MOLECULAR HYSTERESIS: HYDROLOGICALLY-DRIVEN CHANGES IN THE COMPOSITION OF RIVERINE DISSOLVED ORGANIC MATTER

Rainfall-runoff processes have emerged as key controllers of the quantity and quality of terrestrial dissolved organic matter (DOM) exported from the landscape to inland waters and coastal margins. Hydrological events result in increased river discharge and a concomitant release of large amounts of DOM into fluvial networks. This study is part of an NSF Macrosystems Biology project which aims to test the Pulse-Shunt Concept: where rivers are converted from active to passive pipes during high discharge events (“pulse”), transporting labile, terrestrial DOM downstream (“shunt”), and relocating biogeochemical hotspots for DOM from the upper to the lower reaches of the watershed. The primary objective of our study was to track hysteretic changes in riverine DOM molecular composition over the course of several storm events. Samples were collected from nested watersheds in the Passumpsic River catchment, a tributary of the Connecticut River (USA). High resolution monitoring (via in-situ sondes) and high frequency collection of discreet samples (for FT-ICR/MS and other analyses) was necessary to capture short-term, hydrologically-driven variations in DOM concentration and composition. A unique DOM signature, enriched in aliphatic, and potentially biolabile DOM, was observed at the onset of discharge events. During peak discharge, and along the falling limb of the hydrograph, an aromatic, vascular plant and/or soil-derived DOM signature was more prevalent. These initial findings support the pulse-shunt hypothesis, providing evidence for the release of labile forms of DOM into rivers during the onset of storm events, which apparently persists across low-to-high stream orders. Insights into the molecular hysteresis of fluvial DOM spotlights the impact of watershed hydrology on biogeochemical cycling in river networks and coastal regions.