Ocean Acidification

Highlights

• Acidification compresses benthic community isotopic space via carbon homogenization.
• Functional groups show differential isotopic responses consistent with bottom-up trophic reorganization.
• Arbacia lixula shifts from a sponge-dominated diet to herbivory under low pH.
• Paracentrotus lividus maintains stable trophic niche despite acidification.
• Asymmetric grazer responses drive site-specific increases in isotopic niche overlap under acidification.

Abstract

Ocean acidification is expected to alter marine food webs, yet community-level evidence from long-term, naturally acidified systems remains limited. We used stable isotope ratios (δ¹³C and δ¹⁵N) to investigate how natural acidification restructures benthic trophic interactions, from basal resources to consumer niche dynamics, a naturally acidified system (pH gradient: 8.1 to ~7.4; pHT scale) spanning present-day to end-of-century projected conditions (SSP2–4.5 to SSP5–8.5). After accounting for local differences in the isotopic baseline, we found that: (1) community isotopic space contracted under acidification, with the carbon range decreasing as primary producers converged toward ¹³C-depleted values, while the nitrogen range expanded, indicating species-specific trophic adjustments; (2) functional groups responded asymmetrically, with primary producers showing the strongest δ¹³C shifts, filter-feeders remaining relatively stable, and omnivorous consumers showing reduced δ¹³C and δ¹⁵N values; (3) the two dominant sea urchin grazers responded in opposite directions — the macrophyte-based omnivore Paracentrotus lividus maintained stable isotopic values across the pH gradient, whereas the invertebrate-based omnivore Arbacia lixula exhibited pronounced isotopic shifts — resulting in markedly increased, though site-dependent, niche overlap at the most acidified sites (up to 27%); (4) Bayesian mixing models revealed that this convergence was driven by dietary shifts in A. lixula, which transitioned from a predominantly invertebrate-based diet (70–79% animal prey) toward algal-based feeding under acidification (19–24% animal prey), while P. lividus maintained relatively consistent dietary composition (50–70% algae). Together, these results indicate that acidification-driven homogenization of primary producers, combined with asymmetric consumer responses, compresses trophic niche space, with broad implications for the resilience and functioning of benthic ecosystems under future ocean change.

Graphical abstract

Along a natural CO₂ gradient in the Canary Islands (pH 8.1–7.4, pHT scale; spanning present-day to end-of-century projected conditions, SSP2-4.5–SSP5-8.5), ocean acidification compresses community isotopic space through homogenization of primary producers. The two dominant sea urchin grazers respond asymmetrically: Paracentrotus lividus maintains a stable trophic niche, while Arbacia lixula shifts toward algal-based feeding, increasing niche overlap up to 27% at the most acidified sites.

González-Delgado S., Cardona L, Rufino-Navarro A., Hernández J. C. & Pérez-Portela R., 2026. Ocean acidification drives grazer niche convergence and trophic homogenization in benthic communities. Marine Pollution Bulletin 233(2): 120225. doi: 10.1016/j.marpolbul.2026.120225. Article.