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Quantifying spatial variation in isotopic baselines reveals size-based feeding in a model estuarine predator: implications for trophic studies in dynamic ecotones

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Abstract

Nitrogen stable isotopes (δ15N) are typically used to estimate trophic position, providing insight into ecological roles and broader food web structure. Ecological inferences drawn from these estimates rely on quantification of isotopic baselines, i.e., low trophic level organisms reflecting the predominant nitrogen sources that support food web biomass. When baselines vary due to environmental (e.g., temperature) or anthropogenic factors (e.g., nutrient run-off), interpretation of trophic position based on δ15N may not be ecologically sound. Here, we tested the effects of assuming stable versus spatially variable δ15N baselines used to estimate the trophic position of a cosmopolitan estuarine predator—juvenile bull sharks (Carcharhinus leucas). Sampling across the San Antonio Bay system, TX, USA revealed that baseline consumers exhibited spatially variable δ15N values, which were strongly associated with capture salinity representing the influence of anthropogenically introduced nitrogen largely from fluvial run-off. As a result, estimates of bull shark trophic position differed based on baseline assumptions—sharks exhibited an ontogenetic shift in trophic position when spatial variability of baseline δ15N was accounted for, while an uncorrected approach indicated no relationship between body size and trophic position. Diet data supported ontogenetic shifts in bull shark diets, with increased consumption of larger-bodied prey among older individuals. Evaluation of isotopic baselines in spatially dynamic ecosystems like estuaries is essential, especially for highly mobile species like sharks that traverse dynamic isoscapes. A literature review revealed that only 16% of studies leveraging stable isotopes to assess the trophic ecology of sharks have accounted for potential spatial variability of isotopic baselines. As such, greater consideration of variability in isotopic baselines is important moving forward considering the ubiquitous application of this technique by ecologists.

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Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

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Acknowledgements

Thanks to staff from TPWD Coastal Fisheries Port O’Connor Laboratory for providing specimens, and Matt Hamilton for dissecting specimens. Thanks to David Wells for logistical support.

Funding

Funding was provided by the United States Fish and Wildlife Service through Texas Parks and Wildlife Department State Wildlife Grant program (TX-T-177-R-1).

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  1. Marine Biology Department, Texas A & M University at Galveston, 1001 Texas Clipper Rd, Galveston, TX, 77553, USA

    Philip Matich

  2. Department of Biology, University of New Mexico, Albuquerque, NM, 87131, USA

    Oliver N. Shipley

  3. PSL Research University, EPHE-UPVD-CNRS, USR 3278 CRIOBE, Université de Perpignan, 66860, Perpignan, France

    Ornella C. Weideli

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PM designed the study, acquired necessary funding, collected and analyzed samples and data, and wrote the manuscript; ONS analyzed data and wrote the manuscript; OCW wrote the manuscript.

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Correspondence to Philip Matich.

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Matich, P., Shipley, O.N. & Weideli, O.C. Quantifying spatial variation in isotopic baselines reveals size-based feeding in a model estuarine predator: implications for trophic studies in dynamic ecotones. Mar Biol 168, 108 (2021). https://doi.org/10.1007/s00227-021-03920-0

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