Meta-analysis of primary producer amino acid δ15N values and their influence on trophic position estimation. Academic Article uri icon

Abstract

  • 1. Compound-specific
    stable isotope analysis of individual amino acids (CSIA-AA)
    has emerged as a transformative approach to estimate consumer trophic positions
    (TPCSIA) that are internally indexed to primary producer nitrogen isotope
    baselines. Central to accurate TPCSIA e stimation is a n u nderstanding of b eta ( β)
    values—the
    differences between trophic and source AA δ15N values in the primary
    producers at the base of a consumers’ food web. Growing evidence suggests
    higher taxonomic and tissue-specific
    β value variability than typically appreciated.
    2. This meta-analysis
    fulfils a pressing need to comprehensively evaluate relevant
    sources of β value variability and its contribution to TPCSIA uncertainty. We first
    synthesized all published primary producer AA δ15N data to investigate ecologically
    relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode
    of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning
    AA δ15N and β value variability. Lastly, we evaluated the sensitivity of TPCSIA
    estimates to uncertainty in mean βGlx-Phe
    values and Glx-Phe
    trophic discrimination
    factors (TDFGlx-Phe).
    3. We show that variation in βGlx-Phe
    values is two times greater than previously considered,
    with degree of vascularization, not habitat type (terrestrial vs. aquatic),
    providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰;
    non-vascular
    autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily
    contributed to βGlx-Phe
    value variability, but we found no clear distinction
    among C3, C4 and CAM plant βGlx-Phe
    values. Notably, we found that vascular plant
    βGlx-Lys
    values (+2.5 ± 1.6‰) are considerably less variable than βGlx-Phe
    values,
    making Lys a useful AA tracer of primary production sources in terrestrial systems.
    Our multi-trophic
    level sensitivity analyses demonstrate that TPCSIA estimates are
    highly sensitive to changes in both βGlx-Phe
    and TDFGlx-Phe
    values but that the relative
    influence of β values dissipates at higher trophic levels.
    4. Our results highlight that primary producer β values are integral to accurate
    trophic position estimation. We outline four key recommendations for identifying,
    constraining and accounting for β value variability to improve TPCSIA estimation
    accuracy and precision moving forward. We must ultimately expand libraries of
    primary producer AA δ15N values to better understand the mechanistic drivers of
    β value variation.

Publication Date

  • 2021-06-01