Hydrological regime controls DOM and WEOM dynamics in contrasting mid-channel bars: Insights from the middle Yangtze River.

Riverine sandbars are critical ecological units within rivers, yet their biogeochemical responses to dam-regulated hydrological regimes remain poorly understood. This study investigated the divergent dynamics of dissolved organic matter (DOM) and its sediment-bound counterpart, water-extractable organic matter (WEOM), in two geomorphologically contrasting mid-channel bars-Sanba Sandbar (SBS) and Wugui Sandbar (WGS)-in the middle Yangtze River across four hydrological periods. By integrating three-dimensional fluorescence spectroscopy, UV-vis absorption spectroscopy, and partial least squares structural equation modeling (PLS-SEM), we revealed their distinct functional roles. The results showed that the frequently inundated sandbar (SBS) exhibited high sensitivity to hydrological pulses. Its DOM characteristics were cyclically "reset" by the flood peak and subsequently shifted rapidly toward autochthonous sources stimulated by flood-driven nutrient pulses, reflecting a weak biogeochemical "memory." In stark contrast, the relatively stable, vegetated sandbar (WGS) displayed strong biogeochemical resilience. Its response may have been primarily internally mediated, buffering hydrological disturbance and potentially triggering a pronounced "first flush" of stored terrestrial organic matter upon initial inundation, thereby functioning as a biogeochemical processor with a strong "memory." PLS-SEM analysis indicated that at SBS, inundation extent was a direct external driver, whereas at WGS, increased inundation extent may have indirectly regulated organic matter dynamics by altering internal sediment physicochemical conditions. These findings suggest that the hydrological regime is a key driver of DOM and WEOM dynamics in sandbars, while the geomorphic characteristics of sandbars modulate the magnitude of this response. The contrasting behaviors between SBS and WGS provide new insights into the spatiotemporal dynamics of organic matter in large alluvial rivers.