USE OF HIGH TEMPORAL AND SPATIAL RESOLUTION DATA TO EXAMINE HYDROLOGIC CONTROLS ON DISSOLVED ORGANIC MATTER AND MAJOR ION VARIABILITY IN NINE NESTED WATERSHEDS WITHIN THE PASSUMPSIC RIVER WATERSHED, VT, USA

Dissolved organic matter (DOM) is an important biochemical variable in freshwater ecology. It drives ecosystem respiration, transports pollutants, impacts drinking water treatment, and is an integral component of carbon cycling on the planet. Streams link the terrestrial sources of DOM to downstream river networks. DOM concentrations in forested, headwater streams are low during base flow, but increase during flood events, which are expected to occur with greater intensity and frequency as the Earth warms. The goal of this project is to examine the relationship between hydrological and DOM variability across event and seasonal temporal scales in catchments ranging from headwater streams to large rivers. This work is a key step to further elucidate processes controlling DOM flux in fluvial networks and the subsequent response to changing flow regimes.

We analyzed high frequency time series data collected over two years from nine sondes installed in first- to fifth-order streams within the Passumpsic River Watershed, a 1,130 square kilometer largely forested watershed located in northeastern Vermont. The sondes measured fDOM, turbidity, pH, specific conductivity, dissolved oxygen, and temperature at 15 minute intervals. Measurements of fDOM were corrected for turbidity, temperature, and inner-filter effects. Automated samplers were co-located with four of the sondes to collect high frequency grab samples during hydrologic events. Samples were analyzed for DOC concentration and major anions and cations, among other analytes.

For each site, we explored seasonal relationships among DOM, major-ion concentrations, specific conductivity, and discharge. We also delineated individual hydrologic events and characterized hysteresis between solute concentrations and stream discharge for more than 800 unique events across nine sites. In general, DOM concentrations increased with stream discharge while specific conductivity decreased, despite a wide range of median concentrations of DOM and specific conductivity across sites. Initial results indicate that DOM and major ion concentrations are influenced by watershed land cover, specifically wetland extent, and that seasonality, event type, and antecedent moisture conditions are critical in regulating DOM and major ion response to increased streamflow.