ABSTRACT:

Headwater streams are dynamic reactors, where flow largely controls carbon dynamics (C). Riverine C exists largely as dissolved and fine particulate organic matter (DOM<0.7 µm and 0.7 µm<FPOM<1 mm, respectively). Although FPOM contributes significantly to C stocks and stream metabolism, FPOM processing is often unaccounted for in stream C cycling measurements and budgets. To address this knowledge gap, we investigated OM fate across flow conditions at two sites in a forested stream network in Alabama, USA: the upstream headwaters and the downstream outlet of Pendergrass Creek. At each site, we quantified DOM and FPOM in surface water and the streambed as benthic organic matter (BOM) for one year. At both sites, we found that FPOM and BOM diluted with flow and increased during leaf-on, while DOM (as dissolved organic C) did not change with flow and increased during leaf-off. To assess the microbial metabolic activity (MMA) of different OM pools, we conducted resazurin-resorufin incubation assays as a proxy for respiration. Rates of MMA across all three OM pools decreased while particulate C:[N]itrogen ratios increased at higher flows, indicating more reactive OM is stored and respired at low flows. We observed that FPOM accounted for the majority of water column respiration across all flow conditions. Our findings, corroborated by a hydrologic model (Coupled Routing and Excess STorage [CREST]) of flowing network length, suggest that as flow regimes become more extreme with climate change, the quantity and reactivity of OM stored in headwater streams may shift, influencing C emissions and downstream resources.