Climate change and ocean acidification outweigh local stressors in Mediterranean mussels: A multi-method convergence...

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.