Hydrologic and seasonal controls on organic matter pools and transformation in a forested headwater stream network_data

Streams are dynamic reactors, where flow often controls carbon (C) dynamics. 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. 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 FPOM C:[N]itrogen ratios increased at higher flows, indicating more reactive OM is likely stored and respired at low flows. We also observed that FPOM often accounted for the majority of water column respiration potential. Our findings, corroborated by a hydrologic model of flowing network length, suggest that as stream flows decrease with climate change, the quantity of OM stored and respired in headwater streams may increase, influencing C emissions and downstream resources.