Integrating Ontogenetic Shift, Growth and Mortality to Determine a Species' Ecological Role from Isotopic Signatures

<div><p>Understanding species linkages and energy transfer is a basic goal underlying any attempt at ecosystem analysis. Although the first food-web studies were based on gut contents of captured specimens, the assessment of stable isotopes, mainly <i>δ¹³C</i> and <i>δ¹⁵N</i>, has become a standard methodology for wide-range analyses in the last 30 years. Stable isotopes provide information on the trophic level of species, food-web length, and origin of organic matter ingested by consumers. In this study, we analyzed the ontogenetic variability of <i>δ¹³C</i> and <i>δ¹⁵N</i> obtained from samples of three Neotropical fish species: silver sardine (<i>Lycengraulis grossidens</i>, <i>n</i>=46), white lambari (<i>Cyanocharax alburnus</i>, <i>n</i>= 26), and the red-tail lambari (<i>Astyanax fasciatus</i>, <i>n</i>=23) in Pinguela Lagoon, southern Brazil. We developed a new metric, called the Weighted Isotopic Signature (<i>φ¹⁵N</i> or <i>φ¹³C</i>, ‰), that incorporates ontogenetic variability, body growth, and natural mortality into a single number.</p>