Climatological and elevational controls on organic and inorganic nutrients in stream waters, Boulder Creek watershed, Colorado Front Range .

摘要

Due to short growing seasons, sparse vegetation, thin soils, and a harsh climate, the biota of high elevation ecosystems throughout the Colorado Front Range are sensitive to disturbance. These ecosystems are subject to two major drivers of environmental change: directional changes in climate and increasing inorganic nitrogen (N) deposition in wetfall. By analyzing long term climate records and water chemistry data, we investigate the role of climate and N deposition in determining nitrate export from Green Lakes Valley in the Colorado Front Range. We found that streamwater nitrate concentrations observed at the Green Lakes 4 (GL4) outlet have significantly increased by 0.27 mueq L-1 yr-1 between 1985 and 2009. In contrast, there was not an increasing nitrate trend at the subalpine site. Comparing solute chemistry before and after the regional drought that began in 2000, we show a 114% increase in sulfate concentrations, an 82% increase in calcium and a 42% increase in silica concentrations at GL4, suggesting that flowpaths switch towards an increasing contribution from rock glacier melt during dry years. Simple mass balance models indicate that there is a net gain of nitrate between the two highest elevation sites of approximately 0.22 moles NO 3- m-1 d-1, suggesting high nitrification potential in the recently exposed alpine soils. These findings confound emission policies and associated water quality improvement efforts, as climate change and cryosphere melt may affect alpine nitrate concentrations as much, or more than atmospheric deposition trends.;The Landscape Continuum Model (LCM) is a conceptual framework for how mountain ecosystems accumulate and redistribute exogenous and endogenous materials, emphasizing the importance of transport processes and redeposition of nutrients and water. This study tests the LCM by comparing changes in organic and inorganic nutrients in stream waters of headwater catchments along an elevational gradient in the Colorado Front Range. Water samples were simultaneously collected at four gauged headwater catchments, ranging in elevation from 1830 to 3500 m, and analyzed for nitrogen, carbon, and base cation chemistry. Weathering rates, as measured by dissolved silica concentrations, decreased exponentially with increasing elevation, but the relative contributions of different primary materials to the solute load did not change systematically with elevation. We report a strong shift from a dominance of inorganic nutrients in the alpine to greater organic contributions below treeline as indicated by the decreasing nitrate concentrations (21.36 to 0.85 mueqL-1) and increasing dissolved organic carbon concentrations (0.36 to 10.41 mg C L-1) with decreasing elevation. Solute yields did not demonstrate similar patterns, as local geomorphic and climate characteristics confounded general elevation trends. This study expands the scope of the original LCM to include elevation, climate, and geomorphic controls that, in turn, drive the landcover characteristics that account for the difference in solute concentrations and fluxes. The updated conceptual model also provides a useful framework to assess the implications of future disturbances such as mountain pine beetle infestation, increased N deposition and warming air temperatures.