Author: Robert T Hensley,  Aquatic Biogeochemist (NEON)

Description:
The National Ecological Observatory Network (NEON) is a continental scale network of ecological monitoring sites distributed across 19 ecoclimatic domains.  We present sensor-based water chemistry data from two NEON sites in the Southern Rocky Mountains domain, Como Creek and West St. Louis Creek.  These snow melt driven streams are located around 3,000 m (10,000 ft) elevation, making them among the highest instrumented streams in the U.S.  Spring runoff typically begins in April and peaks in June, though the exact timing varies from year-to-year based on accumulated snowpack.  In both streams conductivity is strongly inversely correlated with discharge with minimal hysteresis, suggesting a strictly dilution response.  In contrast, nitrate (NO₃-N) and fluorescent dissolved organic matter (fDOM) peak during the initial onset of snowmelt, producing clockwise C-Q hysteresis over seasonal time scales.  This “first flush” response potentially indicates mobilization of nutrient and carbon stores built up over winter.  However, over shorter time scales (i.e. during pulses within the pulse), NO₃-N and fDOM lag discharge, producing smaller counterclockwise hysteresis within the larger clockwise seasonal loops.  This suggests that while easily mobilized, stores may not be limited to immediately adjacent riparian zones, and require time to reach the streams.  Finally, we observed higher rates of stream metabolism during the receding limb of the snowmelt pulse, likely a response to warmer temperatures. This raises the possibility that the lower concentrations observed during this period may also partly be due to increased biological processing (in the stream itself but also in the contributing area).  Given that approximately ¾ of streamflow in the western U.S. derives from snowmelt, understanding the biogeochemical dynamics of melt pulses is of critical importance, particularly in the face of warming winter temperatures and reduced snowpack.

Category: Scientific Program Abstract > Special Session > SS11 Exploring watersheds under a changing climate via storm-driven solute and sediment retention, transport, and export dynamics