Abstract
Studies of trophic-level material and energy transfers are central to ecology. The use of isotopic tracers has now made it possible to measure trophic transfer efficiencies of important nutrients and to better understand how these materials move through food webs. We analyzed data from thirteen 15N-ammonium tracer addition experiments to quantify N transfer from basal resources to animals in headwater streams with varying physical, chemical, and biological features. N transfer efficiencies from primary uptake compartments (PUCs; heterotrophic microorganisms and primary producers) to primary consumers was lower (mean 11.5%, range <1% to 43%) than N transfer efficiencies from primary consumers to predators (mean 80%, range 5% to >100%). Total N transferred (as a rate) was greater in streams with open compared to closed canopies and overall N transfer efficiency generally followed a similar pattern, although was not statistically significant. We used principal component analysis to condense a suite of site characteristics into two environmental components. Total N uptake rates among trophic levels were best predicted by the component that was correlated with latitude, DIN:SRP, GPP:ER, and percent canopy cover. N transfer efficiency did not respond consistently to environmental variables. Our results suggest that canopy cover influences N movement through stream food webs because light availability and primary production facilitate N transfer to higher trophic levels.
Original language | English |
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Pages (from-to) | 3044-3055 |
Number of pages | 12 |
Journal | Ecology |
Volume | 98 |
Issue number | 12 |
DOIs | |
State | Published - Dec 2017 |
Funding
We thank everyone who participated in the individual tracer experiments used in this analysis. We are grateful for the leadership and friendship of the late Pat Mulholland, whose legacy continues to inspire. This manuscript is the product of a workshop funded by a National Science Foundation grant (NSF-DEB 1052399) to M. R. Whiles and W. K. Dodds. Partial support during manuscript preparation to N. A. Griffiths was from the Department of Energy's Office of Science, Biological and Environmental Research. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. DOE under contract DE-AC05-00OR22725. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. S. M. Collins was supported by a National Science Foundation Postdoctoral Research Fellowship in Biology (DBI-1401954).
Keywords
- N
- food chain efficiency
- food webs
- isotope tracer experiment
- nitrogen
- stream