In this study, we investigated Cd, Cr, Cu, Pb, and Zn in the seagrass Posidonia oceanica (L.) Delile leaves and in the brown algae Cystoseira sp. sampled along a 280-km transect in the Tyrrhenian Sea, from the Ustica to Linosa Islands (Sicily, Italy) with the aim to determine their control charts (baseline levels). By applying the Johnson's (Biometrika 36:149-175, 1949) probabilistic method, we determined the metal concentration overlap ranges in a group of five biomonitors. Here, we propose the use of the indexes of bioaccumulation with respect to the lowest (L'i) and the highest (L i) extreme values of the overlap metal concentration ranges. These indexes allow the identification of the most opportune organism (or a suite of them) to better managing particular environmental conditions. Posidonia leaves have generally high L i indexes for Cd, Cu, Pb, and Zn, and this suggests its use as biomonitor for baseline marine areas. Our results confirm the high aptitude of Patella as a good biomonitor for Cd levels in seawater. From this study, Ustica resulted with higher levels of Cd, Cu, Pb, and Zn than the other Sicilian Islands.