Gabor, Rachel S听1听;听McKnight, Diane M听2
1听University of Colorado Boulder
2听University of Colorado Boulder
Concentrations of DOM have been increasing in many northern latitudes and many hypotheses for this phenomenon suggest that changes in the terrestrial landscape and hydrologic patterns are affecting aquatic systems1. An improved understanding of how terrestrial organic matter enters, and is transformed within, aquatic systems can help us understand the trend of increasing DOM and better predict future water quality dynamics. As part of the Boulder Creek Critical Zone Observatory, weekly samples of DOM in Boulder Creek as well as a first-order tributary were analyzed with fluorescence spectroscopy. In addition, water-soluble organic matter (WSOM) from the soil of the first-order catchment was also analyzed. This multi-year dataset showed a noticeable difference in the chemical character of the stream DOM and soil WSOM, most pronounced in samples from the riparian zone. A tracer study was performed to understand how soil organic matter is processed within a stream to alter its chemical character2. Chloride was used as a conservative tracer and freshly leached WSOM as a reactive tracer and concentrations were kept small enough to not significantly impact stream dynamics, while still being detectable above the background level. Samples were collected at 4 locations downstream during the 45 minute injection, with each location chosen after either a clear or clogged portion of the small alpine stream. The pulse of the conservative tracer, still detectable at the furthest site 85 meters from the injection location, displayed noticeable attenuation, indicating interaction with and temporary storage in the hyporheic zone. The DOM chemical signature from the reactive tracer, as measured with fluorescence spectroscopy, attenuated quickly, and in-stream processing of the WSOM appeared to almost completely remove the tracer signal within 35 m. Modeling with OTIS showed considerable differences between reaches which where clear of debris and reaches which were clear with plant debris. A second, single pulse, tracer study was repeated along a shorter stretch of stream and produced similar results. These results indicate that while the terrestrial environment may be a substantial source of stream DOM, in-stream processing can quickly change the chemical quality of that organic matter after it enters the aquatic system.