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Published March 1, 2018 | Supplemental Material
Journal Article Open

Nitrogen isotopic analysis of carbonate-bound organic matter in modern and fossil fish otoliths

Lueders-Dumont, Jessica A.
Wang, Xingchen T.
Jensen, Olaf P.
Sigman, Daniel M. ORCID icon
Ward, Bess B.

Abstract

The nitrogen isotopic composition (δ^(15)N) of otolith-bound organic matter (OM) is a potential source of information on dietary history of bony fishes. In contrast to the δ^(15)N of white muscle tissue, the most commonly used tissue for ecological studies, the δ^(15)N of otolith-bound OM (δ^(15)N_(oto)) provides a record of whole life history. More importantly, δ^(15)N_(oto) can be measured in contexts where tissue is not available, for example, in otolith archives and sedimentary deposits. The utility and robustness of otolith δ^(15)N analysis was heretofore limited by the low N content of otoliths, which precluded the routine measurement of individual otoliths as well as the thorough cleaning of otolith material prior to analysis. Here, we introduce a new method based on oxidation to nitrate followed by bacterial conversion to N_2O. The method requires 200-fold less N compared to traditional combustion approaches, allowing for thorough pre-cleaning and replicated analysis of individual otoliths of nearly any size. Long term precision of δ^(15)N_(oto) is 0.3‰. Using an internal standard of Atlantic cod (Gadus morhua) otoliths, we examine the parameters of the oxidative cleaning step with regard to oxidant (potassium persulfate and sodium hypochlorite), temperature, and time. We also report initial results that verify the usefulness of δ^(15)N_(oto) for ecological studies. For three salmonid species, left and right otoliths from the same fish are indistinguishable. We find that the δ^(15)N_(oto) of pink salmon (Oncorhynchus gorbuscha) is related to the size of the fish for this species. We find that intra-cohort δ^(15)N_(oto) standard deviation for wild pink salmon, farmed brown trout (Salmo trutta), and farmed rainbow trout (Oncorhynchus mykiss) are all 0.4‰ or less, suggesting that δ^(15)N_(oto) will be valuable for population-level studies. Lastly, our protocol yields reproducible data for both δ^(15)N_(oto) and otolith N content in 17th century Atlantic cod otoliths. We find that 17th century cod are approximately 2 ‰ higher than modern cod, arguably consistent with either the larger size of the otoliths (and thus inferred for the fish) or with changes in baseline (primary producer) δ^(15)N in the modern coastal ocean compared to the past. All told, the results of this study bode well for the utility of otolith-bound δ15N for investigating the environment and ecology of modern and past fish.

Additional Information

© 2018 Elsevier Ltd. Received 13 April 2017, Accepted 2 January 2018, Available online 10 January 2018. We thank Alexa Weigand and Sergey Oleynik for their technical expertise and support in the laboratory. We thank Sophia Myers, who assisted with N isotopic analysis of cod muscle tissue. We thank Professors Nathan Hamilton of the University of Southern Maine and Beverly Johnson of Bates College for providing midden mound otoliths and historical context and we thank Karin Limburg of the College of Environmental Science and Forestry, State University of New York, for comments on an earlier version of the manuscript. We thank Elena Fernandez at the Alaska Department of Fish & Game, in Cordova, Alaska, for providing pink salmon otoliths. We thank Eric Robillard and Sandy Sutherland at the Fishery Biology Program at the NOAA Fisheries Northeast Fisheries Science Center for providing Georges Bank cod otoliths. We thank Steve Eddy and Melissa Malmstedt at the University of Maine Center for Cooperative Aquaculture Research in Franklin, Maine, for providing farm raised cod. We thank Musky Fish Hatchery, Asbury, NJ, for providing farm raised rainbow and brown trout. Additionally, we thank two fish markets: Nassau Seafood, Princeton, NJ and Metropolitan Seafood, Lebanon, NJ, for providing fish from which otoliths were extracted. We thank William Hoffman and Micah Dean at the Massachusetts Division of Marine Fisheries for providing cod otolith weights and cod lengths from the western Gulf of Maine. We also thank two anonymous reviewers for their insightful comments. All research was conducted in accordance with the Princeton University Animal Care and Use protocol (IACUC #1995A-14). This work was supported by the Scott Fund for vertebrate paleontology of the Princeton University Department of Geosciences, the Grand Challenges Program of Princeton University, and the US NSF through grants OCE-1136345 (to BBW and DMS), and OCE-1060947 (to DMS). Author contributions: JLD, BBW, DMS, XTW, OPJ designed experiments. JLD conducted experiments. JLD conducted statistical analyses. JLD, BBW, DMS, XTW, OPJ interpreted data. JLD, DMS, and BBW wrote the paper.

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August 21, 2023
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