Zooplankton stable-isotope patterns in a coastal system reshaped by a recent volcanic eruption

Coastal volcanic eruptions can profoundly modify nearshore geomorphology and physical conditions, yet post-eruption pelagic food-web structure remains poorly documented. Here, we characterize zooplankton stable-isotope patterns (δ 13 C, δ 15 N, δ 34 S) and hydrodynamic context in a coastal system adjacent to lava deltas formed during the 2021 Tajogaite eruption (La Palma, Canary Islands). Zooplankton were sampled 15 and 20 months after the eruption across seasons, depths, and size fractions, and isotopic signatures were analysed in combination with oceanographic observations and Lagrangian particle-tracking simulations. Isotopic values differed consistently among zooplankton size fractions, with larger fractions exhibiting higher δ 15 N and broader isotopic ranges than smaller fractions, reflecting trophic and taxonomic heterogeneity. δ 34 S values fell within the range commonly associated with marine production and were relatively uniform across depths and seasons, within the range expected for coastal marine sulfur sources. Hydrodynamic simulations indicate short residence times for individual particles within the lava-delta area, while also highlighting spatial variability in nearshore retention associated with post-eruption coastal morphology. Together, these results provide a detailed post-eruption description of pelagic isotopic structure and physical context in a recently reconfigured coastal system. Despite substantial geomorphological disturbance, the observed isotopic patterns suggest that pelagic trophic structure remains primarily governed by size-based ecological organization rather than by localized volcanic forcing. While the absence of pre-eruption or unaffected reference data limits causal inference, this study documents baseline conditions that can inform future comparative analyses of volcanic disturbance effects on pelagic ecosystems.