LB 2-17 - Spatiotemporal changes in dissolved organic matter across a reservoir watershed

Dissolved organic matter (DOM) plays an essential role in freshwater ecosystem functioning and aquatic carbon cycling, motivating a need to understand the magnitude of variation in both DOM quantity and quality across ecosystem gradients. DOM processing can vary spatially throughout a watershed, but it remains unresolved the magnitude of variability in DOM quantity and quality processing across the headwaters of a reservoir to its dam. Specifically, substantial DOM processing likely happens along the flowpath between riverine and pelagic zones in reservoirs, but changes in both DOM quantity and quality on this ecosystem gradient are rarely quantified. To examine variation in DOM across a watershed, we monitored DOM spatially across a warm monomictic reservoir located in Roanoke, Virginia, USA that experienced variable hydrology during 2021 and 2022. We sampled fortnightly to seasonally at four inflow streams and at multiple depths at the deepest pelagic site of the reservoir. At each site, samples were collected for analyses of dissolved organic carbon concentration (DOC) and fluorescent DOM (fDOM), which was measured using a spectrofluorometer to determine excitation-emission matrices (EEMs) and metrics of organic carbon quality. Additionally, two high-frequency fDOM sensors were deployed at two depths at the pelagic site, allowing for an improved understanding of temporal variability in DOM quantity between sampling dates. Stream sites showed greater variability in DOM quantity and quality than the pelagic sites in our study reservoir, with stream DOC concentrations ranging from 0.8 - 5.5 mg/L and EEM derived fluorescence index (FI) ranging from 1.4-1.8, with higher values indicating a larger pool of autochthonous-like DOM in fall months. The pelagic reservoir site had lower variability in DOC and fDOM in both the surface (2.1-3.5 mg/L and 3.5-24.6 quinine sulfate units (QSU), respectively) and bottom waters (1.8-3.1 mg/L and 9.4-33.6 QSU, respectively), with the largest changes occurring after fall turnover when DOC and fDOM concentrations increased by over 1.1 mg/L and 10 QSU. Similarly, DOM quality was less variable in the pelagic site compared to the streams with FI ranging from 1.4-1.6, with high values occurring in later summer. Differences in DOM quantity and quality between the streams and pelagic sites highlight the important role reservoirs play in processing DOM. These results highlight the need to study spatial changes in DOM quantity and quality to improve our understanding of how the entire reservoir watershed processes DOM, which has implications for the role of reservoirs in global carbon budgets.