RCCZO -- Nutrient Fluxes, Stream Water Chemistry -- Stream Drying, Wildfire, Surface Water Chemistry -- Reynolds Creek Experimental Watershed -- (2016-2016)

Stream drying and wildfire are projected to increase with climate change in the western United States, and both are likely to influence the patterns and processes explaining stream chemistry. To investigate drying and wildfire effects on stream chemistry (carbon, nutrients, anions, cations, and isotopes), we examined seasonal drying in two intermittent streams in southwestern Idaho, one stream that was unburned and one that burned six months prior. We hypothesized that spatiotemporal patterns of stream chemistry would change due to increased evaporation, groundwater dominance, and autochthonous carbon production as water and carbon sources shifted from snowmelt to low flow conditions. With increased nutrients and sunlight available, we expected greater shifts in the burned stream. To capture spatial stream chemistry patterns, we sampled surface water for a suite of analytes in each stream longitudinally with a high spatial scope (50-meter intervals along ~2500 meters). To capture temporal variation during drying, we sampled each stream in April, May, and June (2016). Patterns and processes influencing stream chemistry were generally similar in both streams, but some were amplified in the burned stream. Mean dissolved inorganic carbon (DIC) concentrations increased with drying by 22% in the unburned and by 3-fold in the burned stream. In contrast, mean total nitrogen (TN) concentrations decreased in both streams, with a 16% TN decrease (mostly DON) in the unburned stream and a 5-fold TN decrease (mostly nitrate) in the burned stream. Contrary to expectations, dissolved organic carbon (DOC) concentrations were longitudinally variable but relatively less temporally variant. In addition, we found weak evidence for evapoconcentration with drying. However, consistent with our expectations, both water isotopes and strontium-DIC ratios indicated stream water shifted towards groundwater-dominance, especially in the burned stream. Fluorescence and absorbance measurements showed considerable longitudinal variation in DOC sourcing with seasonal drying in both streams, and temporal shifts from autochthonous to allochthonous carbon sources in the burned stream. Our findings suggest that the effects of fire may magnify chemistry patterns but not the controls with stream drying. This empirical study contributes to advancing our conceptual stream models that incorporate stream drying, wildfire, and the interplay between them.