Quantifying Nitrate Uptake In An Unsteady, Anabranching, Antarctic Stream
Koch, Joshua C 1 ; McKnight, Diane M 2 ; Neupauer, Roseanna 3 ; Baeseman, Jennifer 4 ; Gooseff, Michael 5 ; Rajagopalan, Balaji 6
1 University of ÃÛÌÇÖ±²¥
2 University of ÃÛÌÇÖ±²¥
3 University of ÃÛÌÇÖ±²¥
4 Kent State University
5 ÃÛÌÇÖ±²¥ School of Mines
6 University of ÃÛÌÇÖ±²¥
In the McMurdo Dry Valleys of Antarctica, biotic uptake limits nitrogen transport in glacial meltwater streams containing abundant cyanobacterial mats. Nitrogen is delivered to lakes predominantly from streams with minimal mat growth, such as Huey Creek. We injected nitrate into Huey Creek to quantify nitrogen uptake in a mat-less stream. Tracer data is typically analyzed with a transient storage model (TSM), which accounts for changes in tracer concentration by modeling storage, lateral inflows, and biological processes. Daily flood pulses in Huey Creek lead to temporal and spatial variations in hyporheic exchange, storage, and nutrient decay, precluding use of a TSM. Instead, we use a groundwater flow model and Principle Component Analyses (PCA) to explore hydrology and biology in Huey Creek. The groundwater model identifies a second storage zone besides the hyporheic zone and suggests significant down-gradient transport of the stored water. The groundwater flow model agrees with tracer breakthrough curves, suggesting a residence time in this second storage zone of approximately 30 hours – two orders of magnitude greater than hyporheic storage measured using a TSM at low flow conditions in Huey Creek (Runkel et al. 1998). Coupled PCAs identify gaining and losing stream reaches consistent with the groundwater model, isolate biological uptake from nitrate variations caused by streamflow variability and tracer dilution, and divide uptake into a flow-induced and flow-independent fraction. Results suggest that the bacteria require approximately 15 hours to fully respond to the nitrate enrichment, and hint that the large storage zone may exert control on storage and uptake of nitrate in Huey Creek.
Runkel R. L., D. M. McKnight, and E. D. Andrews. 1998. Analysis of transient storage subject to unsteady flow: Diel flow variations in an Antarctic stream. J. N. Am. Benthol. Soc 17: 143-154.