Tracing Organic Matter in the Stream of a First-Order Catchment

An improved understanding of the processes by which dissolved organic matter (DOM) enters a stream ecosystem may help explain the trends of increasing DOM concentrations in many northern latitude rivers. To this end, the water-soluble organic matter (WSOM) in the soil and the DOM in the stream of a first-order catchment were studied within the Boulder Creek Critical Zone Observatory. Several years of data showed noticeable differences in the chemical character of the WSOM and DOM, most extreme in soil samples from plots in the riparian zone. A tracer study was performed to understand how soil organic matter may be processed within a stream to alter its chemical character. 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 and after the 45 minute continuous rate 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 transient 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. 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.