DOM dynamics from a long-term optical sensor deployment at Sleepers River, Vermont

Abstract:

Since September 2012, we have monitored in-stream fluorescent dissolved organic matter (fDOM) and turbidity using optical sensors at a 15-min timestep at the outlet to the 41-ha forested W-9 catchment at Sleepers River, Vermont, USA, in the headwaters of the Connecticut River. We corrected the fDOM sensor record for water temperature and turbidity interferences. Turbidity commonly exceeds 50 NTU during events and we applied a local correction to fDOM based on discrete sample analyses. Dissolved organic carbon (DOC) correlated strongly with in situ fDOM (r² = 0.92), and we used this relation to compute a 15-min DOC record and stream DOC export. Sensor-based DOC fluxes were comparable to those computed from conventional sampling and modeling of concentration-discharge (C-Q) relations. However, the high-frequency dataset revealed persistent patterns that may have eluded conventional sampling, including: (1) a general counterclockwise C-Q hysteresis dominated by an extended period of elevated concentrations on the falling limb, suggesting delayed soil- and groundwater DOC contributions; (2) a persistent but low C-Q slope with minimal hysteresis during snowmelt, suggesting both supply and transport limitation; (3) diel variation in DOC limited to the vernal window between snowmelt and leaf-out, when abundant solar radiation reaches the channel; and (4) leaffall-induced DOC increases in autumn both during base flow and events. Finally, we demonstrate how understanding of DOC sources and delivery at the small catchment scale provides insight to patterns in the larger Connecticut River basin.