Using carbon, nitrogen, sulfur, and mercury isotopes to trace elasmobranch foraging habitats in contrasting biogeochemical environments

Abstract

Stable isotopes are well established as routine and reliable tracers of nutrient flux and trophic structure. However, inferring trophic ecology from isotopic data is challenging due to variability at the food web base and systematic differences in biochemical fractionation during metabolism. Analyses of isotope systems from multiple elements with contrasting fractionation drivers may resolve some sources of variance, strengthening connections between measured isotopic variations and inferred ecological processes. This study combines carbon (δ ¹³C), nitrogen (δ ¹⁵N), sulfur (δ ³⁴S), and mercury (Δ ¹⁹⁹Hg/δ ²⁰²Hg) isotopes to investigate trophic niches of coastal and oceanic elasmobranchs across two ecosystems in northwestern Mexico. In the Pacific Ocean, similar δ ¹³C, δ ¹⁵N, Δ ¹⁹⁹Hg, and δ ²⁰²Hg values suggest that elasmobranchs relied on common pelagic resources, likely from upwelling events. In the Gulf of California, coastal species with higher δ ¹³C and δ ¹⁵N values and lower Δ ¹⁹⁹Hg and δ ²⁰²Hg values fed on prey isotopically distinct from those offshore, allowing classification trees to identify foraging habitats more accurately than in the Pacific. Meanwhile, δ ³⁴S values systematically decreased from oceanic to coastal species at both sites and were highlighted as the most discriminative isotopic tracer by random forests. This study advocates for integrating complementary isotopic analyses to better comprehend biogeochemical and ecological mechanisms.

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