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.