Evaluating Controls on Nutrient Retention and Export in Wide and Narrow Valley Segments of a Mountain River Corridor.

Abstract: Over the past few decades, nitrate‐nitrogen (NO₃‐N) concentrations have increased within streams of the central Rockies, a pattern linked to regional N deposition trends. As NO₃‐N concentrations increase, in‐stream biological demand may become saturated and stream N export may increase. In mountain landscapes, streams generally flow through steep, narrow valleys with limited riparian area and strong stream‐hillslope connectivity. Interspersed between the narrow valleys are wide segments where substantial floodplain riparian areas can develop. Here, we coupled measures of stream reach NO₃‐N flux balances with nutrient enrichment experiments along two stream reaches of contrasting valley morphology in Rocky Mountain National Park. The stream reaches were (1) a narrow valley segment with limited floodplain riparian area and (2) a longitudinally adjacent (directly downstream) wide valley segment with extensive floodplain riparian area. We found that in‐stream biological uptake of added NO₃‐N was limited in both segments, presumably as a consequence of saturating conditions. Assessment of mass flux indicated that the narrow valley segment was a consistent source of water and NO₃‐N across flow states, while the wide segment was a sink at high flow and a source at low flow. Due to low in‐stream biological retention, gross gains and losses of water and NO₃‐N to and from the stream exerted primary constraint on segment mass balances. Our results suggest that the exchange of water and nutrients between the stream and adjacent landscape can exert strong control on reach‐scale nutrient export, particularly in streams experiencing or approaching N saturation. [ABSTRACT FROM AUTHOR]