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Title: Comparing compound-specific and bulk stable nitrogen isotope trophic discrimination factors across multiple freshwater fish species and diets

Author
item BLANKE, CHELSEY - University Of Wisconsin
item CHIKARAISHI, YOSHITO - Japan Agency For Marine-Earth Science And Technology (JAMSTEC)
item TAKIZAWA, YUKO - Hokkaido University
item Steffan, Shawn
item DHARAMPAL, PRARTHANA - University Of Wisconsin
item VANDER ZANDEN, M - University Of Wisconsin

Submitted to: Canadian Journal of Fisheries and Aquatic Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2016
Publication Date: 1/16/2017
Publication URL: http://handle.nal.usda.gov/10113/5740426
Citation: Blanke, C.M., Chikaraishi, Y., Takizawa, Y., Steffan, S.A., Dharampal, P.S., Vander Zanden, M.J. 2017. Comparing compound-specific and bulk stable nitrogen isotope trophic discrimination factors across multiple freshwater fish species and diets. Canadian Journal of Fisheries and Aquatic Sciences. 74(8):1291-1297. doi: 10.1139/cjfas-2016-0420.

Interpretive Summary: Estimation of animal trophic position allows for analysis of individual- or population-scale trophic tendency, as well as community-scale assessments of functional diversity. Before a verifiably accurate trophic position estimate can be generated for a wild, free-ranging individual or a population, the degree of intertrophic enrichment must be established for that species. Such intertrophic enrichment is referred to as the “trophic discrimination factor,” and this parameter is a key element of isotope ecology. Our paper reports remarkable consistency in this parameter across four species of freshwater fish. Further, the degree of trophic enrichment was relatively consistent across diets. By examining the main effects and interactive effects of consumer identity and diet, our results are generalizable among freshwater fish taxa in the Great Lakes. Impact: Altogether, these findings substantiate the use of a single, consistent trophic discrimination factor for estimation of trophic position among wild, free-roaming consumers. It is extremely important that the degree of enrichment be relatively constant among species, habitats, and ecosystems, because most consumer groups tend to be highly mobile, consuming resources from a wide, heterogeneous landscape and/or seascape.

Technical Abstract: The use of nitrogen stable isotopes for estimation of animal trophic position has become an indispensable approach in food web ecology. Compound-specific isotope analysis of amino acids is a new approach for estimating trophic position that may overcome key issues associated with nitrogen stable isotope analysis of bulk tissue, including definition of the isotopic baseline and uncertainties associated with trophic discrimination factors (TDF). There is a need for assessment of amino acid trophic discrimination across various ecosystems, species, and diets. We addressed this gap by conducting a laboratory experiment to characterize the variation in compound-specific (TDFCS) and bulk (TDFBulk) trophic discrimination factors of four freshwater fish species fed on three separate, homogeneous diets of known isotopic composition, spanning a range of trophic positions. In addition, we compared TDFCS of fish muscle and scale to evaluate the viability of scales as a proxy for whole-body tissue analyses. Mean TDFBulk was 2.24 ± 0.9 (±1 SD) and there were significant interactive effects of species and diet on TDFBulk (p < 0.001). Mean TDFCS in experimental fish muscle was 6.93 ± 0.78‰ (± 1 SD) and was significantly affected by diet (p = 0.005), decreasing with diet trophic position and lipid content (p = 0.002 and p = 0.007, respectively). Additionally, TDFCS from fish scales were not significantly different from those of muscle tissue (p = 0.68), suggesting that fish scales are a suitable tissue for making food web inferences. We illustrate the advantages of estimating trophic position using compound-specific stable isotope analysis as well as the need for continued investigation of trends in trophic discrimination variation.