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Retention of Lake-derived Nitrogen in Arctic Streams with Different Geomorphic Settings Using a 15N Stable Isotope Tracer

UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
Lindsey Diane Pollard (Creator)
Institution
The University of North Carolina at Greensboro (UNCG )
Web Site: http://library.uncg.edu/
Advisor
Anne Hershey

Abstract: Retention of lake-derived nitrogen (N) was studied in four geomorphically distinct lake outlets located in arctic Alaska. 15N ammonium chloride tracer was experimentally added to Lakes NE-12, GTH 99, GTH 153 and GTH 114 at the beginning of the arctic summer each year from 2005 to 2008 and monitored in Carex, the dominant riparian plant, fine benthic organic matter (FBOM) and seston along the outlet streams. In this study, samples from each compartment were taken pre-enrichment and post-enrichment 2008, then processed and analyzed for δ15N. Pre-enrichment samples measured N retained from 15N enrichment in summers of 2005-2007, and differences between pre-enrichment and post-enrichment samples measured mobility of N over the summer season. A similar decrease in δ15N values of pre- and post-enrichment Carex samples in all lake outlets suggested that N supporting biomass in the 2008 season was derived from previous enrichment seasons. Differences in pre- and post-enrichment δ15N values for FBOM and seston in three of the four outlets suggest mobile particulate matter was only retained during the summer. Tight coupling of FBOM and seston was observed in GTH 99 outlet where both compartments showed similar patterns of enrichment. Lateral retention of 15N was detected up to 2 m from the main channel of GTH 114 outlet. Higher gradient streams had a pronounced difference in longitudinal patterns of FBOM and seston, which was not found in lower gradient streams. However, higher amounts of previously supplied 15N were found in lower gradient streams. Influence of discharge on uptake length (SW) was analyzed for each sampling time period in compartments with at least three significant SW. Discharge did not have a significant effect on SW of pre-enrichment Carex, or post-enrichment FBOM and seston. Exponential regression slopes of uptake from each lake outlet were compared. Results indicated a significant difference between Lake GTH 153 outlet, a beaded stream, and Lake GTH 114 outlet, a meandering wetland stream. These data suggest that differences in stream geomorphology affect temporal dynamics and compartments for N retention. Results from this study give evidence to the importance of arctic lake outlets as N sinks and that geomorphic setting determines the efficiency of N retention.

Additional Information

Publication
Thesis
Language: English
Date: 2010
Keywords
Biology, Limnology
Subjects
Rivers $z Arctic regions.
Nitrogen $x Research.
Stable isotope tracers $x Analysis.
Ammonium compounds.