Mountain Stream Ecosystem Metabolism and Nitrogen Cycling Responses to Hydroclimatic Volatility

Abstract

Changing hydroclimatic conditions increase streamflow variability and add uncertainty to predictions of watershed nutrient export and ecosystem function. Mountain headwater streams are especially sensitive to climate shifts due to their dependence on seasonal snowpack. We explored how changes in mountain precipitation affect the synchrony between hydrologic processes (peak flow and baseflow onset) and biological processes (metabolism and nutrient uptake) that control nitrogen dynamics. We monitored two mountain streams in the Lake Tahoe basin (California and Nevada, USA)—a larger open‐canopy stream and a smaller, shaded stream—over two dry water years and one substantially wetter water year. On average, the large stream received more annual light, was warmer, and had higher inorganic nitrogen demand compared to the small stream. From dry to wet years, the onset of baseflow was delayed by ∼40 days in both streams, shortening the optimal window for benthic algae growth by over a month and substantially reducing the potential for in stream nitrogen uptake. Concurrently, cumulative annual gross primary production declined by 86% (Δ −248 g O 2 m −2  yr −1 ) in the large stream and 93% (Δ −26 g O 2 m −2  yr −1 ) in the small stream. Mean daily ammonium supply increased by 47% (Δ + 0.01 g m −2  d −1 ) in the large stream and by 112% (Δ + 0.16 g m −2  d −1 ) in the small stream. These findings highlight the critical role of streams in regulating nutrient dynamics and show that even neighboring or hydrologically similar streams can exhibit distinct biogeochemical responses to hydroclimatic variability.