Retention of lake-derived 15N in arctic streams in different landscape settings

We studied retention of lake-derived N during summer of 2007 in two geomorphically distinct lake outlets located in arctic Alaska.  ¹⁵N ammonium chloride tracer was experimentally added to Lakes GTH 153 and GTH 114 at the beginning of the arctic summer each year beginning in 2005, and monitored in FBOM and Carex, the dominant riparian plant, along the outlet streams.  In this study, samples of Carex and FBOM were taken pre-enrichment and post-enrichment 2007, then processed and analyzed for del ¹⁵N.  There was a general pattern of decreasing del ¹⁵N with increasing distance from the enrichment source in both compartments and both streams. In pre-enrichment compared to post-enrichment samples, del ¹⁵N was higher in FBOM in GTH 153 outlet, a beaded stream, and in Carex in GTH 114, a meandering wetland stream, suggesting that N derived from the previous growing season was more important to these retention compartments than that derived from the current growing season.  In contrast, del ¹⁵N in Carex in GTH 153 outlet and FBOM in GTH 114 outlet was higher in post-enrichment samples. These data suggest that differences in stream geomorphology affect temporal dynamics and compartments for N retention.