A reference dataset for chemical contamination biomonitoring using mussels in the Mediterranean Sea

This study presents the first Mediterranean-wide assessment based on the complete original MYTILOS database, generated through coordinated mussel transplantation campaigns conducted under a common harmonized monitoring protocol. Although subsets of this dataset have previously supported scientific publications, the complete database has never been analyzed as a single basin-scale synthesis. This synthesis supports the assessment of spatial patterns of marine contamination across the basin. The objectives were to establish Mediterranean-wide baseline contaminant concentrations, assess spatial variability among Mediterranean sub-basins, and identify regional contamination hotspots using the complete MYTILOS database. The analyses were based on data from coordinated mussel transplantation campaigns conducted throughout the Mediterranean Sea between 2004 and 2011. A standardized dataset was produced to ensure consistency across sites, periods, and analytical procedures. Marked spatial heterogeneity was observed across the Mediterranean. Organic contaminants showed stronger variability than trace metals, with higher concentrations in the Adriatic Sea and, to a lesser extent, in the north-western Mediterranean. Hotspots were consistently associated with major river mouths, densely urbanized coastal areas, and industrial-port complexes, reflecting land-sea coupling and anthropogenic pressure. The Adriatic Sea appeared particularly impacted, likely due to its semi-enclosed configuration, limited water renewal, and strong riverine inputs, especially from the Po River system. These conditions, combined with historical and ongoing anthropogenic pressures, contribute to its role as a key accumulation zone for organic contaminants. Overall, this study provides a robust historical baseline for mussel-based biomonitoring in the Mediterranean Sea and highlights persistent, basin-scale contamination patterns driven by natural and anthropogenic gradients.