Network Connectivity and Local Hydrology Drive DOM Composition in a Nonperennial Stream

Abstract Nonperennial streams are dynamic sites of biogeochemical processing, yet much remains to be learned about how hydrologic connectivity at different spatial and temporal scales impacts the composition of dissolved organic matter (DOM). We characterized spatial and temporal patterns of DOM composition in a nonperennial prairie stream network using 3 years of hydrologic data and fluorescence spectroscopy of surface water. Principal component analyses revealed that humic, terrestrial components explained the most variance in DOM composition. These components were highest at sites in the headwaters. In contrast, autochthonous DOM was relatively greater at sites with persistent local‐scale surface water and flowing conditions. Network and local controls (i.e., the degree to which surface water is connected across the network and site‐level water persistence and flow metrics, respectively) interacted to alter DOM composition, with their combined influence depending on the hydrologic phase. High connectivity promoted mixing and transport that diluted local autochthonous signals, whereas low or transitional flows enhanced retention, autochthonous activity, and microbial processing. These findings suggest that hydrologic fragmentation in nonperennial systems modulates not only DOM source and transformation but also its downstream bioavailability, with implications for carbon cycling under shifting climate and flow regimes.