Hydrological effects on dissolved organic matter export from a temperate forested watershed across timescales from minute to season

Abstract:

Storm events are well known as the main driver mobilizing dissolved organic matter (DOM) from terrestrial landscape into fluvial networks. However, the effects of precipitation intensity, duration and frequency on the pattern of DOM export remain unclear. Furthermore, these effects are often compounded with seasonal variation in the sources and production of DOM, leading to changes in the composition and quality of event-exported DOM, while relevant data are limited. We have collected high-resolution (up to every 15 minutes), in-situ data of fluorescence DOM (fDOM) for one year, in tandem with simultaneous logging of water temperature, water level, and conductivity, in a third-order stream draining a temperate forested watershed. The fDOM data were calibrated against laboratory-measured dissolved organic carbon (DOC) concentrations of grab samples which were collected on an hourly to biweekly basis. Grab samples were also analyzed for DOM composition and quality, using excitation emission matrix fluorescence spectra and 28-day biodegradation incubations. Over the course of one year, we captured around 60 storm events, where we mostly observed increases in stream water DOC concentration accompanying rises in water discharge, with a counterclockwise hysteresis. Diurnal DOM variation was controlled primarily by evapotranspiration-driven changes in stream water level, as shown by that the stepwise linear regression model selected water level as the best predictor of DOC concentration (DOC concentration=-63.379* water level+95.309; r=0.732, P<0.001, n=48). We will further analyze and compare DOC exports across time scales ranging from event, month, to season. We predict that extreme storm events with unusual high precipitation intensity, duration and/or frequency can ultimately lead to the depletion of soil DOM and thus a DOC-discharge clockwise hysteresis, accompanied by increasing inputs in biorefractory compounds to streams. These data will be combined with a series of geochemical parameters including δD/δO, cation and nutrient to constrain the shifts in the sources and flow paths of DOM across event sizes and temporal scales.