Scaling up DOM dynamics from the small catchment to a mesoscale basin

Small catchments make up the majority of the landscape. We show here how more than two decades of study of terrestrial to aquatic movement of dissolved organic matter (DOM) at the small catchment scale helps to interpret DOM dynamics in higher order rivers downstream. The 41-ha forested W-9 catchment at Sleepers River Research Watershed lies in the headwaters of the Connecticut River basin in northeastern Vermont. DOM increases with flow, from ~1 mg-C/L at base flow to nearly 20 mg-C/L at the highest flows. Counterclockwise hysteresis in the DOM-discharge relation during hydrologic events suggests that DOM moves slowly relative to water, but that its flux is maintained by post-event elevated groundwater tables. Based on SUVA-254, DOM aromaticity increases along with concentration, consistent with flushing from more surficial organic-rich soil horizons. At high flows this DOM is shunted downstream to higher order streams before it is subject to transformation post-event. The degree of DOM counterclockwise hysteresis increases with basin scale all the way to the mouth of the Connecticut River. At larger scales, long travel times for DOM sourced from the abundant wetlands in the northern part of the basin enhance DOM hysteresis.