02203nas a2200217   4500008004100000245008900041210006900130260001200199300001100211490000800222520155200230653001501782653002601797653002301823653002101846653001801867100001301885700002401898700001701922856004601939       2010                        eng d00aEffect of unsteady flow on nitrate loss in an oligotrophic, glacial meltwater stream0 aEffect of unsteady flow on nitrate loss in an oligotrophic glaci  c03/2010  aG010010 v1153 a<p>The McMurdo Dry Valleys of Antarctica are among the coldest, driest ecosystems on Earth. During the austral summer, glacial meltwater supports cyanobacterial mat communities in some streams, but they are not ubiquitous. We conducted a nitrate (NO<sub>3</sub><sup>-</sup>) enrichment tracer injection in Huey Creek to quantify NO<sub>3</sub><sup>-</sup> loss in a Dry Valley stream where algal mats would not obscure hyporheic microbial processes. Unsteady streamflow led to diel variability in the tracer concentration and in surface/subsurface water and solute exchange. Subsequently, concentrations of NO<sub>3</sub><sup>-</sup>, nitrite (NO<sub>2</sub><sup>-</sup>), ammonium (NH<sub>4</sub><sup>+</sup>), and dissolved organic carbon (DOC) varied significantly during the injection, with a net loss of NO<sub>3</sub><sup>-</sup>, NO<sub>2</sub><sup>-</sup>, and DOC, and production of nitrous oxide. These mass changes within a reach were often coincident with high streamflows. Reactivity also coincided with the highest DOC concentrations, suggesting that DOC is the primary limitation to heterotrophic microbial activity in the stream. Together, streamflow and DOC availability create the hot spots and hot moments that dominate NO<sub>3</sub><sup>-</sup> reactivity and removal in this polar desert ecosystem. The combination of spatially and temporally variable hyporheic dynamics and solute availability underscore the limitations of common nutrient uptake metrics and transient storage models when unsteady flow conditions exist.</p>10aAntarctica10ahot spots/hot moments10ahyporheic exchange10anitrogen cycling10aunsteady flow1 aKoch, J.1 aMcKnight, Diane, M.1 aBaeseman, J.  uhttp://doi.wiley.com/10.1029/2009JG001030