analysis (PARAFAC) to ascertain the response mechanisms of DOM/SDOM distribution patterns in different submerged extents of the Sanba Sandbar (SBS) and Wugui Sandbar (WGS). The results indicated that the source and component characteristics of DOM/SDOM from different submerged extents in typical sandbars exhibited some differences and similarities. The sediment conditions of the local sandbars, physicochemical parameters of the water, and human activities jointly controlled the DOM/SDOM characteristics of the local sandbars. The DOM of long-term submerged sandbars is generated by mixed endogenous and exogenous inputs; its fractions are more complex, and protein-like proteins are the main components; however, periodically submerged sandbars are dominated by inputs from terrestrial sources, and DOM is relatively more strongly humified, hydrophobic, and aromatic. Sediment dissolved organic matter (SDOM) is an important organic component in sediments, which has an important impact on the ecological environment of the water, the material cycle and the migration of pollutants. The endogenous characteristics of SDOM of sandbars with different submerged extents are evident, mostly in the form of land-source inputs; SDOM is similarly dominated by protein-like humus with a relatively high percentage on periodically submerged sandbars. However, the SDOM component of long-term submerged sandbars is more complex than that of periodically submerged sandbars, and SDOM has more hydrophobic and aromatic properties. This study is the first to analyze the DOM and SDOM of typical sandbars in the middle reaches of the Yangtze River, offering new insights into the characteristics and influencing factors of DOM and SDOM in this region, as well as its distribution in large rivers.