Ocean Acidification

Highlights

• Climate change explains ∼40% of biological stress variance in Mediterranean mussels.
• Ocean acidification drives 67% of metal bioavailability changes via pH-metal speciation.
• 64% of climate effects on biomarkers operate indirectly through metal bioaccumulation.
• Multi-method convergence (LMG, GAM, SEM, Bayesian Networks) confirms stressor ranking.
• Only SSP1-2.6 keeps biological stress below the chronic-stress threshold through 2050.

Abstract

Marine coastal ecosystems face concurrent pressure from climate change and anthropogenic contamination, yet their relative contributions to biological stress remain poorly quantified. Here we present a decade-long (2014–2023) biomonitoring study on Mytilus galloprovincialis in the Ligurian Sea (NW Mediterranean), integrating quarterly biomarker measurements, heavy metal bioaccumulation data (12 metals), and high-resolution oceanographic records at a control site (Gorgona Island Marine Protected Area) and an offshore regasification terminal.

Biological stress variance was partitioned using five complementary analytical frameworks — Lindeman–Merenda–Gold (LMG) variance decomposition, Hierarchical and Generalised Additive Models (HGAM/GAM), Structural Equation Modelling (SEM), and Bayesian Networks — applied to four biomarkers: DNA damage, lysosomal membrane stability, gill tissue integrity, and immune response. A campaign-specific T0 baseline normalisation isolated environmental signals from initial population variability.

Climate change emerged as the dominant driver, consistently explaining ∼40% of variance across all methods, significantly exceeding metal bioaccumulation (∼30%), terminal influence (∼13%), and seasonal effects (∼2%). Ocean acidification was the primary climate mechanism, influencing 67% of analysed metals. Causal mediation analysis revealed that 64% of the climate effect operates indirectly through enhanced metal bioaccumulation (Climate→Metals→Biomarkers), while 36% acts directly. Climate and biological stress indices co-varied strongly (ρ = 0.78, p < 0.001), with marine heatwaves coinciding with peak biomarker responses.

Under IPCC Shared Socioeconomic Pathway (SSP) scenarios, the Biological Stress Index is projected to cross chronic-stress thresholds by 2035–2040 under the high-emission scenario (SSP5-8.5) and the intermediate-emission scenario (SSP2-4.5), with only the low-emission scenario (SSP1-2.6) maintaining stress below critical levels through 2050.

These findings challenge pollution-centric monitoring paradigms and demonstrate that CO₂ mitigation now constitutes the highest-leverage intervention for marine invertebrate health in the Mediterranean.

Graphical abstract

This graphical abstract illustrates the main findings of a decade-long field study (2014–2023) on the effects of climate change and metal contamination on Mytilus galloprovincialis in the Ligurian Sea (NW Mediterranean). Three panels summarise the causal chain from environmental drivers to biological outcomes. The first panel depicts the key oceanographic trends recorded at the study site: ocean warming (+0.41°C/decade), acidification (−0.020 pH units/decade), deoxygenation, and a doubling of marine heatwave frequency after 2018. The second panel shows how pH decline enhances the bioavailability of 67% of the metals analysed, driving a predominantly indirect pathway (64%) from climate stressors to biological stress, mediated by metal bioaccumulation, as revealed by structural equation modelling and Bayesian network analysis. The final panel presents the four biomarkers used to compute the Biological Stress Index (BSI), its strong temporal correlation with the Climate Change Index (ρ = 0.78), and BSI projections to 2050 under three IPCC emission scenarios, showing that only SSP1-2.6 keeps BSI below the chronic-stress threshold throughout the projection period.

Gaion A., Guidi P., Scatena G., Sartori D., Pittaluga G. B., Spinelli O., Dell’Ira S., Pacciardi L., Palumbo M., Bernardeschi M., Frenzilli G. & De Biasi A. M., 2026. Climate change and ocean acidification outweigh local stressors in Mediterranean mussels: a multi-method convergence analysis. Environmental Pollution 406: 128668. doi: 10.1016/j.envpol.2026.128668. Article.