Ocean acidification drives grazer niche convergence and trophic homogenization in benthic communities

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.