Network Connectivity and Local Hydrology Drive DOM Composition in a Non-Perennial Stream

Nonperennial streams are dynamic sites of biogeochemical processing, yet much remains to be learned about how hydrologic connectivity impacts the composition of dissolved organic matter (DOM) at the network and local scale. We characterized spatial and temporal patterns of DOM composition in a non-perennial prairie stream network using three years of hydrologic data and surface water fluorescence spectroscopy through repeated sampling. Principal component analyses were used to reduce dimensionality and uncover major controls of DOM composition. Humic, terrestrial components were found to explain the most variance in the model. Humic, terrestrial components of DOM were controlled by network conditions, and were highest during dry down phases. Microbial components of DOM were associated with local flow conditions and water persistence. DOM was more microbially derived and autochthonous at sites that remained persistently wet and flowing. The interaction between network and local scales and their impact on DOM composition is phase dependent, such that local microbial activity can be reduced during high connectivity (due to transport) but enhanced during low or transitional flows (due to retention). 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.