Urban proximity threatens marine protected areas: evidence from PAHs and fecal sterols in mussels at Cagarras Archipelago

Polycyclic aromatic hydrocarbons (PAHs) and sterol biomarkers were quantified in Perna perna mussels from the Cagarras Islands Natural Monument (MONA), a federally designated no-take marine protected area off Rio de Janeiro, Brazil. Mussels collected in 2022 exhibited parental and alkylated PAH concentrations (197–1765 ng g −1 dw) up to five- and twelvefold higher than values reported a decade earlier, indicating a marked increase in contamination within this protected coastal system. The predominance of 2–4-ring PAHs and diagnostic ratios revealed mixed petrogenic and pyrogenic signatures consistent with chronic petroleum-derived and combustion-related inputs. Although carcinogenic PAHs were detected in all samples, BaP, Σ4PAHs, and BaP-equivalent toxic values remained below current Brazilian and European regulatory thresholds for seafood safety. Total sterol concentrations ranged from 545 to 3712 μg g −1 dw. Coprostanol was consistently detected, confirming sewage-derived organic inputs, while sitosterol suggests additional biogenic contributions associated with seabird populations inhabiting the archipelago. The co-occurrence of petroleum- and wastewater-related tracers indicates that MONA, despite its strict protection status, remains environmentally connected to the surrounding metropolitan coastal system. These findings demonstrate that no-take designation does not ensure chemical isolation in urban-embedded MPAs and support the incorporation of standardized contaminant biomonitoring into adaptive coastal management and conservation performance assessment frameworks. • PAH concentrations in MONA mussels increased up to twelvefold. • Mixed petrogenic and pyrogenic PAH sources were identified. • Coprostanol confirms chronic sewage influence in a no-take MPA. • MONA remains chemically connected to the metropolitan seascape. • Results support standardized contaminant biomonitoring in MPAs.