SPATIOTEMPORAL CONTROLS ON STREAM NITRATE IN EAST RIVER, COLORADO

Nitrate (NO₃) is the dominant source of nitrogen in stream ecosystems and is an important nutrient for instream and riparian biota. In high-mountain watersheds where ecosystems are often NO₃ limited, changes in N deposition, weathering rates, and anthropogenic activity over the past decades are altering instream NO₃ concentrations. Numerous mechanisms control the spatial and temporal concentration and fluxes of NO₃ making it challenging to track sources and manage water quality. This study examines how NO₃ concentrations in the East River and its tributary, the Slate River, both headwater tributaries to the Colorado River, vary over time and space. Despite their geographic proximity, the streams have significant differences in NO₃ inputs; the East River has a lithologic source of N, while the Slate River lacks a lithological source and is directly impacted by anthropogenic inputs through tourism and agriculture in the town of Crested Butte. The variety and contrast between N sources in these streams make them ideal for studying NO₃ dynamics in montane systems, and thus provide a source of transferable knowledge. We find that average NO₃ concentrations in both streams have been decreasing over time driven by the decreases in atmospheric deposition. Further, we found that seasonal trends in concentration differ between the two streams; winter concentrations have been decreasing over time in the East yet remain stagnant in the Slate, and summer concentrations have been decreasing over time in the Slate yet remain stagnant in the East. This suggests that seasonal trends show greater variation in signals than long-term trends and differ between the two catchments. The winter trend in the Slate suggests a consistent input from anthropogenic activity such as tourism and could be augmented by decreased primary production, and the summer trend in the East suggests consistent input from its lithological N source. Overall, our findings suggest that changes in stream NO₃ in high-mountain watersheds are exhibited by different signals across time and space, meaning that temporal and spatial scales need to be considered when examining particular controls on stream NO₃, such as the effects of a warming climate.