A research team from the University of Las Palmas de Gran Canaria sampled mesozooplankton in the Franja Marina de Fuencaliente, a Special Area of Conservation on La Palma’s western flank, fifteen and twenty months after the eruption ended. Working aboard the research vessels ACUIPALMA and TRAMES 1, they collected plankton with a 200-micrometer WP-2 net at eighteen stations arranged along three transects perpendicular to the coast, hauling vertically from depths of 10, 30, 50, or 100 meters depending on the local bathymetry. Each catch was split in two: one portion for taxonomic identification, the other frozen for stable-isotope analysis.

The analytical core of the study rested on three isotopes measured in three size fractions of zooplankton, from 200 to 500 micrometers, 500 to 1000 micrometers, and larger than 1000 micrometers. Carbon isotopes, expressed as δ13C, trace the sources and pathways of organic matter; nitrogen isotopes, δ15N, rise predictably with each step up the food chain and therefore serve as a proxy for trophic position; and sulfur isotopes, δ34S, offer clues about whether the sulfur in an animal’s tissues came from marine pelagic production or from benthic, seafloor-influenced sources. Measurements were performed with isotope-ratio mass spectrometry at the University of A Coruña, with analytical precision below 0.1 per mil for all three isotopes.

The clearest signal in the data was size. The smallest fraction, dominated by copepods and their eggs, consistently showed the lowest δ15N values, while the two larger fractions carried higher nitrogen isotope signatures and broader isotopic ranges, reflecting a mixture of trophic levels and taxonomic groups. A two-way analysis of variance confirmed that size fraction was by far the dominant source of isotopic variability, with a highly significant effect (F = 19.57, P < 0.0001).

The sampling season, winter versus spring-summer, had a smaller but statistically significant effect, and the interaction between size and season was also significant, meaning that seasonal shifts in isotope values differed among the size classes. Post hoc comparisons showed the 200-micrometer fraction differed significantly from both larger fractions, while the 500- and 1000-micrometer fractions were statistically indistinguishable from each other. Estimated trophic position followed the same size gradient.

Using a baseline anchored to the smallest fraction, treated as primary consumers, and a standard trophic enrichment factor of 3.4 per mil, the team calculated median trophic positions of roughly 1.9 to 2.0 for the smallest zooplankton, around 2.1 to 2.2 for the intermediate fraction, and 2.2 to 2.35 for the largest. The largest fraction also showed the widest spread of estimates, consistent with its taxonomic diversity, which in spring and summer included chaetognaths, siphonophores, salps, fish larvae, and decapod larvae alongside the ever-present copepods. Sulfur told a subtler story.

Zooplankton δ34S values averaged 21.43 per mil and fell squarely within the range typically reported for marine pelagic consumers, varying little across depths and seasons. In a volcanic coastal setting, sulfur signatures could in principle record inputs from fresh lava substrates, sediment resuspension, or microbial sulfur cycling in the water column. The authors are careful on this point: without isotopic measurements of the benthic substrates, sediments, or primary producers themselves, the δ34S values can only be described as consistent with a coastal environment influenced by benthic processes, not as direct proof that sulfur from the new lava deltas is flowing into the pelagic food web.

To understand the physical stage on which these plankton drift, the team turned to Lagrangian particle-tracking simulations using the Ichthyop modeling framework. They compared virtual particle trajectories under the pre-eruption coastline with those under the post-eruption configuration created by the lava deltas. Ocean currents came from the Copernicus Marine Environment Monitoring Service at roughly 2.8-kilometer resolution, which the researchers artificially refined to about 28 meters to capture the fine-scale coastline changes.

One thousand virtual particles were released north of the deltas and tracked for twenty-four hours on January 1, 2023, a date chosen because the new coastal morphology had by then fully stabilized. The simulations showed that individual particles spend only short periods within the immediate lava-delta area, consistent with the vigorous circulation typical of steep island coasts. In both scenarios, most particles reached the coastline within hours, but the timing differed: under the post-eruption configuration, a greater proportion of particles hit the coast between two and eight hours, particularly near the southern lava delta.

This means the new volcanic morphology does reshape nearshore transport pathways and retention patterns, even if it does not trap drifting organisms for long. The authors stress that these simulations provide physical context only; wind forcing, horizontal diffusion, and biological behaviors were deliberately excluded, and no causal link between hydrodynamics and isotopic composition is claimed. Seasonal contrasts were evident but did not overwhelm the size-based structure.

Winter samples generally showed lower δ13C and δ15N values and narrower ranges, while spring-summer samples displayed higher values and broader distributions, along with higher and more uniform δ34S. Seawater chemistry collected during the winter cruise reinforced the picture of a strongly stratified coastal system: surface waters carried elevated δ2H and δ18O values, an evaporation signature with a slope below the Global Meteoric Water Line and low d-excess of around 3 per mil, together with elevated ammonium reaching 1.7 micromolar at 10 meters depth. Yet despite these seasonal differences in community composition and water properties, the size-structured isotopic pattern held across both cruises, suggesting that zooplankton integrate feeding signals over days to weeks and over spatial scales larger than any single simulation can capture.

The most important caveat, and the one the authors emphasize throughout, is that no pre-eruption or unaffected reference data exist for this system. The study therefore cannot test whether the eruption changed the food web; it can only describe what the pelagic isotopic structure looks like under the new coastal configuration. What it does show is striking in its own right: a year and a half after lava remade the shoreline, the zooplankton community’s isotopic architecture looks governed primarily by the universal size-based organization of pelagic ecosystems rather than by any localized volcanic fingerprint.

By pairing isotope data with high-resolution bathymetry, satellite-derived environmental records from 2019 to 2024, and particle-tracking hydrodynamics, the team has built the baseline against which future time-series and comparative studies, on La Palma and other volcanically reshaped coastlines, will be able to measure how, and how fast, pelagic food webs respond when a volcano rewrites the map.

Subject of Research: Post-eruption zooplankton stable-isotope structure and coastal hydrodynamics near the 2021 Tajogaite lava deltas, La Palma

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

Article References: Herrera, I., García-Mendoza, A., & Haroun, R. (2026). Zooplankton stable-isotope patterns in a coastal system reshaped by a recent volcanic eruption. Frontiers in Marine Science, 13, 1803898. https://doi.org/10.3389/fmars.2026.1803898

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1803898

Keywords: zooplankton, stable isotopes, Tajogaite eruption, La Palma, lava deltas, pelagic food web, trophic position, sulfur isotopes, Lagrangian particle tracking, coastal hydrodynamics, volcanic disturbance, Canary Islands